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.

**MAJOR WARM-UP ON THE WAY FOR THE END OF THE SCHOOL YEAR**

**MAJOR WARM-UP ON THE WAY FOR THE END OF THE SCHOOL YEAR**

Finally.. After months of rather cold, dreary, wet, and sometimes snowy weather, we are finally looking at the possibility of a long stretch of warm-weather, which doesn't look like its going to come to an end any time soon. In fact, for the foreseeable future (10-14 days), temperatures should remain above normal, with highs ranging from the mid-upper 70's, to the mid-80's. In fact, I would not doubt that during one or two of the warmer periods that we see a 90 degree day.

With this stretch of warmer weather, we can also expect periodic showers and thunderstorms, as shortwaves ride along quickly in the weaker upper level jet stream. Chances of thunderstorms will remain across the area everyday through at least Sunday. Although these chances for thunderstorms are relatively miniscule, they will still provide the area with brief heavy rainfall in isolated areas.

Let's look at the details now:

1.) Starting tomorrow, we can expect temperatures to reach the mid-70's and warmer for the foreseeable future. Models are in very good agreement on this (with the exception of the GFS, which will be explained later).

2.) A weak cold front will arrive in the area by late Wednesday night, which will drop temperatures from near 84 down to 77-78. Thunderstorms will accompany this front with scattered rainfall most likely at this time.

3.) Longer-range forecast models are currently predicting that this warmth will last through at least the weekend, with temperatures even rising back into the mid-upper 80's by Sunday/Monday. This is where model discrepancies come into play.

A.) The GFS is showing a strong low pressure system taking shape across the Northern Plains, driving a very warm-air mass into the area. If the GFS were to verify (as well as its winds and boundary layer forecasts), I would expect temperatures possibly exceeding 85-88 on Sunday. It also indicates that dew points will approach the MID-70's. This will create a very humid, and unstable air mass in the vicinity of a **possible** strong cold front Sunday night; allowing for a potential severe weather outbreak. Don't worry though.. This solution isn't all that likely.

B.) The ECMWF shows a relatively "lax" and moderate pattern. It indicates that we will periodic shortwaves move across the United States, which will bring scattered thunderstorm development. It is not expecting any major storm systems, and no waves of cool air. It is the warmest solution, as it keeps temperatures in the 70's, 80's, and possibly 90's for the entirety of its 10-day forecast.

 At this point, I would give more weight to the ECMWF, as the overall pattern seems to be the case of what it is showing. Furthermore, its ensembles are in general agreement with it and the general pattern. It would suggest that later in the period, a strong trough will dig into the West, forcing height rises in the East, and thus a potential "heat-wave" by mid-next week.

Although I don't see any major rainfall events headed our way, scattered showers and thunderstorms on Thursday, Friday, and Saturday could lead to brief periods of heavy rainfall, and thus "ruining" some plans.

And as far as I can see, temperatures look to remain in the lower 80's and upper 70's for the next two weeks, with possibly warmer weather as well.

Cut-off Low in the Plains; Normal, Moderate Pattern on the Horizon

**Post frequency will begin to go up as soon as school gets out. It has been quite hectic for me lately.**

The main story it seems of the past week, has been one of normal early May warmth, all the way to record-breaking May snowstorms. Why is this occurring?

The image above is a chart from the 500 millibar level, which is normally about 5600 meters above the earth's surface. As you can clearly see, there is a large upper level, and nearly stationary low sitting over the Mississippi Valley region. At the same time, you will notice that this same low is producing a band of "retrograding" showers and thunderstorms, which have made very little eastward progress since this morning. In fact, they aren't expected to make ANY eastward progress within the next 48 hours, before dissipating. This means it is very likely that the most rainfall that we see is little more than a trace of precipitation.

Even so.. Conditions will become slightly cooler tomorrow. Temperatures will drop into the upper 60's and lower 70's.

For the next 10 days or so, expect rather tranquil and pleasant weather conditions, with temperatures hovering in the low to mid 70's. Late next week, the possibility of thunderstorms arrives again as a series of weak shortwaves approach the area from the NW.

And then.. The model differences come into play. The GFS is forecasting the continuation of a relatively warm pattern, and in fact, shows the first potential "heat" wave of the season coming for the Midwest by May 15. While I doubt it will get warmer than the middle to upper 80's, this is definitely something to watch.

On the otherhand, the ECMWF forecasts that a deeper trough could come into play in Southeast Canada, which could lead to a series of "backdoor" cold fronts which bring periods of relatively cooler weather (highs in the 60's). In fact, it forecasts that we could have a few additional frosts before mid-month, according to the model.

