Area Forecast Discussion

KEY MESSAGES

- A cold front will slowly drift southward through the region, bringing widespread showers and thunderstorms late Sunday into Monday.

- The pattern shift will result in cooler, more seasonable temperatures through next week.

SHORT TERM /THROUGH SUNDAY/

Issued at 236 PM CDT Sat Sep 19 2026

A cold front will begin to sink southward through the later half of the weekend with increasing chances for showers and thunderstorms Sunday into Monday. Precipitation potential peaks (70-90%) late Sunday night into Monday, when coverage is expected to become widespread. This will mark the beginning of a pattern shift that will include much cooler, more seasonable temperatures behind the cold front.

Extensive cloud cover coincides with a stationary boundary that has occasionally meandered north/south over the course of the last few days or so. South of the boundary, weak mid-level impulses have given rise to isolated showers and a few thunderstorms. Cells east of the Mississippi River may briefly show some intensity this afternoon with a brief, localized downpour, gusty winds, and lightning. However, this potential looks short-lived with weak vertical and directional shear (<20 kts) profiles. The stationary boundary to the north will finally begin to drift southward Sunday as mid/upper level ridging begins to break down and a longwave upper level trough approaches from the Plains. Showers and a few thunderstorms generally track west to east within the westerly flow, some of which skim the northern sections of Missouri/west central Illinois by early Sunday morning, roughly along and north of Hwy 36 and I-72. South of the front, showers/thunderstorms are expected to be more isolated in nature with similar characteristics as today's isolated activity with little change in shear/weak westerly flow. Localized heavy rainfall, gusty winds, and lightning will accompany more robust thunderstorms, but coverage remains limited where CAPE values around 2000 J/kg might otherwise support stronger thunderstorms. Severe potential is low and isolated. Therefore, severe potential is not being widely messaged.

Increasing chances will track along the front as a second upper shortwave rounds the mid-level ridge and ejects northeastward from the southern Plains into Missouri and Illinois late Sunday. Ascent increases with the arrival of the parent upper trough Sunday night into Monday with deep saturation and broken/overcast skies. Much of the ascent is elevated with mean surface instability limited to around 500-1000 J/kg, so expect showers and a few elevated thunderstorms through Monday. The greatest uncertainty, with regard to temperatures, follows the progression of the boundary Sunday (north of I-70), dictating where surface heating is able to achieve 90s for highs. Otherwise, a much cooler air mass brings needed relief for all areas by Monday. The warmest temperatures (80s) linger over southeast Missouri and southwest Illinois, while 60s and 70s run along and north of I-70. probabilistic LREF data shows aerial averages of 0.25-1.00 of total QPF by late Monday evening. Everyone gets in on some rainfall, though higher totals are favored around locations that get under thunderstorms supported by PWATS above 1.5 inches.

LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/

Issued at 236 PM CDT Sat Sep 19 2026

For now, it's beginning to look as though the anomalous heat and humidity will give way to the seasonal transition over the course of the next week. This cool down may seem cool or even chilly to some, as we shave 20-30 degrees from both high and low temperatures for several days.

Surface high pressure and mid-level ridging develop over the Great Lakes into the southeastern Canadian Provinces as the axis to an upper ridge over the Plains edge eastward toward the Mississippi River Valley. Global guidance (ECM/GFS) both show some version of this progression, though they begin to diverge in its evolution from mid to late week. GFS shows the upper wave getting pinched off and momentarily becoming stationary west of Lake Michigan, while ECM is more progressive, taking the upper wave into the Ohio Valley. In both scenarios, easterly flow is funneled into the region with limited moisture and mid-level temperatures in the single digits to around 10C. Despite some of the spread in the long range synoptic evolution, surface temperature spreads are pretty reasonable, running around 5 degrees through much of the period. Additionally, long range teleconnection outlooks give a bit of a boost in a shift toward sustaining fall-like temperatures as projections show the PNA sharply trending more positive and AO favoring negative trends. This would lean in favor of eastern CONUS troughing with northwesterly flow maintaining more of a fall-like feel through the later part of the month into early October.

AVIATION /00Z TAFS THROUGH 06Z MONDAY/

Issued at 604 PM CDT Sat Sep 19 2026

The chances for showers will begin to increase with this set of TAFS including at UIN late tonight into Sunday with a chance (30-40%) between 11-15Z increasing to 40-60% though 18Z. There is an additional chance at UIN, but confidence is too low in timing to include in the TAF at this time. MVFR conditions can be expected with this activity, particularly late on Sunday morning. There are also low chances (20-30%) at COU/JEF between 12-15Z. Chances remain low at the St. Louis metro terminals until after 00Z on Sunday evening when I have added a PROB30 group starting at 02Z. I have only included showers at this point as the coverage of thunderstorms is expected to be limited, though I cannot rule out thunderstorms at any of the sites during the time I have included rain chances. Winds will generally be light, particularly outside of thunderstorms.

LSX WATCHES / WARNINGS / ADVISORIES

MO...Heat Advisory until 8 PM CDT this evening for MOZ047>049- 059>065-072>075-084-085-099. IL...Heat Advisory until 8 PM CDT this evening for ILZ064-069-070- 074-079-100>102.