Significant Storm System: Blizzard, severe weather AND heavy rain?

If you haven't heard already, the storm system that has been predicted on the models since last Sunday and Monday has finally consummated, and occluded in the Midwest. It is currently weakening very slowly as we speak. But even so, it packed a pretty significant punch to the Midwest.

Just to recap the last few days of weather from this storm system:

1.) It produced feet of snow and impassable travel conditions Thursday night in parts of the Rocky mountains.

2.) It strengthened further Friday morning into the afternoon, producing high winds, extremely heavy snowfall, and cold air across South Dakota and Nebraska. Snow totals have exceeded 45 inches in some areas, with the vast majority of the region receiving well over a foot of snow. The heavy, wet nature of the snow has caused a lot of tree damage and quite a few power-outages.

3.) On the NW side of the storm, within the blizzard, high winds became so strong that it broke records across South Dakota. For instance, Rapid City, SD observed wind speeds exceeding 70 MPH during the highest part of the storm! This caused widespread whiteout conditions within the heavy snow-bands, as well as causing interstate highways to be nearly impassable.

4.) In the warm-sector, heavy rain, flash-flooding and severe weather has been prevalent from Iowa and Missouri into the upper Ohio Valley. Yesterday afternoon, parts of NW Ohio recorded several inches of rain due to training thunderstorms, and most areas saw at least a period of heavy rain. In Iowa and Missouri, large, violent tornadoes destroyed several towns and caused many injuries.

5.) The storm system is still expected to produce very heavy rain across a large part of the Midwest, even with the storm system weakening. This is because of its interaction with the moisture plume associated with Karen, and the fact that it is slowing down significantly.

Take a look at the latest visible satellite imagery:

Notice the spiral band shape of the entire cyclone. Clearly, this indicates that the entire system has already occluded, and is beginning to weaken. Also notice the prevalence of convection on the eastern flank. What this means is that we should see a strong increase in thunderstorm activity over the next hours, and days perhaps if computer models are right.

The strong, closed-low in the Midwest will continue to propagate slowly eastward, drawing rich, Gulf moisture from the south, and causing repeated rounds of showers and thunderstorms. Today, expect showers and thunderstorms to rapidly build-up due to the marginally unstable and humid air mass in place. These storms will move rather slowly, allowing for the possibility of heavy rains before the main events comes under way.

By later this weekend, expect the showers and thunderstorms to organize into more organized structures, and allowing for the possibility of repeated waves of heavy rain.  It will then become possible that we see several inches of rain through Monday.

WINTER OUTLOOK COMING IN MID-LATE OCTOBER

I'm currently planning on releasing my preliminary winter-outlook for the season towards the end of October, or possibly early-mid November. It will all depend upon how certain I am of the winter forecast. 

Storm System Potential: Model Consensus

After many consecutive runs of indicating some kind of "major" storm system later this week, models have finally reached general agreement on the track of the storm system itself. Other issues such as intensity, precipitation, and thunderstorms still need to be sorted out, but over all, we are reaching a model consensus on when and where this storm will form. The timing issues that need to be sorted out are related to the mesoscale movement of the cold front into the area, which will be discussed on Tuesday and Wednesday.

And now, on to the model run-down.

12z GFS takes a shift west and south with Saturday's system, while at the same time weakening its overall amplitude and anomalous structure [instead of having a 540-546 DM core at 500 MB, it has a 552-558 DM core]. It will be some time before the GFS quits shifting the intensity and track of the system, but I think I can get a general track and intensity of the system.

The storm will likely track from Western Kansas and Eastern Colorado into Eastern Minnesota and NW Wisconsin. From both the latest runs of the GFS and the ECMWF, it appears the storm will have a maximum strength at occlusion near 1000-996 MB [if the wave is as amplified as currently forecast; right now, there are only moderate differences on the issue of amplitude; all that needs to be watched now is differences in run to run consistency] in the Western Great Lakes region before translating eastward across the Great Lakes and into Ontario. 

I'll keep watching this, especially on WV imagery, as that will help determine how well the models are currently handling the wave.

More details coming later on this potential storm system.

POTENTIAL STORM SYSTEM LATER THIS WEEK AND WEEKEND

 While other people are going gung-ho on winter outlooks.. I decided I would post something on the upcoming storm system.

Take a look at the 12z GFS and 12z ECMWF: They are coming into agreement on a rather large trough pivoting into the Midwest and amplifying by late this week and early weekend. A strong cold front will plow through the Midwest [regardless of how amplified the surface reflection becomes], causing sharp temperature drops across the front. This could quite possibly be the first major storm system of the season if everything pans out correctly. So far, the strength of the upper level wave and the surface low has been increasing between runs, so I wouldn't doubt that we see at least a slight similar trend to continue. Even without an amplified surface trough, both models are still predicting a strong cold front to move through the Midwest and eventually the East.

The key to predicting this system will most likely be dependent upon which model is over-developing/under-developing the system. At this time, I would say that a consensus of both the 12z GFS and 12z ECMWF with some uncertainty is our best bet. I think we should keep an eye on this system.

