Current Conditions at Sanborn Field
Monday: Sunny. High: 90
Monday Night: Mostly clear. Low: 68
Tuesday: Becoming partly cloudy in the afternoon. High: 88
Tuesday Night: Becoming cloudy. Rain likely. Low: 65
Wednesday: Cloudy in the morning, becoming partly cloudy in the afternoon. High: 80
Discussion:
Monday and Tuesday are looking to be more beautiful days. Temperatures will remain warm for mid-September. However, after what seems like an eternity of clear skies and pleasant temperatures, a weak cold front will bring with it more "unsettled" weather Tuesday night into early Wednesday morning. Clouds will be on the increase Tuesday afternoon into Tuesday evening eventually giving way to showers overnight Tuesday into early Wednesday. Showers should taper off after sunrise with skies partially clearing by the afternoon. Temperatures behind the front will be noticeably cooler than the low 90s we have seen these past several days.
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Forecasters: Cade, Baker, Travis
Date Issued: 09/13/21 10:30 AM CST
Technical discussion:
After an extended period of quiet weather, the main focus of this forecast period is a weak cold front expected to blow through early Wednesday morning bringing with it chances for rain. The GFS was mainly used as the NAM was far too cool for daytime highs and overnight lows. GFS MOS was consulted for temperatures as well as the GEFS to judge model performance.
A strong upper-level ridge over the southern CONUS has kept large-scale, upper-level flow over the Mid-Mississippi River Valley zonal for several days. GFS 250-hPa winds show the jet stream and the associated storm track remaining in southern Canada for the remainder of Monday and into Tuesday. Tuesday night into Wednesday, a shortwave can be seen in the 250-hPa height field (much less so at 500 hPa). A weak jet streak forms over the Great Lakes region putting the Missouri, Illinois region in its poleward entrance region of enhanced divergence. At 500 hPa this disturbance shows up in the absolute vorticity field, but is more subtle highlight its weak nature.
GFS plots of 700-hPa RH depicts the landfall of Tropical Storm Nicholas in TX. This system should stay well to the south due to the aforementioned shortwave's propagation southeast. Tuesday night into Wednesday morning, saturation over Missouri increases with the approaching system. This saturation collocated with rising motion dynamically supports overnight showers. This is supported by GFS soundings with near complete saturation of the column overnight. According to these model soundings, sufficient MUCAPE of ~600 J/kg suggest that thunderstorms ahead of the approaching front are a possibility. However, seeing as the MUCAPE is nominal at best, and the event is still 48 hours out from our latest model output, have decided to refrain from including thunderstorms in the general forecast.
MSLP and surface winds place the timing of frontal passage near 06z Wednesday. GFS 850-hPa and surface maps of height and MSLP respectively, shows the quick build in of high pressure behind the front. This will push rain out of the area by sunrise on Wednesday and allow skies to clear partially by the afternoon. The resultant temperatures for Wednesday will be noticeably cooler than the summer-like low 90s we've seen these past few days.
Later shifts should pay close attention to model soundings to assess the potential for thunderstorms in addition to showers.
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