New and Interesting Developments

As I said yesterday, the forecast was incredibly likely to change, and it has! Over the past several model runs, the models have trended to a much more amplified ridge developing over the area, which seems likely, given the strength of the jet streak which will amplify the Northern Plains trough, introducing what I like to call the "teeter totter" effect.  What happens is that when a jet streak or a strong trough moves into the West, usually strong height falls (equivalent to falling pressure over time) accompany them, causing a deepening of the trough.  This strong trough then progresses eastward, "squeezing" the ridge, eventually causing it to amplify.  The reason I call it the "Teeter Totter" effect, is because usually when there are height falls in the west, there are usually height rises occurring in the East, kind of like a kid sitting on the end of a teeter totter, causing the other end to go up.  Even with the ridge developing, I do expect that a weakening shortwave will move north of the area into Canada.  This will cause a cold front to approach to the area, but not pass through.  The good news about this front is that it COULD produce rain (even if it does it will probably occur west of here), given enough moisture and lift, which will not be in good supply over the area.  If any of you ever happen to look at recent model runs, you will notice that these models are forecasting that dew points into the lower 70's by Sunday afternoon. Do not believe this for one minute, because most models have well known moisture biases, which can cause major problems with convective feedback.  When models forecast more moisture than there actually will be, and since instability is related to the low level moisture profile, this will create a bias towards higher instability and thus more widespread thunderstorm activity.  In actuality, the models are having a problem with this insane drought, due to the fact that they are overestimating evapo-transpiration from crops such as corn, soybeans, and wheat.  The ground is so dry and the plants are so dry right now, that the models moisture parameterization schemes implement this into the  model, causing an extreme overestimation in moisture.  Therefore, I do not believe that thunderstorm activity will be very widespread, if there are thunderstorms that develop.  My second reason for not anticipating thunderstorms, is the fact that a strong cap will still be located over the region (even though it could be effectively reduced depending on where the shortwave tracks).  This will effectively block most thunderstorm activity.  If there is widespread thunderstorm activity it will be to the northwest of here in Illinois and Wisconsin on Sunday afternoon and evening.  The third reason is for the fact that over the past few weeks and months, models have consistently overdone convective potential.  There are a multitude of other reasons, but those are based on my opinions on how the situation is going to develop, not on actual facts. For these reasons, I don't think we will have as much cloud cover on Sunday, allowing temperatures to reach the lower 90's
     As for most of next week, the ridge appears to be stronger than what I thought yesterday.  Ridging will begin to build back in by Monday, sending temperatures back into the low to mid 90's.  On Tuesday through at least Thursday, temperatures could soar into the middle 90's to near 96 (according to the HPC and other guidance) with nighttime lows in the lower 70's. By Friday, the ridge will break with the approach of an upper level trough, as blocking breaks down in the Atlantic. After a few cooler days in the 80's looks likely, with highs returning to the 90's soon after.

