Area Forecast Discussion

KEY MESSAGES

- Cooler and less humid weather is expected Wednesday and Thursday under the influence of a Canadian high pressure system.

- The next chance for rain will be late Thursday night through Friday as an upper level disturbance moves across the area.

SHORT TERM /THROUGH WEDNESDAY/

Issued at 220 PM CDT Tue Jul 21 2026

Cooler and drier air is moving into the Mid Mississippi Valley behind a cold front this afternoon. Dew point temperatures have already fallen into the 60s for much of the area north of I-70. There will still be a chance for thunderstorm development for the next couple of hours across Reynolds, Madison, and Iron counties just ahead of the front, but the front should move through by 22-23Z which will bring the threat for storms to an end. The remainder of the short term through Thursday will be quiet with highs falling to 5-10 degrees below normal in the low to mid 80s. Dew point temperatures will also be 10+ degrees lower Wednesday and Thursday in the mid 50s to low 60s in most locations. This will produce much more comfortable weather in the wake of our recent heat.

LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/

Issued at 220 PM CDT Tue Jul 21 2026

Global guidance brings the upper level low which is currently near the New Mexico/Arizona border northward around the upper level high and then into the central Plains Thursday night. The wave is picked up in the mean northwest flow aloft and it moves southeast into the Mississippi Valley by late Friday. Models are showing some weak to moderate lo level cyclogenesis over the central Plains in response to the upper forcing. The gradient between the high over the Ohio Valley and the developing low over the Plains produces a 30-40kt low level jet late Thursday night that shifts into the lower Missouri and Mid Mississippi Valley Friday. This results in what is likely be the best chance for precipitation for our area over the next 7 days. The highest chance for rain remains along and west of the Mississippi as high pressure to the east will continue to funnel drier air into the region which undercuts the moisture convergence produced by the low level jet. Clouds and precip also limit daytime heating on Friday with most locations struggling to reach the the mid and upper 70s.

The short wave continues to move southeast away from the area Friday night, and the upper ridge builds east into the Mississippi Valley Saturday. This will bring warmer temperatures and higher humidity back to Missouri and Illinois for the weekend, although there's some question as to how fast the warmup will occur. A little over 60% of the LREF members keep max temperatures below 90 on Saturday. However some of the warmer 40% are very warm with highs over 95. These are the outliers...though the operational GFS is among them with 850mb temps around 22C which mixes down to 95F at the surface. NBM temps in the mid 80s to around 90 look reasonable given the uncertainty for Saturday's highs. The upper ridge continues building eastward through the weekend into next week, although deterministic guidance shows another weak short wave moving across the Mississippi Valley through Iowa into central Illinois. This attenuates the ridge somewhat, but it doesn't look strong enough to spare our area from another period of mid 90s temperatures along with low to mid 70s dew points. This would put us squarely back into Heat Advisory territory for most of the area for at least Sunday and Monday if these conditions are realized.

AVIATION /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 611 PM CDT Tue Jul 21 2026

This evening and tonight: A cold front has pushed south of all of the terminals this evening with VFR conditions in place. A few transient wind gusts will be possible for the next hour or so as weak cold air advection persists, but the PBL will likely quickly decouple with north winds relaxing.

Wednesday: Surface high pressure will move southeast out of North Dakota with winds from the north around 10 kt. Latest forecast soundings from the GFS, RRFS, and HRRR all show a rather robust cumulus field forming in the afternoon and this makes sense as 850 mb moisture never fully clears Missouri. Either way, cigs will be VFR.

LSX WATCHES / WARNINGS / ADVISORIES

MO...Heat Advisory until 7 PM CDT this evening for MOZ041-047>051- 059>065-072>075-084-085-099. IL...Heat Advisory until 7 PM CDT this evening for ILZ069-074-079- 100>102.