Current Conditions at Sanborn Field
Monday: Partly cloudy skies. High: 89
Monday Night: Becoming cloudy. Showers and thunderstorms likely. Low: 59
Tuesday: Becoming partly cloudy. High: 70
Tuesday Night: Skies becoming clear. Low: 50
Wednesday: Clear skies. High: 70
Discussion:
A taste of fall is finally coming to mid-Missouri. One more day of summer-like temperatures is in store for Monday, but a drastic change will come Monday night into Tuesday as a cold front approaches the region. Monday night into Tuesday, showers and thunderstorms are likely between 10pm and 3am. Skies are slower to clear out Tuesday, but partial clearing is expected nonetheless. Afternoon high temperatures going forward will be pleasant, ranging in the upper 60s and lower 70s. Tuesday night looks to be the coolest night we've seen this month potentially reaching down into the upper 40s. High pressure builds in completely by Wednesday keeping our skies clear.
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Forecasters: Baker, Cade, Travis
Date Issued: 09/20/21 10:45 AM CST
Technical discussion:
A large scale pattern shift is in store for the Midwest after what seemed like endless summer-like temperatures and dry weather. Our main concern is an amplified shortwave trough that will be digging into the Upper Midwest Monday night into Tuesday. Both the GFS and NAM had good consensus in the strength and timing of the system, but the NAM displayed its usual aggressiveness in afternoon high temperatures thus pushing us towards primarily GFS guidance.
Analysis loops of the upper-level flow has shown almost nothing but zonal flow these past several days, but the GFS depicts an amplified shortwave trough at 350 hPa and an associated dip in the jet stream digging into the Upper Midwest and Mid-Mississippi River Valley Monday night into Tuesday morning. This will help bring some cold air south into the region. GFS 500-hPa absolute vorticity shows the center of the upper low passing over Missouri Tuesday into Wednesday, another suggestion that a colder airmass will set up.
Much is the same story in the height field at 700 hPa. Saturated air will be setting up over the Great Lakes region extending SW along the cold front draped along this same region. The bulk of the moisture looks to stay off to the NE, thus a large, widespread rain event looks unlikely. That being said, sufficient moisture alongside frontogenetic lift seen in the GFS vertical motion field will likely result in a line of showers and thunderstorms extending SW along the front with little trailing stratiform rain.
Sufficient, but not overly plentiful moisture, is backed up by the presence of a present, but not overly impressive LLJ at 850 hPa ahead of the front. This will allow daytime highs to inch slightly above Sunday's high, but its weaker nature does not inspire much confidence of a Monday high significantly greater than Sunday's.
GFS plots of MSLP, 1000-500-hPa thickness, and sfc winds show an unmistakable cold front set to pass through Columbia between 03z and 06z Tuesday. Around this same time, associated convection will blow through the region as well. The HRRR simulated reflectivity field lines up well with the timing of the frontal passage suggested by the GFS. The HRRR may be slightly slow to push the precip out of the region when compared to the movement of the front by GFS.
GFS soundings support the HRRR's idea of a line of thunderstorms at the time of frontal passage. Soundings depict rapid moistening of the column to near full saturation by 06z then quickly drying out post frontal passage. Moderate instability (~900-1000 J/kg of CAPE), PWATs of ~1.5 inches, and K-indices >35 all suggest heavy rain associated with some of these storms. Areas where the heaviest rain sets up could see locally higher rainfall accumulations, but the quick movement of the front should push the line through quickly. Soundings are quick to dry out enough for no precipitation, but they do suggest that partial cloud cover could remain into Tuesday. The extent of this cloud cover will determine if the daytime high needs to be adjusted lower.
By Tuesday night, skies will clear allowing for ample radiational cooling. Tuesday night being the first clear night post frontal passage, expect a chilly start to Wednesday. Have started warmer with a low of 50 degrees for Tuesday night, but thinking that future shifts will need to adjust this lower as model guidance progresses. Surface high pressure builds in by Wednesday and will keep skies clear and temperatures pleasant.
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