Warm, muggy conditions will last through Monday afternoon, but a cold front is on its way into the area this evening. As the front passes through, a few thunderstorms cannot be ruled out. Chances are very high that most of us remain dry. Any storms that do form, however, will have the potential to produce damaging winds and large hail. Garden variety showers will move in for Tuesday, which - combined with cloud cover - will keep temperatures much chillier and more reminiscent of March than May. Dry and mainly sunny weather returns for Wednesday before another quick-hitting chance of rain arrives Thursday. Temperatures look to remain much below average for this time of year, with highs in the 50s and 60s from Tuesday on.
The primary concern of the near term is a small chance for showers and thunderstorms Monday afternoon and evening. A surface cold front is approaching mid-MO; the latest METAR data suggest this front is located just west of COU. A complex of thunderstorms is currently (as of ~20:30 UTC) ongoing across southeastern IA and far western IL, while a few pop-up showers are trying to get their act together just east of COU. These have so far been unsuccessful - despite SBCAPE values on the order of 1500 j/kg - as neither FGEN nor moisture convergence are that impressive in mid-MO.
For the remainder of the afternoon and evening, the surface front will continue to translate eastward and ultimately pass through COU sometime before 00 UTC; likely between 21 and 23 UTC. Chances for isolated convection will remain along and ahead of this surface front, though - once again - forcing just does not look very impressive. Ensemble models and short-range CAMs are insistent upon only very low probabilities of precipitation at COU, an assessment that I am inclined to agree with given the history of convective development this afternoon. Nevertheless, the SPC still places COU in a slight risk of severe weather for the rest of the afternoon, so the chance of severe wind or hail is not zero. However, it is very low.
Overnight, a positively-tilted upper-level trough currently located over the Rocky Mountains will swing out into the central CONUS and take on a neutral tilt. This will encourage the formation of a brief surface circulation in northern AR and southeastern MO. A large convective complex will likely be ongoing in that area by Tuesday morning, which may help contribute to the temporary deepening of this surface circulation via latent heat release. Moisture will be present on the northern side of this circulation, and models indicate the presence of saturation and lift in the column by ~12 UTC Tuesday. That will lead to a band of stratiform rain over mid-MO, with several hours of light to moderate rain possible Tuesday morning. Short-range CAMs seem to resolve this; the GFS places Tuesday's rain further south. Given the accuracy with which the HRRR has handled this afternoon's setup thus far, this forecast tends to reflect its solution of rain and overcast skies. Given potent CAA and overcast skies with rainfall, temperatures Tuesday will remain very chilly this time of year - the upper 50s to near 60.
Clouds will clear Tuesday evening, leading to a mostly clear sky Tuesday night into Wednesday morning. This will provide ideal conditions for temperatures to tank, with lows potentially scraping 40 by Wednesday at sunrise. Mainly sunny skies are anticipated Tuesday as a surface high passes through, though a few high clouds will be possible. Temperatures will be slow to recover given the cooler regime, and highs should remain in the low to mid 60s.
Wednesday night into Thursday, a shortwave associated with the same longwave trough moving towards mid-MO this afternoon will pass overhead. Ensemble models indicate POPs at 30-40%, while the deterministic GFS and NAM both provide minor amounts of QPF in mid-MO Wednesday night into Thursday. At the very least, the shortwave will lead to an increase in cloudiness, which - combined with a continued regime of CAA, will only serve to keep temperatures from rebounding further. Thus, after Monday evening, we are headed right back into a markedly colder than average pattern with several quick-hitting rain chances.