At this point, it is unreasonable to make any kind of long-range predictions based upon models. What I CAN do is make predictions based upon current observations.

1.) The MJO, which is the main driving force behind the "nature" (speed, curvature, and the wave number) of the Pacific jet stream will become very active, and head into phases 4, 5 and 6 by mid-May. This would allow for a zonal Pacific pattern, but with an active jet stream, which will not allow for the development of any significant troughs or ridges, leading to a warmer pattern.

2.) The continuing weak La Nina/Neutral ENSO points against a cooler May/Summer period.

3.) The drought in the West.

Why should I say the drought in the West? Specifically because whenever the core of the heat across North America focuses in the Rocky Mountain region, it almost always is pushed eastward by an active Pacific jet, which is why I believe there will be several periods of warmth, with increasing frequency into June. And that is when we will see our first heat waves much like last year. But the thing about this season is that it will be much wetter, with a much higher frequency of storm complexes and severe thunderstorms.

More details will be coming in a few weeks, when I create my official summer outlook.

SEVERE WEATHER THREAT.. MORE DETAILS LATER

I am extremely sorry for the lack of updates!

THIS IS AN IMPORTANT UPDATE!

Strong to severe thunderstorms are possible tonight, tomorrow, and tomorrow night, based upon the latest trends. Models have continued to shift northward with this very dangerous severe weather threat. In fact, it would NOT surprise me if the SPC expands the MDT (Moderate) risk into most of the area (it currently resides west of our area), and places a high-end risk for Western Indiana/Eastern Illinois. At this time, based upon the latest observational and model trends, it looks likely that the warm front will surge northward this evening and tonight. Currently, the warm front resides in Central Indiana, and should continue pushing northward, before evaporational cooling caused by heavy thunderstorms forces the warm front to remain stationary during the night. THIS IS WHERE THE FORECAST GETS VERY TRICKY! If thunderstorms do NOT form as expected, and head mainly north of us, I will expect a very quick passage of the warm front late this evening and early tomorrow morning. The warm front will bring very warm, and humid air to the region, with dew points approaching the mid-upper 60's tomorrow area wide. High temperatures under partly to mostly cloudy skies will approach 75-80 degrees, ESPECIALLY south of U.S. Route 6.

But.. For this post, I will deal with tonight's "precarious" threat. The SPC has issued a tornado watch for Western and North Central Indiana, all the way into Eastern Illinois. This deals with the very DANGEROUS threat for low-level vorticity and rotation along the warm front to couple with thunderstorms to produce tornadoes, and large hail (due to the very stable nature of the low level atmosphere, damaging winds will not be a primary concern.). But this is for Western Indiana. These storms will travel east-northeast into the region, producing heavy showers and thunderstorms later this evening. Although I don't expect much, if numerous thunderstorms develop, we could be looking at heavy rain tonight, instead of a drier night with a few showers. I will keep watch on this threat. Don't expect much rain, if any, before the 8 p.m. timeframe though. 

Current radar imagery

As for tomorrow.. It's looking increasingly likely (as stated before), that dew points will increase into the upper 60's by late afternoon, with temperatures in the upper 70's. If this occurs, we can expect moderate to strong instability across the region, with the potential for discrete supercell thunderstorms, and squall line segments. THIS WILL ALL DEPEND UPON THUNDERSTORM DEVELOPMENT TONIGHT AND THE MOVEMENT OF THE WARM FRONT. Don't get me wrong... The strongest potential WILL remain farther west at this time, but severe thunderstorms will still impact the area. I will continue to watch this, as the main threat could expand east (especially due to the very progressive nature of the cold front). Deep-layer shear will also be very conducive to tornadic development, and so we will have to watch for this threat. Other than that, tomorrow will be a very warm, humid day.

MORE UPDATES LATER.. STAY TUNED.

Thunderstorm, and Severe Storm Potential Wednesday

FIRST POSSIBILITY OF SEVERE WEATHER FOR THE SEASON LIKELY WEDNESDAY AFTERNOON AND EVENING:

The Storm Prediction Center in Norman, Oklahoma has placed most of the area, especially south of Route 6, in a slight risk zone for severe weather, usually indicating that we have a 15-30% chance of seeing any type of severe weather within 10 or so miles of any given location. By the usual standards of probability within the atmosphere, this is fairly significant (although the chance is much higher farther south and west).