12z GFS 850 TEMP map

12z GFS 500 REL. VORTICITY

12z ECMWF

WX Analysis

***CURRENT WX ANALYSIS***

Even though the weather over the next few days won't necessarily be "exciting", I'm still going to be watching out for the potential of scattered to numerous showers and thunderstorms during the day Sunday, due to the influence of a strong and amplified short-wave trough.

A number of factors will be in play during this event, including but not limited to:

High moisture content in the atmosphere [PWATs will approach 1.9 IN, which is about the 99th percentile for late September.].

A strong, driving cold front, with strong temperature advection behind it [even though the temperature difference behind the cold front will only drop about 10-12 degrees or so].

Weak-moderate upper level dynamics and jet core circulations. This will all be dependent upon the eventual track of the short-wave trough, as well as how quickly the storm system de-amplifies. [See image below].

Weak mid-level instability [which will ultimately lead to the possibility of thunder and thunderstorms].

Because of the weaker levels of instability, I do not expect to see much in the way of thunderstorm activity, although it does remain to be a possibility.

As for the upcoming pattern, some very significant changes could possibly occur, and it will all be dependent upon the movement of the incredible Alaskan vortex [which can be seen on the image below]. If the Alaskan vortex continues to shift eastward and retains its shape, we have a decent chance at seeing a highly amplified pattern. If the Alaskan vortex only breaks down into a series of smaller waves, we will certainly see a more *active* pattern, but it will flatten out the pattern into a more zonal, and definitely less active pattern eventually. Considering the model variability on this issue, it remains to be seen what exactly will happen. Continue to follow my page for further details in upcoming days.

Record-Breaking Heat for September

After looking at the current analysis, and the short-range forecasts, I think I have a better idea about how intense this brief burst of heat will be.

First, let's look at the analysis maps:

   

Clearly, the ridge has already built into the East, and has become firmly established ahead of a strengthening shortwave in the Northern Plains. Along with this, an intense "nose" of hot, dry air from the West is surging north and eastward across the Central Plains, and into the region. By tomorrow, 850 MB temperatures are expected to approach 22-24 degrees Celsius.

Given the above factors, its becoming clearer that dry, sunny conditions, as well as gusty winds will allow deep mixing within the boundary layer. Because of this deep mixing, probably to the 825-775 MB levels, hot, dry air from the mid-levels will mix to the surface, allowing temperatures to soar to even higher levels. In fact, because of this, the NAM seems to indicate that high temperatures approaching the UPPER 90's and lower 100's ARE possible if we can get enough mixing to occur in the low-levels.

Mixing dry adiabatically from 800 MB yields a surface temperature of about 37-40 degrees CELSIUS! If this were to occur, our surface temperatures would approach 100 in many areas. However, this may not occur if mid-level temperatures cool off, and/or PBL moisture increases during the daytime. Either way, its going to be a hot and "steamy" day tomorrow.

As for the conditions for the rest of the week, more details will be coming tomorrow.

Brief Record-Setting Heat; Chilly Fall Weather to Follow

As many of you probably have heard from your local meteorologists, a brief period of hot weather is on the way this week. In fact, temperatures may even spike as high as 96 across the area on Tuesday. Thankfully, a series of strong cold fronts will follow this brief surge of hot air, causing temperatures to drop nearly 25 degrees over a 48 hour period. On the cusp of the cold front will come scattered showers and thunderstorms Wednesday and Wednesday night. The coverage of thunderstorm activity will be dependent upon how strong differential vorticity advection becomes over the area ahead of the digging trough, as well as the depth of low level moisture and ultimately instability.

So, let's get into the details.

Here are the current analysis charts, for the surface and 500 MB respectively:

On the surface chart, you can clearly see the larger than normal temperature differences between the cloudy, cool and muggy lower Great Lakes and the warm, dry Great Plains. Temperatures across Nebraska, Kansas, parts of Missouri and Oklahoma are soaring past 95 degrees, with rather dry conditions, while the humidity remains across parts of Missouri and east of the Mississippi River, with rather cooler air in place.

But as you can see from the 500 MB chart, the heat ridge will be shifting east ahead of a weaker shortwave [the same shortwave that will ultimately usher in the development of a new Eastern Canadian trough later in the week], causing a dramatic rise in temperatures across the Ohio Valley and East by later this week. In fact, temperatures could be so high as to set record-high temperatures in places like Chicago, Indianapolis, Detroit, etc. Around here, temperatures are expected to range in between 93-96 degrees.

What are the main factors that are allowing this burst of heat?

• Ultimately, the strong EML advection from the Desert Southwest, aided by the strengthening gradient around the surface high in the plains. Because of how warm the West has been lately, this dry hot air will eventually mix to the surface, where the cooler, moist air resides and cause extremely warm temperatures. 
• Dry soil conditions- For the past three to four weeks, the area has yet to see little IF ANY rain. Grounds are becoming cracked and dry, making it increasingly difficult to harvest produce from fields and gardens. Even so, clear sunny conditions will contribute to "baking the soil" and allowing temperatures to soar far passed what they would normally reach under similar conditions.
• Clear skies and mid-level subsidence will contribute as well, especially before the ridge begins to be "squelched" by the developing upper level trough that will descend from Canada by Wednesday.
• Strong boundary layer wind and mixing will transport hot, dry air from immediately above the PBL and into the surface layer, producing rather hot conditions. This will also allow temperatures to increase quickly Tuesday morning.