New and Difficult Weather Pattern

        Originally, this next "interesting" weather pattern looked like a classic heat wave/ridge style pattern when I last had a blog post several days ago. Things have changed significantly since then, with models trending towards, a Rex block pattern.  In a Rex blocking pattern, a significantly large trough of low pressure has cut itself (but not completely, as in the case of a cut-off low) off from the main jet stream flow, shoving itself into the middle of a ridge.  As long as there is no warm air advection into the trough, the blocking pattern can persist for quite a while.  As the ridge strengthens, the low "shoves" itself underneath the entire thing, causing the entire pattern to retrograde, or move in a direction opposite of the normal motion (which is moving eastward).  This kind of block is amplified by the fact that upper level blocking, even though it is still weak, is still persisting over Greenland.  This should cause a situation for our area that is extremely similar to the Greek story about Tantalus, who was in a lake of water up to his neck, but every time he tried to reach down for a drink, the water level would drop, not allowing him to get a drink. Around, a large upper level trough is going to build into the Northern Plains over the weekend, as a series of shortwaves "runs" into the amplifying ridge, which will cause the ridge to deamplify for a few days.  This series of shortwaves will carve out a rather large longwave trough over the Northern Plains, which will allow for a series of storm systems to traverse the upper plains. At the same time that the trough tries to dig into the Midwest and Great Lakes, the Rex blocking pattern will set up in the western Atlantic and near the East Coast.  This will cause these same storm systems to move across the same areas, Minnesota and South Dakota, causing several very heavy rain events. At the same, the heat wave will build north and westward into our area, with 500 millibar heights building into the 5900-5940 meter range, which is about as high as 500 millibar heights get (the highest 500 millibar heights were recorded last summer over Wichita, with heights of over 6000 meters above sea level!).  This will eventually lead to a strengthening of the heat by next Tuesday and Wednesday, with temperatures rising back into the mid 90's by Wednesday and Thursday of next week.  After that, there is incredible model uncertainty as to what will happen (there is a lot of uncertainty with what will happen this weekend!), but some of the models are indicating that the longwave trough will weaken and move over the area, causing temperatures to cool back into the 80's by next weekend.  Other models are indicating that either nothing will happen or keep flipping back and forth between solutions!  Ensemble consensus indicates that the longwave trough will retrograde back into the western United States (according to the HPC, or the Hydrometeorological Prediction Center), forcing the ridge to reamplify over the area.  In my opinion, the models are certainly having problems with the blocking pattern setting up and their forecasts should be considered with a grain of salt.  At this time, the GFS is probably the most progressive, while the ECMWF is probably the most amplified. If I had to use one model with the best forecast, I would say the ECMWF, as it seems to be handling the pattern quite well, as a Rex block will not allow the longwave patterns to progress very easily.  I would not take even the ECMWF for face value, as it breaks down the pattern quite rapidly, even though the Pacific ridge should retrograde, which would mean the entire pattern should shift west, including the trough in the Northern Plains, which would eventually cause the ridge axis to move back over the region.  I will have to keep updated on the pattern as it very confusing and not giving many meteorologists confidence.  There is always the possibility for cooler and wetter weather (hopefully)!
           As for this weekend, a shortwave will rapidly move through the upper Midwest by Saturday and Sunday, which could force a rather weak cold front close to the area.  Due to the amplifying ridge, I would highly doubt that the cold front will make it much past Chicago.  From my understanding, the only reason the NWS dropped temperatures into the upper 80's and lower 90's for Sunday and Monday is because of increasing clouds and lower low level temperatures associated with the approach of the cold front.  If the models end up having their well known moist bias, then they are likely having convective issues. Since they could be overestimating convection development, they may be forcing the cold front to far south, which could have a rather large effect on the entire weather pattern, especially with high temperatures and precipitation (for all of you who are hoping for rain, don't get your hopes up, rain chances for this weekend are less than 5%, in other words, don't expect rain). With a continuation of the drought (in fact, there is a moderate drought over most of the area, with the development of severe drought likely soon), expect high temperatures to be warmer than what is expected, and (possibly) cooler lows depending on the moisture profile of the low levels. Other than that, expect nice, sunny weather, with much warm and drier conditions developing. For more updates, I should have another blog post by tomorrow or Saturday, or you can come talk to me personally and I will answer your questions (usually I am at the West Bend News from 9 a.m. to 4 p.m.).

Rainfall not as promising

I hate to be the messenger of bad news, but I have to give you the truth. Heavy rainfall for Monday does not look near as promising as it did yesterday.  In fact, some parts of the area may only receive a tenth of an inch by Monday night. Here's what's likely to happen. Right now, a rather weak upper level shortwave is located over the Gulf Coast, while a strong upper level trough is swinging into the Northern Plains, forcing a cold front eastward, which at the same time is sparking severe weather over Minnesota. The weak shortwave near the Gulf Coast is currently "trying" to move eastward, but its forward progress is being halted by the strong upper level ridge which is currently located over the area.  This is forcing to slowly move northward, especially in advance of the upper level trough over the Northern Rockies.  Overnight, this will cause the deep layer mean wind to becoming southerly, as the upper level shortwave is absorbed into the main upper level storm system.  These southerly winds will "pump" an incredibly moisture "rich" air mass into the region, with precipitable water (the total amount of "useable" water vapor in the atmosphere above a location, usually it is a good indication of heavy rainfall) values approaching 2 inches by the morning hours. Convergence from a surface trough and weak upper level forcing (along with moisture convergence along the initial surge of rich moisture) will cause the development of scattered showers tomorrow morning.  At this point, thunderstorms are unlikely tomorrow morning (from 8-noon is the best chance for rain), due to the advection of a mid level cap into the region, limiting atmospheric instability.  After this first wave of rain, skies should clear for a while before the initial storm system arrives, as a pocket of drier air aloft arrives in the area.  As for afternoon and evening, if the initial moisture surge has not shifted eastward yet, the cold front should be able to produce a few showers and thunderstorms across the area, but they will not likely be numerous, although some could be strong (assuming they do form). As for temperatures, depending on the amount of rainfall and cloud cover, temperatures should range from the low to mid 80's across the area.  Current model trends have been toward the moisture surge shifting eastward rapidly, bringing a rapid end to the rainfall, or the rainfall completely missing us.  Right now, I very skeptical of this, as the NAM may be having convective feedback issues associated with the closed low over the Gulf of Mexico, pushing the low too far east.  For now, we will have to see because this is a very unusual situation.  The cold front will push through sometime Monday night, still allowing warm overnight temperatures in the upper 60's.  By Tuesday, much cooler air will arrive with highs nearing the lower 80's (depending on the amount of rainfall we receive Monday).  Wednesday and Thursday, temperatures should start out in the middle 70's for highs on Wednesday, then jumping upward into the lower 80's.  By the weekend, highs will likely be in the 90's yet again as yet another ridge of high pressure builds into the area.