At this time, there are still too many model discrepancies to make a sound judgement of the forecast, but the best guess at this time is that DISCRETE SUPERCELLS are possible. Why? Because low level helicity (rotating winds in reference to a storm-relative framework with height in the atmosphere) will be on the increase (due to our proximity to the warm front), deep-layer shear will be rather strong (in the 40-50 knot; ideal for classic supercells), and instability will range from 1500-2000 J/kg on Wednesday afternoon (J/kg is a measure of the amount of potential energy in the atmosphere; another way of measuring atmospheric instability). What is a discrete supercell? A discrete cell is a storm that is rather isolated and scattered. There may be numerous in a region, but a discrete storm is one that can be differentiated from other storms (and is therefore not part of a storm complex or system such as a squall line). They can be violent producers of severe weather, and have been known to do quite a bit of damage in this area. Any type of severe weather will be possible from these storms, from damaging winds and large hail, to frequent lightning and tornadoes. Tornadoes look to be a problem, especially if they interact with the warm front. Hail will not be as much of a concern unless CAPE increases, and moisture amounts decrease. This doesn't look to happen, so I would elevate the tornado and wind risks over the hail risks. Widespread damaging winds would be more likely if the squall line from the west arrives earlier. This doesn't appear all that likely at this time, but it is possible. Damaging winds in these discrete supercells could be threatening, especially if evaporational cooling and momentum transfer can occur with steeper-lapse rates in the low levels. Dry air will be non-existent from this storm, so I would not be as worried about intense downbursts or microbursts. At this time, the most prevalent "danger" might be tornadoes and damaging winds. This will be monitored. 















Forecast soundings from the NAM indicate a fairly moist, and moderately unstable atmosphere on Wednesday afternoon, precluding the possibility of such dangerous storms. At this time though, the NWS and SPC believe that the NAM may be reflecting its bias towards more instability, and thus, should probably be disregarded. It is possible that we see this kind of instability, then we WILL see some intense thunderstorms, but its highly unlikely right now. 

Before you get the idea that we have a high chance of severe weather, let me emphasize that it is still early in the season, and it may be that the NAM model is overestimating instability and thunderstorm coverage. If so.. The thunderstorm threat, and especially severe threat, will be greatly diminished. REMEMBER, I do NOT expect widespread severe weather. Only the POSSIBILITY of severe weather. As we get closer to Wednesday, details will become clearer in regards to the type of weather, and the possiblities. 

Other than that, if severe weather doesn't happen, expect scattered thunderstorms to become more likely as the afternoon progresses Wednesday. Temperatures will likely approach the mid-70's, especially south of Route 6, making for a very warm, and humid day. Thunderstorms will become numerous to widespread during the evening and overnight hours, producing heavy rain, and frequent lightning. It appears that rainfall amounts may approach 1 to 2 inches, but most areas will likely see 0.5 to 1 inch of rain.

Rain will continue on Thursday as well as the system exists the region. By Friday, conditions will continue to clear as cooler air arrives. But don't worry, temperatures will begin to rebound by the end of the weekend and next week.

POTENTIAL WINTER STORM... AGAIN

This looks to be our *last* winter storm of the season, as northern stream blocking and the Pacific longwave patterns WILL be shifting after this storm. Warmer weather looks to be well on its way by early-mid April.

Here are some of the main points, with more details below them:

1.) Track of the low is becoming more confident. The low looks to track through the Ohio Valley in an ideal position to bring the area heavy snow, ice, and gusty winds. 

- Current ensemble consensus is shown in this map:

There are several arguments in favor of going against the ECMWF (which is shown below):
      - Blocking is too strong on the model. This is a result of its amplification bias, which tends to split the jet stream apart too "much" and for too long of a time because it amplifies the Polar blocking pattern more so than it needs too. 
      - The ECMWF has been showing a southern bias all winter (which is unusual), and therefore, its likely that its both too weak, and too far south. 

GFS (below the ECMWF chart) vs. ECMWF

12z ECMWF has the primary low down near the Gulf Coast.

12z GFS has a more reasonable solution with the primary low in Kentucky, before transferring to the secondary farther east.

This is the 06z GFS, which is a bit further north than the 12z GFS is. These discrepancies are being resolved though, and we now have general model agreement (disregarding the ECMWF).

2.) Moisture will be quite sufficient with this system. The system itself will tap into the Subtropical Jet, which will provide it with more than enough moisture to produce heavy precipitation.

      - The GFS has circumvented the deepest moisture further to the southeast due to the development of the secondary low off of the coast of the Carolinas. The development of the secondary low will determine the extent of the precipitation due to the fact that if the secondary low forms much quicker than expected, the flow of moisture will be cut-off into the Ohio Valley. This will depend heavily on the strength of the blocking, and strength of the low pressure system itself, which is discussed below.