12z GFS forecast at hour 120 at 500 MB

Ultimately, these factors will allow temperatures to soar well-above normal, and possibly break records in many different areas. In fact, I would not doubt that most of our current computer guidance is UNDERESTIMATING this period of heat. From my own experience, usually brief-excursions of heat usually lead to rather unusual temperature anomalies, so it would not surprise me if MOST areas reach the mid-90's and possibly the UPPER-90's. However, given this summer's rather cool 'record', I don't think it would be wise to go with temperatures above 95/96 degrees this time around. Even so, the possibility is still on the table and needs to be monitored.

However, because of hot, dry air mixing down from the mid-levels, we do not need to prepare for "unbearable" humidity levels. In fact, I think its rather likely that dew points will only approach slightly uncomfortable levels Tuesday afternoon, possibly in the mid-upper 60's [which is rather high for this time of year, but not unbearable]. Dew points will then increase into the lower 70's Wednesday afternoon as moisture pools along the approaching Canadian front.

After the cold front passes through, the weather will turn rather chilly across the area, leaving temperatures in the LOW-70's and possibly upper 60's for a few days. In fact, the above GFS forecast clearly shows that the first strong longwave trough will set up across the East and Maritime Canada later this week, keeping an attendant high pressure system across the area and the associated chilly Canadian air mass.

As for the longer range forecast, details are rather lacking, but more and more signs are coming up that are pointing to a chilly middle part and end to September but a possibly warmer October. However, don't let this fool you about winter; signs are showing that November could be our transition month, much like November 2010 [Thanksgiving week went from being in the upper 60's with heavy rain and thunderstorms (we ended up with about four inches of rain that week) to the upper 20's and lake effect snow showers by the end of it], which eventually lead to a shockingly cold and snowy winter for most of the United States. More details on what I am expecting for the long-range outlook will likely come by the end of September either on my blog or in the newspaper.

Current Wx Analysis; Mundane Forecast

After a period of warm, very humid weather last week, things are settling down again, into a much cooler and drier pattern than in recent days. Temperatures will remain in the mid-70's to low 80's, under mostly sunny skies.

As for the current analysis, we currently have a large 500 MB ridge over the Central Plains, with geopotential heights approaching 594-596 DM heights at the core of the ridge.

And here is the corresponding surface chart:

Current surface maps suggest that a large surface high is slowing moving eastward across the Midwest and lower Great Lakes. This will provide a cool and dry surface air mass for the next few days, leading to chilly mornings, and nice, warm afternoons.

As for a brief look at the medium range, it appears that this pattern will continue through the end of the week before ridging attempts to build back into the region. This will allow for a STORMIER pattern to finally set up across the United States. And when I say "stormier" I am not referring to thunderstorm activity, but rather the overall movement and placement of upper level disturbances, which will finally lead to some increase in activity in our weather pattern. We will continue to see more upper level disturbances as upper level flow increases, and shortwaves become more prevalent. As this occurs, we need to watch for the potential of late season severe weather episodes, due to upper level jet dynamics, surface low development and moderate instability. I will continue to watch this new pattern as it unfolds, but it could speak volumes for what will happen later this fall, particularly in November. Because if it unfolds the way that we are currently expecting it to, we could be in for a much more active winter than in the past two seasons. In fact, this is also best reflected by current SST anomalies in the Pacific, the MJO cycle, and the long-range PDO cycle.

Current Weather Analysis and the Upcoming Week

After several days of cooler than normal temperatures, temperatures will begin to return back to normal, and even above normal by this week. In fact, it appears we may have a quick burst of "heat", with temperatures possibly hitting the upper 80's and the lower 90's. Not only this, but moist, tropical air will accompany this surge of warmth, and most likely cause scattered showers and thunderstorms.

But currently, we have a very weak upper level pattern in place, that is barely defined, and very zonal/progressive. A weak upper level low is currently in place over the mid-Mississippi and Tennessee Valleys, diverting moist flow from the lower Great Lakes and into the East [and subsequently around the ridge in the Central Plains]. As soon as this weak upper level feature breaks down, that is when we should begin to see the humidity rise again.

The MOS forecasts are indicating that, specifically at Fort Wayne, temperatures will gradually rise this week from the low-mid 80's to the upper 80's, although there is obvious disagreement between the two models on this issue.

Here's what will happen:

The weak upper level "height depression" will begin to weaken early this week, allowing the broad [and massive] Western ridge to shift east. By Tuesday and Wednesday, the 588 DM line should be well east of the area [which is usually when its known that a "heat dome" is over us], and allow a surge of warmth and moisture into the area. Temperatures will rise into the mid-upper 80's and dew points into the upper 60's.

By the end of the week, temperatures should continue to rise into the upper 80's/possible lower 90's as the massive high pressure system shifts offshore. Dew points will continue rising as well.

After this week, it is uncertain of what will happen but it appears that ridging may in fact build back into the Midwest and East the following week. It appears that we may have a very warm end to the summer before Fall begins.

Current Weather Analysis and Brief Glimpse at the Long Range

Once again.. I'm sorry for the lack of updates, but they will be much more frequent as we progress over the next few weeks.

