textproduct: St. Louis

KEY MESSAGES

- Summer-like heat continues today with relief from a cold front starting Sunday.

- More widespread showers and thunderstorms are expected Sunday night into Monday.

SHORT TERM /THROUGH SUNDAY/

Issued at 343 AM CDT Sat Sep 19 2026

An upper level ridge continues to reside over the central CONUS, which has been the driver behind the heat and humidity lately. The front that brought some subtle heat relief to northern areas and spotty showers and thunderstorms has retreated north, allowing the hot and humid air mass to return over the entire forecast area today. Ensemble guidance indicates that 850mb temperatures are 21-22 degrees C (>95th percentile of climatology) translating to mid to upper 90s surface temperatures in full mixing. With deeper mixing, dewpoints are expected to be lower in the mid to upper 60s leading to lower heat index values around 100 across the region. An upper level tough that is currently over the western CONUS will move to the east through the day today and into Sunday, eventually pushing a cold front into our area beginning Sunday. Temperatures on Sunday will still be hot and humid south of the front as it begins to push through the region. This will lower high temperatures into the 80s in northern MO, while southeastern MO will remain in the 90s. However, modest humidity will prevent more widespread 100+ heat index values. As a result no changes are being made to the Heat Advisory as it aligns well with the forecast.

The low-level jet is expected to develop some showers and thunderstorms in eastern KS overnight tonight, but are expected to significantly weaken by the time they reach central MO. HREF guidance shows that there is about a 10-40% chance that some convective remnants could translate through northeastern MO. Otherwise, today and Sunday are looking to remain dry.

LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/

Issued at 343 AM CDT Sat Sep 19 2026

The light at the end of this hot tunnel is in sight. Cluster analysis continues to show uncertainty on the speed of the front with much cooler temperatures behind it. The relief will begin in northern MO on Sunday during the day and is expected to be completely through the area by Tuesday. Current NBM guidance interquartile range for the high temperatures on Monday reaches 10 degrees due to the uncertainty in the fronts exact position and speed. Behind the front temperatures are expected to be much cooler, and latest NBM guidance continues to trend cooler than previous days. For example, NBM has dropped Tuesday max temperature forecasts around 8 degrees over the last 48 hours, now with highs only in the upper 60s to low 70s. This air mass will dominate for the rest of the week with a gradual warming through the 70s.

The highest chance for showers and thunderstorms in the next week will be Sunday night into Monday morning behind the cold front. While the confidence is low on the exact timing and location of this convection, there is a 60-80% chance across the region of measurable rainfall during this period, and 30-60% chance of seeing 0.25 inches of rain.

AVIATION /00Z TAFS THROUGH 06Z SUNDAY/

Issued at 1027 PM CDT Fri Sep 18 2026

Mainly dry and VFR conditions are expected through the period. Light winds are expected through mid-late morning on Saturday before turning out of the southwest 5-10 knots.

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.