06z GFS 6 hour precipitation forecast at hour 102

12z GFS trended slower with the system, but with heavier precipitation in the Ohio Valley/Lower Great Lakes. This needs to be watched.

      - The GGEM is also quite moist with this system, bringing heavy snow banding, but mainly for areas along and east of North-Central Indiana (Fort Wayne, Toledo, Southeast Michigan, NW Ohio, etc.). 

3.) Cold air will definitely be in place. Based on the latest forecasted thermal profiles, they look to be nearly IDEAL for snow late Saturday night through Sunday. 850 millibar temperatures will be in the -3 to -8 degree range across the area during the snowstorm, which is more than sufficient for excellent ice crystal production ("efficient" snowfall).

      - The 06z and 12z GFS forecasts clearly show the ideal thermal profiles for snow AREAWIDE. Although it is possible that sleet, freezing rain, and even plain rain mix in (especially in the initial stages of the storm), the majority of the time, precipitation will be in the form of snow. Surface temperatures during the times of heaviest snow on both GFS runs were below 30, while 850 millibar temperatures were in the range of -4 to -6. Low level thermal properties is also trending colder on the models, so I don't see precipitation type being a major issue with this winter storm.

   

4.) Strength of this system is still in question, because it will depend on the strength of the downstream blocking pattern, the strength of the lead shortwave, and how quickly a secondary low pressure system develops to our east, just off of the coast of North Carolina. Other than that, this system looks to fairly strong, and probably strong enough to produce strong, gusty winds. 

      -It's likely that the system maintains a strength in between 993 and 997 millibars, before the primary takes over. This is all depending on the strength of the lead shortwave, which could be underestimated, especially in a pattern like this. I won't make a call like that though, as its only based on speculation. This will be resolved in coming days.

That's about all of the details I will provide now.

Also, I will be leaving this Friday afternoon for the wonderful state of Florida. NO updates will be released after that until I have a chance to access the internet on Tuesday. For updates on this potential winter storm, refer to your local broadcast meteorologist, and/or visit the NWS's website at www.crh.noaa.gov/iwx for further details.

Don't forget to like NWS Northern Indiana on Facebook as well for the latest updates.

Remember.. This event is FAR from set in stone, and some details, especially with regards to initial precipitation types and amounts are still in question, but its ENTIRELY possible that we see at least moderate snow amounts (3-6 inches of snow), and moderate icing amounts (0.05 to 0.25 inches of ice.).

Overview of the Rain Event; Week's Outlook through March 19

I'm very sorry for not posting any updates this weekend, but now I will be posting more often.

As for today, periods of rain moved through the area for most of the morning and afternoon hours. At KFWA (Fort Wayne International Airport), approximately 0.5 inches of rain has fallen in the past 8 hours, but I will have to wait until the official release of today's weather observations later tonight to confirm this. Right now, rain has stopped over most of the area, mainly all areas west of Defiance, as the cold front slides through. Expect temperatures to fall back into the 30's as the cold front pushes through, and possibly an increase in gusty winds for a short time. 

Last hour's radar imagery

Tomorrow, expect much cooler temperatures than this morning, as a shallow polar air mass settles into the area for a brief time. Temperatures will dip into the lower 40's and upper 30's tomorrow areawide. By Wednesday, temperatures should drop even further to the mid and upper 30's under partly to mostly cloudy skies. An upper level trough will continue building as it digs into the area, allowing a reinforcement of the cold air Wednesday afternoon. Although it will not be significant, it could allow nighttime lows to drop below 20. By Thursday and Friday, the upper level trough will lift out of the region, bringing in a much warmer air mass, and a much flatter jet stream pattern. With this in place, a few weak shortwaves rounding the edge of a flat upper level ridge across the American Southwest could allow for periods of gusty winds, and even the chance for showers, especially if enough moisture gets involved. Temperatures will also begin approaching the middle 50's as well by Friday and Saturday.

After that, it is rather uncertain what will happen, given the relative lack of agreement among computer models, but its likely that we continue in a flatter, more zonal and progressive pattern, allowing for much more spring-like weather. In fact, its likely to stay quite dry through the end of next week. With a strong Pacific jet, and the Alaskan vortex building into Western North America, a -PNA pattern will likely develop. Even with a very strong blocking regime, the -PNA and the Alaskan vortex will nullify any development of an East Coast trough. This will also allow for warmer conditions, and the potential for the Bermuda High/SE ridge to redevelop across the Southeast, further enforcing the northward placement of a warmer, moist air mass. This, I believe, will allow for the beginning of spring. Have a great day!

GFS Hour 108 forecast for 850 millibar temperatures.