In other news, I'll start with the current weather analysis. Over the past few hours, a few scattered showers and thunderstorms have popped up over the area. Some of these have produced heavy rainfall and gusty winds over isolated area. This is currently happening in a moderately unstable air mass across the area, in the wake of the cold front passage. As drier and cooler air moves in later this afternoon and tonight in the vicinity of a cooler Canadian high, instability will decrease allowing for much clearer conditions for tomorrow.

In the upcoming pattern, the vast majority of medium and long-range guidance continue to indicate a cooler, and possibly wetter pattern to continue, as an H5 trough descends into the Eastern United States [an H5 trough is a 500 millibar trough], possibly bringing a more unstable pattern in regards to storm systems. This pattern will likely continue through most of the early Fall season as well.

More updates will come later tonight and tomorrow. Have a good evening.

Update to the Long-Range Pattern: Heat Not Out of the Question, but Less Likely

After yesterday's blog post on the "potential" for heat in the medium and long-range, I'd like to issue an addendum to that post, summarizing what the latest models have changed in regards to the time period in question; 120-168 hours out.

The latest models have shifted westward in regards to the placement of the upper level ridge. Yesterday, the 00z and 12z runs of the ECMWF were indicating that an amplified upper-level ridge would shift eastward with time, shifting the heat dome eastward into the region.

Today's model runs have shifted the dome westward, with the area on the periphery of a sharp [yet very "shallow"] trough over the East and Appalachians. This will cause a sharper temperature gradient between temperatures in the 90's to our West and 70's/80's to our east. What's very likely to happen here is that the area has temperatures "juggling" between 85-90, and then possibly a few days with temperatures exceeding 90. It will all depend upon the axis of the upper-level ridge, which at this time appears to set itself up across the Northern Plains.


Here's a look at the ensemble consensus and the ECMWF from the WPC:

For the next few days, the upper level height pattern will flatten, allowing clouds and the upper-level cold pool to clear out of the area, as well as allow for warmer temperatures. In fact, within the next four days or so, we could hit the lower 90's [if enough insolation is realized].

On the otherhand, a shortwave trough dropping out of Canada will amplify the ridge, and cause it to retrograde. It's entirely plausible that with the very moist and unstable air mass in place that we see numerous showers and thunderstorms from this shortwave. Even so.. It's influence will be minimal on the area and for a short period of time. After it shifts east, the ridge will attempt to edge eastward, but very slowly. It will not be until the next shortwave trough shifts out of the Western trough, and causes the ridge to amplify that we get a chance with the heat. If this does not happen, then its not likely at all that we see any kind of marginal "heat" [90+].

I'll continue to monitor the situation for now.. More details will either come early tomorrow morning, or it'll have to wait a week. I'm going to be gone for most of next week and the following weekend, so don't expect any updates.

Have a great evening!

The Possibility of Heat Returns; Mundane Warm and Humid Pattern in the Short-Range

Before I get to the longer-range forecast and its implications for the rest of July, let's look over the details of the shorter-range forecast, and what it means for the next several days.

First and foremost, the Bermuda high is "attempting" to push westward from the Western Atlantic on to the East Coast. At the same time, this Bermuda high is pumping extremely warm and moist tropical air into a large portion of the Eastern United States. Because of this warm, moist, and at times, unstable flow, the mundane pattern of warmth and humidity will continue for at least the next 3-5 days, with the possibility of daily thunderstorm activity in the tropical air mass.

The Bermuda high is being forced westward because of the breakdown of the "split" trough that has been sitting over parts of the East and Midwest for days, bringing heavy showers and thunderstorms at times. This pattern will end soon, giving way to diurnally-driven showers and thunderstorms, and then to a drier pattern in the long-range.

As for now, expect increasing humidity and day-time temperatures in the coming week, with high temperatures averaging around 85-90 with dew points in the low to mid 70's. This will allow for isolated thunderstorms to develop each day in the unstable air mass.

As for the long-range, there is some model disagreement, but the overall consensus is as follows:

1.) The Bermuda High begins to shift back east, and causes the height pattern at 500 millibars to flatten out across the United States.

2.) At the same time, a shortwave ejects from the main Alaskan vortex [which is trying to hold together], and flattens the Western ridge, while sending a surge of warmth eastward [about mid-late next week; possibly allowing temperatures to hit 90].

3.) The shortwave then deepens weak troughing in the far East along Maritime Canada and the East Coast. This causes a re-amplification of the upper level pattern, and causes the ridge out West to strengthen.

4.) By late next week, and the weekend, a secondary stronger wave ejecting from the main Alaskan vortex [and subsequently weakening it as well] will force the ridge to move eastward, and amplify. This will cause mid-level subsidence to strengthen, with compressional heating, daytime clearing, and thus much warmer air temperatures.

At the same time, ridge heights may build as high as 5940 meters or more according to the ECMWF, which could allow 90+ surface temperatures. Even then, the humidity levels will linger, with dew points well above 70 likely.

Even in this case, we still have model disagreement. Some of the ensemble members have a much flatter height pattern in the long-range, and thus, not as warm of surface conditions [but still rather warm]. I will continue to monitor this to see how the models change their forecast in coming days, but I do think its becoming apparent that a much warmer pattern is headed our way for many areas west of the Appalachians.

