Monday, April 19, 2021

 

 Current Conditions at Sanborn Field

Freeze Warning in effect from April 21, 01:00AM CDT until April 21, 09:00AM CDT


 Monday Night -
Mostly cloudy with a chance of rain late. Low: 32-36. 


 Tuesday - Snow likely, mixed with rain. High: 38-42. 


 Tuesday Night - Decreasing clouds. A freeze is likely. Low: 28-32.


 Wednesday - Partly sunny. AM frost. High: 48-52. 

 


 Thursday - Partly sunny. AM frost. High: 54-58.

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Discussion: 

After a pleasant Monday, things will turn much colder and wetter for mid-MO. Overnight tonight, temperatures will drop into the middle 30s behind a cold front. Rain showers are possible towards daybreak. Rain should quickly switch to snow Tuesday morning, and a period of snow several hours long is possible during the first half of the day. Visibility will likely be reduced during this time. Thanks to very warm ground temperatures, accumulations will be limited and confined mainly to grassy surfaces. On surfaces where snow does build up, anywhere from a coating to 2 inches is possible. Travel impacts will be limited, but anyone traveling is advised to use caution - particularly on bridges and overpasses. Tuesday night into Wednesday, this very cold weather pattern will lead mid-MO to experience a freeze. That will kill sensitive vegetation, so be sure to bring in any plants that cannot handle the cold. We will see things get down to near freezing again Thursday morning before things finally begin to moderate late in the week.

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Forecaster: Vanderpool
Issued: 5:00 PM CDT 19 April 2021
 
Technical Discussion (The nerdy stuff we are discussing in class):


An overall remarkable setup is in store over the next couple of days, especially given the fact that May is less than two weeks away. Snow is likely for Tuesday, and frost/freeze conditions will likely follow for both Wednesday morning and Thursday morning. As far as late-season winter events go, this is looking very impressive (if unwelcome). 

 

18 UTC WPC sfc analysis reveals a cold front stretching through central MO. This front had already passed through COU by 20 UTC, as evinced by a strong northwesterly breeze. However, the true chilly air lagged a few dozen miles behind the front. Nevertheless, temperatures this afternoon and evening will embark on a steady (and fairly rapid) decline. 

 

This front is preparing mid-MO for what is likely to turn out to be several hours of snowfall Tuesday morning. Overnight, a positively-tilted mid-level trof (most visible at 500-mb) situated currently over the northern Rockies will rapidly swing out into the Midwest. As it does to, it will quickly become neutral and then negatively tilted. While the development of an organized circulation is not expected, this quickly-moving trof will serve to bolster frontogenesis at 850 and 700-mb, as well as advect some decent low-level moisture into mid-MO. Although the surface front has already passed COU, it appears that the front aloft lags far behind. It will be FGEN in the lower and middle levels of the atmosphere, combined with a narrow band of moisture, that will lead to very high precipitation chances in mid-MO for Tuesday. 


Currently, the 18 UTC NAM, 18 UTC HRRR, and 12 UTC GFS are all in excellent agreement on the timing, track, and mode of precipitation. Initiation of precipitation is likely at COU between 09 UTC and 12 UTC Tuesday morning. This may begin as rain or a rain/snow mix before quickly switching over to entirely snow. Model soundings from the NAM 3km and the RAP indicate that surface temperatures ahead of the main precip band will probably be in the middle to upper 30s, though the introduction of precipitation into the lower levels will cause the temperature and dewpoint profiles to "zipper" and likely reach an equilibrium wetbulb temperature of 33 or 34 degrees shortly after 12 UTC. That will be more than sufficient for snow to make it to the ground with minimal melting, given freezing levels only a few hundred feet high. Owing to significant cloud ice and excellent lift throughout the column, snow - perhaps heavy at times - will be likely as the precipitation band passes overhead. 


In terms of impacts and accumulation, we will be working with ground temperatures in the lower 50s; the Capen Park weather station currently indicates subsurface temperatures between 51 and 55 degrees. This, combined with our warm weather on Monday, will serve to limit how much snow can accumulate. Nevertheless, the strong forcing coupled with deep saturation and a favorable environment for dendritic growth may lead to rather high snowfall rates. If snow can fall at a high rate for several hours Tuesday morning, it will likely be able to overcome surface warmth and produce anywhere from a trace to 2 inches of heavy, wet accumulation. I think roads will remain mainly wet, but elevated surfaces - including bridges and overpasses - may see minor slush accumulations. Those with travel plans Tuesday (especially in the first half of the day) will want to be careful. In addition, heavy snow rates may reduce visibility and make driving hazardous even without significant accumulations. SREF and GEFS means both prog around 4 inches of snow accumulation (assuming a 10:1 ratio) though I suspect that ratios will be lower than this, probably between 5:1 and 8:1, and - combined with warm surface temperatures - accumulations may be only 25-50% of what actually ends up falling. An estimate of a coating to 2 inches on grassy and elevated surfaces seems in order. 


The primary band of precipitation should exit COU between 19 and 22 UTC Tuesday. Stray rain showers will still be possible after the band has passed, owing to leftover saturation and steep low-level lapse rates. However, these will be scattered and unlikely to contribute to further precip accumulations. 


Attention will then turn to the potential for a freeze (and maybe a hard freeze) Tuesday night into Wednesday morning. Model soundings indicate clearing skies shortly after sunset Tuesday, which does not bode well for mid-MO. Clearing skies coupled within an environment of stout CAA will lead to rapidly-falling surface temperatures. SREF means have COU falling to 24 degrees by Wednesday morning, which would smash the old record of 29 set in 1907. GEFS means are a bit more conservative with the magnitude of the cool-down, sitting at ~28. The extended HRRR would keep mid-MO largely between 30 and 32. In any regards, a freeze is almost certain to occur. There is a decent chance that temperatures fall into the 20s, and a small chance that things get as low as the middle 20s. If any snow can remain on the ground through Tuesday night, this would only serve to cause temperatures to sink even lower. Because of the overwhelming model support for a freeze, a Freeze Warning is in effect from 1 AM Wednesday to 9 AM Wednesday. 


For Wednesday, mid-MO will remain stuck in a regime of CAA and northerly flow aloft. Moisture caught up in the upper-level trof may also wrap around and create a small chance for rain showers. This moisture will allow for at least some cloud cover, which - combined with the already chilly airmass in place - will keep things far below seasonal averages. Another frost or light freeze is anticipated Wednesday night into Thursday morning as the winter-like airmass remains firmly in place. 

 

For Thursday, things finally look to regain some semblance of normal. Westerly flow aloft looks to become established, and the surface high responsible for a very cold couple of nights will start to shift east. However, the vestiges of the very chilly airmass will remain in place. Given the potential for at least broken cloud cover, temperatures Thursday will still remain at least 5 to 10 degrees below seasonal averages. Additionally, upper-level moisture will be increasing towards the evening hours as our next disturbance rolls in.


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