Area Forecast Discussion

KEY MESSAGES

- A notable warmup begins on Tuesday lasting through Thursday, with temperatures 5-10 degrees above average.

- Widespread showers and thunderstorms are expected Thursday night into early Friday with potential for additional development Friday afternoon/evening.

SHORT TERM

(Through Late Tuesday Night) Issued at 251 PM CDT Mon Apr 20 2026

A surface high, that was centered across the Great Lakes this morning, has slid southeast throughout the day and is now positioned near the Ohio River Valley. The progression of this surface feature has allowed area surface flow to veer from easterly to southerly with return flow commencing earlier across central MO. This, coupled with abundant solar insolation under a sunny sky, has led to afternoon temperatures reaching the low/mid 70s across MO, with mid/upper 60s across IL. Even with a clear sky persisting into tonight, nocturnal cooling will be limited, thanks to continued low- level southerly winds bringing WAA along with PBL mixing, keeping temperatures largely in the lower 50s before sunrise on Tuesday.

Recent GOES-19 mid-level water vapor imagery reveals broad ridging across the Intermountain West with amplified troughing on either side, just off the West Coast and across the eastern United States. This large-scale pattern places our region under mid/upper-level northwesterly flow, which limits chances for precipitation across the area over the next few days. A mid-level shortwave and surface reflected low far to the north across Ontario will send a weak cold front southward, approaching the area on Tuesday. The approaching boundary will tighten the surface pressure gradient, leading to increased low-level southwesterly flow, gusting to 20-30mph by the afternoon on Tuesday. A setup like this favors downsloping off the Ozark Plateau, which coupled with steady WAA, will work to boost temperatures. As a result, Tuesday should be near 10 degrees warmer than today, with afternoon highs expected to reach the upper 70s/lower 80s areawide.

High-resolution guidance is revealing the weak boundary stalling just to the north across southern IA/northern IL Tuesday evening. Nearly all of deterministic guidance have a very subtle mid-level shortwave propagating across the area Tuesday evening within the mid/upper-level northwesterly flow. As this moves overhead, height falls aloft may be just enough to produce sufficient lift to help initiate showers and isolated thunderstorms along the stalled boundary. Low-level southerly flow colliding into the boundary results in abundant convergence leading to moisture pooling across this area. As a result, REFS/HREF mean CAPE values of 1,000-1,500 J/kg exist along with mean 0-6km Bulk Shear around 30kts, indicating the environment should be supportive for thunderstorms. However, forecast soundings across that area reveal a 750mb cap/inversion that may limit shower/thunderstorm coverage. The most recent CAMs indicate scattered shower and thunderstorm development around sunset on Tuesday, across far northern MO and west-central IL, with decreasing coverage as precipitation slides southeastward into a less favorable environment. As a result, a low chance (10-20%) for scattered showers and thunderstorms exists across northern MO/west- central IL Tuesday evening into Tuesday night.

Peine

LONG TERM

(Wednesday through Next Monday) Issued at 251 PM CDT Mon Apr 20 2026

By Wednesday, the mid-level ridge out west will be slowly sliding eastward with the ridge axis nearing the Great Plains. The continuation of mid/upper-level northwesterly flow will help keep the area dry Wednesday into Thursday, even though low-level southerly warm/moist advection will be persisting as well. Guidance reveals the mid-level ridge axis advancing across the area Wednesday night, leading to the beginning of deep southwesterly flow on Thursday. It appears that Thursday will be the warmest day of the week, with areawide highs in the low 80s, as mean 850mb temperatures progged by the LREF may reach the mid teens (C), which is near the 90th climatological percentile.

An active weather pattern returns near the end of the week as longwave troughing settles in across the Intermountain West behind the departing ridge to the east. Long-range guidance reveals a closed low within the longwave trough ejecting in to the High Plains Thursday. Guidance is in good consensus that this closed low rapidly lifts north into the Upper Great Plains as a Fujiwhara effect occurs between it and other lobes of energy swinging around the base of the nearly stationary longwave trough. With the ejection of the robust closed mid/upper-level low, surface cyclogenesis ensues across the Upper Great Plains Thursday afternoon that sends a cold front eastward across the Central Great Plains. Ahead of this front, on Thursday, the highest LREF mean SBCAPE approaches 1,000 J/kg across central MO with 0-500mb Wind Shear near 30kts. As it stands now, the greatest threat for strong to severe thunderstorms would be across western into central MO, with gradual weakening thunderstorms with eastward extent, a result of waning instability into the night.

It is important to note that the exact timing/orientation of this frontal passage remains uncertain given the plethora of influencing variables at this range. What's more certain, is that shower and thunderstorm chances, along with the potential for severe weather, increase as the front approaches, which currently appears to be late Thursday evening into Friday morning. Some long-range guidance reveals this cold front pivoting clockwise and settling in a southwest-northeast orientation near the area on Friday. The location of this boundary on Friday afternoon/evening will greatly influence where the best chances for strong to severe thunderstorms is, which varies significantly in guidance. With the potential for a stalled boundary around the region Friday and into the weekend, along with multiple shortwaves swinging around the base of the longwave into the Plains, a wet and active pattern may setup from Friday into early next week. The latest outlook from the Climate Prediction Center highlights this well, where the 6-10 day precipitation outlook places our area in a 50% chance for above normal rainfall.

Peine

AVIATION

(For the 18z TAFs through 18z Tuesday Afternoon) Issued at 1247 PM CDT Mon Apr 20 2026

VFR conditions will prevail for the entire TAF package. Light easterly/southeasterly winds will gradually become increasingly southerly with gusty southwesterly winds on Tuesday.

Peine

LSX WATCHES / WARNINGS / ADVISORIES

MO...None. IL...None.