The trend in the PNA index indicates that a warmer pattern is a possibility later next week.

Upcoming Pattern; Heat on the Horizon?

The beginning of summer has know passed, and we are entering into the middle period of summer. Even so.. We have seen very little heat, but longer periods of cooler, wetter weather. This summer/late spring has been quite unusual in this regard.. In fact, June ended as one of the wettest Junes on record, with nearly six inches of rain.

Even so.. It has been rather cool ever since a strong cold front pushed through. In the wake of the cold front, a closed upper level trough formed, and closed off from the main flow over Canada, allowing a highly anomalous pattern for this time of year. It was something that was VERY unusual.

So what is likely to happen over the next several weeks? Well, models have remained fairly consistent in bringing in a warmer, but more zonal pattern to the United States. For the beginning of next week, both the ECMWF and GFS indicate that a strong ridge will form over the East Coast [594 dm+], allowing 850 temps to soar passed 18-20 degrees Celsius. This indicates that surface temperatures will likely exceed 85-90 a few times next week. But at the same time, due to an incredibly saturated ground, very high dew points are likely, with dew points possibly exceeding 75 next week. This allow for the development of daily showers and thunderstorms as well.

After this, the models show somewhat of a "flattening" of the Eastern ridge, which will give way to a pattern more favorable for MCS thunderstorm complexes. I also believe that with this flattening, the Midwest/Rockies ridge may surge eastward once again, and allow for a period of well above normal temperatures [temperatures possibly exceeding 95 for a few days]. If this does not happen, I would expect the continuation of a warm, and very humid pattern, with the possibility of numerous thunderstorms.

More details about the medium and long-range coming tomorrow...

UPCOMING LONG-RANGE PATTERN

I'm extremely sorry for the long hiatus.. I've been forgetting to post on the blog, and instead I've been posting on the Facebook page. I will be posting much more frequently on the blog though.

As for this update, it won't necessarily be all that long, as I do not have much to discuss, but I would like to discuss what I believe will happen in the short-term and long-term forecasts.

As for the short-term...

It does look like 90-degree weather will persist through at least Wednesday. So far, many places in the region have hit 90 on both Saturday and Sunday, and today will just keep that streak going [specifically if current trends in cloud cover persist].

By tomorrow and Wednesday, the area could be looking at temperatures in the lower 90's with heat indices near or exceeding 100. Dew points will increase into the low-mid 70's, especially on Wednesday and Thursday.


By Thursday, expect high temperatures to drop into the upper 80's, as the cold front approaches and as cloud cover increases across the area. Dew points will also be rather high, allowing for scattered thunderstorms to develop.

From Friday through next Tuesday, I expect temperatures to recede back into the lower 80's with more pleasant conditions. I expect that this pattern will continue through July 4th, before the Western ridge flattens once again, and shifts back east. I find it highly likely that 90-degree weather returns to the area for the week of July 8th through the 12th, and possibly persisting longer. This goes along with my general idea that as the summer progresses, our bouts of heat will become more intense and of longer duration. I expect this year to see the vast majority of our 90-degree days at the end of July through late August and early September.

Vorticity: Atmospheric Motion, Shortwaves, and the Vorticity Equation

I'm going to take a brief break from writing in my usual style of blog posts; i.e. forecasts and outlooks, and begin writing about certain meteorological topics; some technical, others not so much. But these topics will help those who want to understand forecasting more to understand certain atmospheric processes which are important to dynamic meteorology.

Vorticity Equation

In this series of blog posts, we will be looking at three different "versions" of the vorticity equation; examining the terms, and how they can be applied to our understanding of atmospheric motion and forecasting.

The first vorticity equation we will be looking at is the height-coordinate form of the vorticity equation. As you can see, it is really straightforward, and is in no need of converting from one coordinate system to the other. So, let's go through the terms and identify them first.

1.) First term on the left:

This term is the Lagrangian time derivative of absolute vorticity. Thus, it is time derivative equation of the change in absolute vorticity FOLLOWING the flow, instead of at a point location. In other words, it is showing the change in absolute vorticity of an parcel which is moving along with the "stream".

2.) First term on the right:

This is the convergence/divergence term. Convergence acting upon a moving column of air will cause its area to decrease, thus causing a correspondence of increasing relative vorticity (assuming that it remains at a constant latitude and therefore, the Coriolis parameter is constant). Increasing relative vorticity leads to stronger convergence, and thus, with no other forces acting upon the parcel, relative vorticity would increase ad infinitum. 

 

The term also shows that divergence causes a decrease in cyclonic relative vorticity, and an increase in anti-cyclonic relative vorticity. Divergence acting upon an air column will cause it to expand, and thus, due to the conservation of angular momentum, must have a corresponding decrease in relative vorticity.

 

3.) Second term on the right:

This portion of the vorticity equation outlines how vortex tube tilting affects vertical absolute vorticity.

Here is an example image:

From this, we can deduce that when the atmosphere has strong vertical shear, that horizontal vortex or vorticity tubes develop (theoretically). When differential vertical motion occurs in the vicinity, this will tilt the vorticity tubes into the vertical, introducing a number of effects, which cause the upper level pattern to modify (an increase in vorticity, by the geostrophic approximation, must be followed by a response to restore geostrophic balance. To do this, upper level height falls must occur, and the upper level wind patterns must acquire cyclonic curvature to compensate for the increase in vorticity).  

