textproduct: St. Louis

KEY MESSAGES

- Moderating temperatures are expected through Tuesday ahead of a cold front. There is a chance (30-70%) of showers and thunderstorms Tuesday afternoon/evening with the frontal passage.

- After a brief cool down Wednesday - Thursday, above-normal temperatures are forecast beginning Friday.

SHORT TERM

(Through Late Tonight) Issued at 236 AM CDT Mon May 11 2026

A surface ridge of high pressure will remain draped across the bi- state area today, helping provide a dry day with plenty of sunshine. High temperatures are expected to top out mainly in the mid 70s, or right about average for the middle of May.

The aforementioned surface ridge is forecast to push off into the Ohio Valley late tonight, with southerly return flow beginning in its wake. This should help lead to a milder night, with lows ranging from the upper 40s to mid 50s. The coolest conditions are expected to be across portions of southeast Missouri and southwest Illinois where winds will stay light/variable the longest.

Gosselin

LONG TERM

(Tuesday through Sunday) Issued at 236 AM CDT Mon May 11 2026

(Tuesday - Tuesday Night)

A quick warmup will occur on Tuesday as strong low-level warm air advection continues ahead of an approaching cold front. High temperatures mostly in the low 80s are forecast, or about 5-10 degrees above normal.

Convective initiation will be the focus late Tuesday afternoon/evening along the cold front. Thunderstorms tend to initiate where surface convergence and mid/upper level forcing for ascent is sufficiently strong, and this should be no exception. While the strongest surface convergence and mid/upper level forcing for ascent will be across the Great Lakes, showers and thunderstorms are more likely than not to develop as far southwest as near/east of the Mississippi River. Deep-layer shear will be quite strong (40-45 knots), but there is uncertainty in moisture return/instability. There really isn't a deep Gulf connection by any means, with more "recycled" Gulf moisture trying to advect northeastward and pool along the boundary. Relatively modest dewpoints (mid to upper 50s) are most likely along the front. Midlevel lapse rates are also not expected to be too steep, which also should help limit instability. Probabilities for at least 500 J/kg of SBCAPE on the 0Z HREF peak out in the 30-60% range across parts of central/northeast Missouri, but fall to 10-20% for at least 1000 J/kg. The (relatively) higher instability is also forecast to be further to the south and west, where convective initiation is more doubtful as well. Therefore, there still does not seem to be a clear signal for any organized strong to severe thunderstorms in our area. Small hail and perhaps some gusty winds (if the storms are surface based) are possible, but severe hail likely would necessitate supercells to develop. A supercell or two cannot be ruled out, but this does not seem to be a likely scenario. While the effective shear should be more than high enough to support supercells, instability probably will be too low. The supercell composite parameter (SCP) forecast values, even on the bullish GFS with respect to instability, largely remain near or below 1.

(Wednesday - Thursday)

A brief cooldown is expected behind the cold front through Thursday. The air mass has trended back slightly cooler compared to 24 hours ago. The incoming air mass is not too noteworthy, but does seem supportive of near to slightly below normal temperatures. The incoming surface high on the NAEFS is near 1020 hPa, or ~90th percentile of climatology. Temperature anomalies at 850 hPa on both the GEFS and EPS also are in the -3 to -6C range, with surface dewpoints (40s) in the 10th-25th percentile range. High temperatures each afternoon are forecast to be mostly in the 70s, with an east (cooler) to west (warmer) gradient each day. Lows Wednesday night/Thursday morning are expected to be in the 40s, or about 5-8F below normal.

(Thursday Night - Sunday)

Another warmup will commence Thursday night as low-level warm air advection intensifies on the western periphery of the departing surface ridge. There is very high confidence in well-above normal low temperatures through the end of the forecast period, but there continues to be a lot of uncertainty with specifics on high temperatures each day. The mid/upper level pattern late week into this weekend is very likely to be quasi-zonal, with multiple weak/subtle midlevel shortwave troughs potentially moving across the Plains through the mid-Mississippi Valley. High temperatures are most likely going to be in the 80s, though a well-timed shortwave could yield enough cloud cover and rain chances to keep highs in the mid to upper 70s for portions of the area. The chances of 90+ degree highs remains low at this time. The air mass moving into the area looks warm, but not overly impressive. Temperatures at 850 hPa on the LREF climb closer to +15C, but that is ~90th percentile of climatology. The mid/upper level ridging also is expected to stay to our south, with 500-hPa heights staying well below the 90th percentile. Probabilities for 90+ degree highs each day on the LREF remain at or below 15% and look much more representative compared to the NBM.

Gosselin

AVIATION

(For the 06z TAFs through 06z Monday Night) Issued at 906 PM CDT Sun May 10 2026

Dry and VFR conditions are expected through the period with light winds.

Britt

LSX WATCHES/WARNINGS/ADVISORIES

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