Area Forecast Discussion

KEY MESSAGES

- Unseasonably hot and humid weather is expected this week with heat index values reaching 105F+ on Tuesday mainly south of I-70.

- Scattered showers and thunderstorms are forecast on Tuesday. A few may be strong to marginally severe with gusty winds the main threat.

SHORT TERM /THROUGH WEDNESDAY/

Issued at 228 PM CDT Mon Sep 14 2026

A surface warm front is marching northeastward, and is nearing central/southeast Missouri as of 1800 UTC. This boundary should clear the CWA overnight tonight into Tuesday morning from west to east. Warmer and more humid air moves in behind this front, with temperatures currently in the 90s in southwest Missouri with dewpoints in the upper 60s. Widespread low 70s for dewpoints are not too far behind into southeast Kansas and northeast Oklahoma. That is the air mass heading our way on Tuesday. In other words, pure summer and nothing that is remotely typical for the middle of September is imminent beginning Tuesday. This incoming air mass remains quite extraordinary in many ways, including: 850-hPa temperatures above +20C (>99th percentile of climatology), record-breaking 500-hPa heights/ridge, and near-record dewpoints. High temperatures may also threaten daily records at St. Louis (KSTL) and Columbia (KCOU) Tuesday afternoon as readings top out in the mid to upper 90s southeast of a slow-moving cold front. Please refer to the CLIMATE section below for more details on daily high temperature records Tuesday through Saturday. These temperatures combined with aforementioned dewpoints in the low 70s means widespread 100-105F+ degree peak heat index values. A heat advisory was issued in areas where widespread magnitude (105F+) criteria is likely to be reached on Tuesday, which is for areas mainly south of the Missouri River and south of I-70 in Illinois. NOTE: This is the latest heat advisory from our office dating back to at least 2004, and likely quite a bit longer. As was discussed yesterday, it is incredibly rare for 105F+ heat index values this late in the year. For St. Louis, (since 1930) this would only be the third day with a peak heat index at least of 105 degrees on or after 9/15 (105F on 9/17/1931 and 106F on 9/19/1972).

By mid morning on Tuesday, the cold front is expected to enter portions of northeast Missouri and west-central Illinois. Scattered showers and a few thunderstorms are forecast along/behind this boundary as it slowly progresses to the southeast with time. A weakening low-level jet and associated moisture convergence suggest diminishing coverage and overall organization late morning toward midday. The most likely scenario is for an outflow boundary to push southeastward toward the I-44/I-70 corridors in Missouri and Illinois respectively by early-mid afternoon. This boundary in most CAMs serves the focus for scattered showers and thunderstorms through the evening hours. The higher shear lags behind the cold front, so mainly pulse convection with a low threat for damaging winds (via downbursts most likely) is expected. The joint probability for at least 1500 J/kg of SBCAPE and >25 knots of deep- layer shear only peaks out 35-45% on the HREF, and that is right near the front itself. Lower probabilities exist to the southeast where there seems to be a stronger signal for convective initiation.

The coverage of showers and thunderstorms should increase Tuesday night as moisture convergence strengthens on the nose of a ~25 knot southwesterly low-level jet. The best chances (60-80%) of showers and thunderstorms will be along/poleward of the effective front. At this time, that boundary is likely to be somewhere across southeast Missouri into southwest Illinois, so the highest PoPs are located along/north of I-70. These rain chances will continue at least through Wednesday morning, and really lessens the confidence in specific temperature and heat index values. The low-level jet also does not weaken much into the early afternoon, so scattered showers and storms may continue. If this occurs, temperatures and heat index values may be at least several degrees too high in the current forecast. There is a pretty good chance that Wednesday is actually the "coolest" day through Saturday. Even lingering mid/high level clouds would probably be enough to keep heat index values below 100 degrees. That is the reason the heat advisory only includes Tuesday at this time.

LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/

Issued at 228 PM CDT Mon Sep 14 2026

(Wednesday Night - Saturday Night)

The main source of uncertainty is how the quasi-stationary front evolves through the end of the week. Deterministic guidance has shifted slightly toward an overall placement more poleward than yesterday, with the front shifting northward Wednesday night into Thursday. I am a bit skeptical of this scenario given a lack of leeside cyclogenesis to the west and the possibility of convection helping push outflow farther south. That being said, there are rising mid/upper level height rises, so there may be some merit to a shift back northward. The quasi-stationary front should remain near northeast Missouri and west-central Illinois through Thursday night. There is a better signal for it to push northward (and north of our area entirely) Friday into Friday night as a surface low moves northeast across the north-central Plains. South of the boundary, near-record heat and humidity is expected to continue with highs well into the 90s forecast through Saturday. The most uncertainty remains across northeast Missouri and west-central Illinois regarding temperatures due to their expected close proximity to the boundary and higher chances of showers and thunderstorms (as well as cloud debris). This uncertainty is illustrated well looking at the inter-quartile range (IQR) differences for high temperatures Thursday-Saturday from the NBM. At KUIN, the IQR spread is 5-8+ degrees while the spread is only about 2-4F at KFAM well south of the boundary.

(Sunday - Next Monday)

There may be light at the end of the proverbial tunnel looking ahead to early next week. While there is not surprisingly a lot of spread in the specifics, all four clusters from the LREF of the 500-hPa height pattern show significant weakening of the mid/upper level ridge. Flow aloft across the mid-Mississippi Valley transitions to more zonal, with an increasing signal for a cold frontal passage sometime Sunday/Sunday night. The specifics with the air mass behind this boundary are not yet known, but it does appear likely that high and low temperatures should get at least much closer to normal. After ~4 straight hot and humid days for some locations, near normal would be quite the change. While there is still a lot of spread, even the 75th percentile from the NBM for high temperatures next Monday are in the 80s so it should be at least 10 degrees cooler. If the cooler EPS solution verifies, widespread 70s for highs would be possible (closer to the 25th percentile of the NBM).

AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 1057 PM CDT Mon Sep 14 2026

VFR conditions will continue through the night. There will be some LLWS for the UIN terminal as a result of a southwesterly low-level jet. An outflow boundary is expected to settle hear the St. Louis metro in the afternoon which will allow for the chance of showers and thunderstorms to develop near STL, SUS, and CPS. Additionally as a cold front will make it's way through the region Tuesday evening and into the night, it creates rain chances for UIN, COU, and JEF and potentially additional chances for STL, SUS, and CPS. There is uncertainty in the exact timing and coverage of the showers and thunderstorms within the model guidance, but development is expected. The PROB30s included in the TAFs highlight when confidence is at its highest of when the terminal could be impacted.

CLIMATE

Issued at 231 PM CDT Mon Sep 14 2026

Daily record high temperatures may be threatened for both St. Louis (KSTL) and Columbia (KCOU) this week. NOTE: Records at Quincy (KUIN) are not shown below due to cooler temperatures forecast:

9/15 9/16 9/17 9/18 9/19 KSTL 101/1939 98/1954 97/1931 97/1925 96/1955 KCOU 101/1893 99/1954 98/1895 98/1893 98/2022

LSX WATCHES / WARNINGS / ADVISORIES

MO...Heat Advisory from 11 AM to 7 PM CDT Tuesday for MOZ047>049- 059>065-072>075-084-085-099. IL...Heat Advisory from 11 AM to 7 PM CDT Tuesday for ILZ064-069- 070-074-079-100>102.