4.) Third term on the right:

This is the trickiest part of the vorticity equation, and perhaps the hardest one to understand. This term shows us that absolute vorticity can also change with time due to density differences in the x and y direction, and differences in the gradients. This term is positive when there is significant "density" advection, or if considering it on a constant pressure plane, thermal advection. This is the solenoidal term, and the circulation is related to the density of air. This last term is the main reason as to why the height-coordinate version of the absolute vorticity equation is not used in applied meteorology.

In coming blog posts, I will explain more of the isentropic form of the vorticity equation,  AND the pressure-coordinate form of the vorticity equation.

But what application can this have for meteorology? Well.. There are several things to note:

1.) The first term is significant, in that if we can identify regions of upward vertical motion, we can predict how the absolute vorticity tendencies will evolve with time. This is significant when forecasting cyclogenesis and the strengthening of a storm system. If upper level analysis indicates short-wave ridging ahead of a trough, its likely due to strong divergence AND warm-air advection at lower levels (because divergence causes surface convergence, which strengthens the low-level flow, and thus warm-air advection).

2.) The second term is significant, because this indicates to us how jet streaks, and changes in the thermal wind can cause shortwaves and troughs to develop out of originally straight, and "fast" upper level flow. When vorticity is tilted in the vertical by differential vertical motion, this leads to the development of shortwave troughs and "eddies" in the jet stream.

3.) The third term is not as significant for forecasting, because this equation is not widely used (and, if we apply scale analysis, we can see that this term is usually very small compared to the others), but when ever thermal advection occurs on the horizontal and even on the vertical scale, some sort of solenoidal circulation will develop; An excellent example of this is the sea-breeze circulation.

This is part 1 of the series on vorticity, shortwaves, and atmospheric motion. Tomorrow, we will be discussing the pressure-coordinate form of the equation, and how it can applied to forecasting. By the third article, we will then begin discussing the relationship between these equations, and the Quasi-geostrophic omega equation.

Heavy Rainfall Threat Around the Corner

Synopsis

 A certain situation is beginning to arise this weekend which will set up the possibility of heavy showers and thunderstorms from Thursday night through Sunday afternoon. There are several factors that will come into play:

1.) A closed low over the Central Plains and Midwest will begin to open-up in response to a digging shortwave in the Hudson Bay vortex. This will cause the ejection of a series of shortwaves around a positively-tilted trough, and allow for periods of thunderstorms.

2.)  In response to this deepening trough, the low-level "Gulf stream" or low-level jet will begin to strengthen, providing direct transport of a very moist and tropical air mass. Dew points will approach the upper 60's and lower 70's, with mid-level lapse rates approaching the moist adiabatic lapse rate (with at times a nearly saturated mid-level profile).

3.) The slow-moving nature of the trough, and frontal zone, as well as fairly weak upper level winds will allow for the possibility of slow-moving and training thunderstorm complexes. MCS development will also be likely Thursday night and Friday night as the low-level jet strengthens. 

Thus, these factors will come together to produce the possibility for a multi-day heavy rain event, culminating on Saturday night, and likely leading to the possibility of severe weather at times. 

 

 Rainfall threat/probability

As we can see from the above "factors and ingredients".. The possibility remains for flooding and heavy rainfall this weekend. Let's take a look at what the models are saying.


1.) A very rich and deep GOM tropical air mass will be entrenched across the area Friday through Saturday. PWAT (Precipitable Water) values will exceed 1.5-2 inches, indicating a rather incredibly moist atmosphere, and the potential for heavy rainfall, if deep-layer moisture convergence and efficency is also involved. 

2.) A positively-tilted, slow-moving upper level trough will be the main "controller" of this weather situation. The slow-moving trough will only allow for weaker low-level convergence along the stalling surface front, while upper level divergence from the upper level jet core could allow for multiple thunderstorm complexes to develop parallel to the front in a moderately-unstable and moist air mass. Weaker winds aloft will allow for training thunderstorms, especially in tandem with strong storm-relative inflow from the low-level jet.

3.) These periods of thunderstorms will not likely be continuous, given the lack of strong upper level forcing and only modest instability at times. They will develop at peak heating times, and organize into complexes in regions of stronger wind-shear. And with this, comes the possibility of highly localized flash-flooding.. Some areas will see lower rainfall amounts near a half inch, while other areas could see as much as four inches of rain (depending on how thunderstorms train).


Models currently vary on how much rainfall will fall across the region, and given the isolated nature of thunderstorms, its going to be hard to pinpoint a general range of rainfall amounts, but at this time, I will have to agree with the WPC (former HPC), who are forecasting a general 2-3 inches of rainfall areawide. While I do not see such rainfall amounts being "widespread", I do see a wide variety of rainfall amounts, ranging from a half an inch to over 4 inches possible. Heavy rainfall on this order could cause regions of minor to moderate flooding.

 

Severe Weather

Although I do not expect the severe weather threat to be nearly as substantial as the threat over the Central Plains today or tomorrow, there is still the possibility for hail-wielding or thunderstorms with damaging winds from Thursday night through Saturday. This will all hinge upon the moisture-content of the lower atmosphere, solar insolation, and the depth of the PBL on both Thursday, Friday and Saturday.

