textproduct: St. Louis

KEY MESSAGES

- Isolated showers and thunderstorms are possible late tonight into Wednesday morning.

- More widespread thunderstorms are expected Wednesday afternoon and Wednesday night. A few storms may produce damaging winds and locally heavy rainfall. Isolated large hail and a brief tornado are also possible.

SHORT TERM /THROUGH WEDNESDAY NIGHT/

Issued at 316 PM CDT Tue Aug 4 2026

A cold front stretching through Iowa into southeast Nebraska will drift into Missouri tonight. The front and its low level baroclinic zone will provide a focus for thunderstorm development through the short term. Could see some isolated convection creep into northeast Missouri/west central Illinois even before 12Z Wednesday, although all indications are that this convection will be relatively weak and sparse over our area. Models suggest more widespread development Wednesday afternoon as a mid/upper level short wave trough moves from the Great Plains into the Midwest. The upper level wave will produce broad-scale lift along with steepening mid-level lapse rates to support afternoon convection. There's still some uncertainty on where convective initiation will occur, but most models are pointing toward northwest or north central Missouri between 17-19Z. Could see 1500-2000 J/Kg CAPE Wednesday afternoon with 35-40kts of shear. Could see a few supercells initially, but CAMs show individual cells quickly merging into line segments and clusters. Think the primary threat will be damaging winds, but some hail will also be possible. 0-1km helicity is also forecast to be 100+ m2/s2 so can't rule out a tornado or two as well.

The focus turns to potential for heavy rain on Wednesday night as 30- 35kt low level jet develops over Missouri. Deterministic guidance shows moderate to strong moisture convergence along the low level baroclinic zone on the nose of the jet, and all models print out widespread QPF, mainly north of I-70. HREF mean precipitable water values are in the 1.8-2 inch range south of the front, and the 6hr LPMM values of 2-3 inches with spots of 5+ inches. However, 1 hour flash flood guidance is 2+ inches, and it increases to 3 an 4+ inches at 3 and 6 hours respectively. Given the environment, kinematics, and the signals in the QPF, heavy rain is a definite concern for Wednesday night, but the relatively dry antecedent ground conditions should allow for a good deal of rain to be absorbed before any flooding begins. If the signals in guidance remain this strong though, a flood watch may be needed for Wednesday night.

LONG TERM /THURSDAY THROUGH TUESDAY/

Issued at 316 PM CDT Tue Aug 4 2026

Medium range guidance shows a somewhat blocky pattern with a ridge over the Desert Southwest, another ridge in the western Atlantic, and a long wave trough sitting over the Midwest and Mississippi Valley in between. The pattern tries to become more progressive and zonal late in the weekend, but there aren't any obvious broad-scale forcing mechanisms to move the surface front. In this case, the position of the effective front will probably be determined by outflow from convection along it. Temperatures will therefore be difficult to forecast for Thursday into the weekend. Current indications are that Thursday and Friday will be relatively cool in the low to mid 80s with clouds and intermittent showers and a few thunderstorms. However the 75th percentile of high temperatures from the LREF is just under 90 degrees, which is certainly possible if the front is farther north than expected. The Bermuda High tries to build back to the west into the gulf and the southern U.S. late Friday into Saturday which would tend to bring southerly flow back to the Mid Mississippi Valley, and push the surface front north away from our area. This will bring temperatures back into the upper 80s to low and mid 90s by Sunday/Monday. The prevailing southerly flow will also bring higher humidity back to the Mid Mississippi Valley and a threat for heat index values above 100 degrees.

AVIATION /18Z TAFS THROUGH 00Z THURSDAY/

Issued at 1228 PM CDT Tue Aug 4 2026

VFR flight conditions and light southerly flow is expected to prevail at least through this evening. May see isolated to widely scattered showers and thunderstorms develop ahead of a cold front late tonight across parts of central and northeast Missouri into west central Illinois. Storms late tonight into Wednesday morning are not expected to be particularly strong, although some could briefly reduce the visibility to 3SM and lower in heavy downpours. Another round of thunderstorms is expected Thursday afternoon.

LSX WATCHES/WARNINGS/ADVISORIES

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