1.) The #1 factor in this event will be instability. Dew points are indicated to approach the upper 60's and lower 70's, with a very moist profile throughout the entire atmosphere by Friday. Although mid-level lapse rates will be lower due to the tropical influence, surface heating and mixing will allow for MLCAPE to approach 2000 J/kg both tomorrow and possibly Friday. This will be heavily dependent on surface heating, as mentioned before. Models continue to develop scattered numerous showers and thunderstorms Thursday afternoon through Saturday, but I disagree with their assessment. The models, the NAM especially, seem to be overdeveloping convection, and developing convection too far away from the front itself. Thus, it seems to me that models are too cool with surface temperatures. I believe that if cloud cover is not widespread tomorrow or Friday, temperatures could reach the upper 80's on both days. If this is the case, expect substantial instability.


2.) 0-6 km bulk shear will be lacking through most of the event (unless, like I described before, a jet streak develops along the eastern flank of the trough), and thus storm organization will be limited to localized shear along boundaries, and the development of shear due to storm-scale processes. Thus, any kind of organized storm development will also be limited to multi-cellular storms and bow-echoes.


3.) With the above factors, I think the severe weather threat in itself will be limited, especially if we see a longer duration of cloud cover tomorrow, Friday, and Saturday. But even so, we need to be on the look out for strong to severe thunderstorms.

Warm Week Ahead After Cooler Weekend...

After a very cool, and somewhat rainy, Memorial Day weekend. expect a nice reprieve from this dreary weather into a very warm, sunny, and possibly humid weather pattern. Temperatures will increase into the mid and upper 80's under the influence of a strengthening upper level ridge, and with increasing flow from the Gulf of Mexico, the warmer temperatures combined with higher moisture could lead to widespread thunderstorm development by the weekend.

Since we all know that good things can't last forever, I'm here to also inform you that this period of "hot" weather will not likely last all that long. A weak trough looks to become "stuck" in an increasingly "split-flow" pattern, which will allow for a cooler "pocket" of air to drift around the Great Lakes region by Sunday and Monday. This will cause temperatures to drop back into the mid-70's for the end of the weekend.

BUT, do not despair. Long-range models have consistently indicated that an even warmer blast of air is on the way, starting next Tuesday. As the weak trough exits the region, ridging will build in from the West, sending the hot EML eastward. This will override a cooler, but moist surface air mass, but as soon as full insolation commences, mixing in the PBL will draw some of this warmer air downward, and allowing hot temperatures to prevail once again. It appears very likely that 850 millibar temperatures may approach or exceed 20 degrees Celsius during this next surge of heat; making it all the more likely that we exceed 90 degrees.

But what does this mean for the rest of the summer? Based on the current trends, its becoming apparently obvious that the rest of the summer may not be as "incredibly hot" as it has been in the previous three years. Although this is subject to change, its likely that instead of seeing average temperatures above normal by 5-7 degrees, its likely that our summertime average will only be 1-3 degrees above average. Now, it is too early to judge how the rest of the summer will turn out, especially in light of the fact of what happened last year (very, very cold beginning to June; but the rest of the summer turned out to be outrageously hot), but I think its safe to assume that this type of pattern will continue for the foreseeable future. This means that June will likely see a fair number of 90 degree days (3-5 days?), while July will probably see slightly more than normal 90 degree days (12-15 days?). Even so... I expect that this summer will feature a "backloaded" summer; i.e. it will remain quite hot even into September, unlike the previous three years.

Warmth, Humidity, and Thunderstorms On the Way

For the past several days, I have been monitoring the potential for much warmer weather, and along with it, the chance for strong to severe thunderstorms.

Although it doesn't look like the area will see any widespread severe thunderstorms, instability on Monday evening and Tuesday afternoon may be sufficient for the development of a few severe thunderstorms, especially Tuesday. This will be highly dependent on the coverage of rainfall Tuesday morning and early afternoon, as well as the proximity of the low-pressure system itself.

So here's the scoop:

Let's take a look at current observations and satellite trends before we go any further.

 

This image is the current surface observation chart across the lower Great Lakes and the Ohio Valley region. If you are more experienced in meteorology, you've probably noted the strong wind-shift and convergence zone which is draped across Northern Indiana and Ohio, as well as the "actual" warm front, which is further south. The actual warm front is demarcating the arrival of tropical, moist air. Although there is very little temperature contrast across the front as of right now, the warm front should strengthen later this evening and night. This will occur as the EML, or Elevated Mixed-Layer, is advected from the Southern Plains into parts of the Ohio Valley in advance of a major cyclone.

There are a number of reasons why I do not expect this front to advance much today:

1.) East-northeasterly winds north of the warm frontal boundary (where the wind-shift and convergence zone is located) will keep advecting drier air from a weaker Canadian high into far Northern Indiana and Ohio.

2.) Although I do expect some progress northward of the front, it doesn't look likely, and thus the reason why instability and moisture will be limited. With this in mind, do not expect anything more than a few showers this morning and afternoon. Other than that, I would just expect partly cloudy skies.

3.) The low-pressure system in the Central Plains has not yet consolidated, and has yet to develop significantly, and thus why the warm front is moving so slowly. Once the cyclone develops, the warm front will begin to shift northward quite rapidly.

The latest GOES sounding also gives a clear indication as to the strength of this warm front. We can clearly see the warm frontal inversion by looking at the moisture and temperature profile. Based upon the current height of the frontal inversion, this suggests that the actual front is not too far away, so it could very well pass through the area today.

 

The above image is current infrared satellite imagery. Based upon this, it doesn't look like there will be much rainfall or even shower activity later today. With instability on the lower side, low-level moisture scarce, and no upper-level forcing mechanism, precipitation is not likely today.

 

One of the more prominent features on the analysis charts this morning is the building upper level ridge across the United States, in advance of a strong Pacific jet streak, which is causing a major shortwave trough to dig. Although this is happening slowly right now, accelerated height falls later this evening will lead to further development of the upper level disturbance, which will cause the cyclone to strengthen considerably.

So.. Let's summarize the main features on the weather charts this morning, and I will elaborate on what this means for the next few days.

1.) A large synoptic scale ridge is building across the Central Plains and Midwest, and into the Great Lakes and Ohio Valley. This will lead to further northward movement of the warm front, and thus well-above normal temperatures by tomorrow and Monday. Ridging should cause weak subsidence to develop over the area tomorrow, and thus, little or no thunderstorm activity, even though instability will be in the 1500-2500 J/kg range.

2.) Current satellite imagery suggests that precipitation will be scarce or non-existent today, so expect a nice, dry and warm Saturday! Looking at other parts of the Midwest, we can see that thunderstorm complexes are developing north of the warm front and north of the low pressure system. These thunderstorm complexes will lead to the development of outflow boundaries, and the possibility of thunderstorm development south of the current clusters of thunderstorms. But due to the building ridge, subsidence should limit thunderstorm development into Monday. 

3.) Surface observations and the GOES sounding suggests that the warm front is approaching, but is being hindered by dry northeasterly winds on its northern side. This suggests that the warm front will make little progress today. Even so.. If diurnal heating is more than expected, I would expect that the warm front pushes north of the area for at least a short while, which could allow for thunderstorm development in the humid air mass.

Now.. For the forecast through Monday.

As of right now, there isn't much model agreement on the timing and strength of the cyclone and upper level disturbance that will impact the Midwest from Sunday through Wednesday. Model agreement is especially lacking between global and mesoscale models. This kind of disagreement between global and mesoscale models is always expected, but it certainly hinders the forecast process.

1.) Given the NAM's track record with forecasts beyond 48 hours, I would expect that the ECMWF and GFS are more correct with their portrayal of a slower track of the low-pressure system on Monday. It is certainly likely that the NAM begins to trend toward this solution over the next three runs.

2.) The overall pattern suggests that the GFS and ECMWF solution is much more likely. This is likely for several reasons as well:

A.) The upper level wave that is digging in the West due to the Pacific jet streak is building very slowly. Upper-level height change analysis suggests this as well, with only 30-60 decameter changes in 500 millibar height in the last 24 hours. This doesn't appear likely to change at least until later Sunday afternoon when the surface cyclone begins to intensify.

B.) The jet stream itself is very weak, and thus not able to move storm systems very quickly. And given the shallow nature of the Pacific cold front behind the system, its also not likely to move very quickly. This gives credence to the global model solution.

This is the GFS model forecast for late Monday evening of surface dew point temperatures.

With upper level ridging in place, the warm EML will be advected from the Southern Plains into the Ohio Valley and Great Lakes region by Monday. The warm EML will allow 850 millibar temperatures to climb to near 18-19 degrees Celsius across parts of the area. With deep-mixing of the boundary layer, this could lead to high temperatures close to or over 90 on Monday. This in combination with dew points in the upper 60's to the lower 70's will lead to high instability; possibly in the range of 2500-4000 J/kg. This will likely lead to at least some thunderstorm development Monday night, but I will have to monitor the situation, as it is extremely uncertain. 

By Tuesday, we can definitely expect the arrival of more widespread and numerous thunderstorms across the area as the Pacific cold front approaches the area.

The main factor in whether we see severe thunderstorms or not on Tuesday is instability. On the one hand.. We have the NAM, which predicts a faster arrival of the Pacific cold front, and severely limits instability. On the otherhand, we have the GFS showing instability across the area in the range of 2000-3000 J/kg, which would definitely lead to the significant possibility of severe thunderstorms. At this time, it is uncertain, given the GFS's "moist" bias, and the NAM's bias towards faster solutions.

By Wednesday, the Pacific cold front will pass through the area slowly, dropping temperatures into the upper 70's. You can also expect scattered showers and thunderstorms throughout the day, but I don't expect it to be entirely a washout.

From Thursday through Memorial Day, much more quiet weather will settle in to the United States as a 1030 millibar Canadian high descends into the Midwest. This will keep temperatures in the lower to mid 70's. After this.. It is uncertain of what exactly will happen, but its likely that another surge of warmth will come this way. I do not agree with the long-range model solutions of a more "blocked" and amplified upper level pattern, as it does not fit with the pattern that has been observed for the past few weeks. Even if that were to happen, I would continue expecting high temperatures in the mid-70's.