639
FXUS63 KLSX 130837
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
337 AM CDT Thu Aug 13 2026

.KEY MESSAGES...
- Dangerous heat and humidity is expected to continue into at
  least Saturday.

- Relief from the heat and humidity is expected early next week
  with the passing of a cold front Sunday into Monday.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 335 AM CDT Thu Aug 13 2026

Current radar indicates isolated showers and thunderstorms
around the I-70 corridor and St. Louis metropolitan area. With
plenty of surface moisture for this ongoing convection, a
stationary front positioned over the region is providing the
lift needed to spark these isolated showers and thunderstorms.
This front is expected to shift to the north caused by low-level
southerly flow and a building ridge, and as it does it will
cause additional isolated convection along it as it moves north.

There is additional convection over east central NE that is
getting support from a strong low-level jet for it to continue.
Similar to yesterday, the driver for this system will be a
ridge-runner shortwave that will allow the convection to
travel through IA. IA contains favorable instability and shear
conditions for the continued support for convection. On top of
that, during the afternoon it will meet the northward moving
front to provide additional lift for the abundant surface
moisture (>70F dewpoints) present in central IA. As this system
moves across eastern IA and northern IL it will cause an outflow
boundary to propagate south into northeastern MO and west
central IL, which will provide lift for a threat of severe
weather again today. SPC has placed our northern and
northeastern counties into a Marginal Risk for this afternoon,
primarily for damaging wind gusts from the strong downdrafts
associated with the thunderstorms. Despite a similar pattern to
yesterday, the threat is lower today due to subsidence aloft
caused by the building ridge.

The dangerous heat and humidity continues. Models continue to be
in consensus that the ridge will build north into the weekend
allowing for the hot and humid air mass to continue its
residence over the central CONUS. Low to mid-90s are likely,
with a 60-90% chance of high temperatures being greater than 95
F along and south of the I-70 corridor for both today and
tomorrow. Dewpoints are also expected to remain in the low to
mid-70s causing dewpoints ranging 100-105 F. Clouds associated
with the rounds of convection will cause cooler temperatures in
the northern portion of our forecast area.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 335 AM CDT Thu Aug 13 2026

Looking into the weekend, ensemble guidance continues to
indicate a continuance of heat and humidity into at least
Saturday. NBM guidance provides a 60-90% chance of high
temperatures Saturday being 95 F or higher along with a 40-50%
chance of dewpoints being greater than 70 F along I-70 and south
both Saturday and Sunday. Heat headlines were not extended into
Sunday due to some uncertainty in the exact timing of a cold
front Sunday into Monday, but also a chance of another
convective system. A few deterministic models do present similar
synoptic patterns that could allow for a repeat of yesterday and
today to occur Saturday night and into Sunday: a ridge-running
convective system that may cast clouds and outflow over parts of
the region. If that were to occur, then temperatures on Sunday
could be low enough to not necessitate heat headlines.

On the brighter side, ensemble guidance is continuing to show
strong signs of a cold front moving through the region at the
beginning of next week, bringing temperatures and humidity
closer to seasonal normals. Cluster analysis is in agreement
that heights will fall and bring in northerly flow starting
Sunday night into Monday. How long this cool down will last is
uncertain. Beyond Tuesday, guidance indicates a range of
possibilities. On one hand another ridge could build far north
over the central CONUS bringing another hot and humid air mass,
while on the other hand a weaker ridge to the southwest could
occur that may allow for an additional cold front to keep
temperatures near normal. As a result, both the LREF and NBM
have a 10-15 degree spread in their high temperatures for
Wednesday and beyond.

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1025 PM CDT Wed Aug 12 2026

While mainly dry conditions are expected through the period at
COU/JEF, there will be a low chance (20-30%) of showers and
thunderstorms at UIN/STL/SUS/CPS. Have only included a PROB30
group at UIN where I have the highest confidence in the timing.
Otherwise, winds will remain light and variable through midday
on Thursday before becoming southerly under 10 knots.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 9 PM CDT Saturday for MOZ034>036-042-052.
     Extreme Heat Warning until 9 PM CDT Saturday for MOZ041-
     047>051-059>065-072>075-084-085-099.
IL...Heat Advisory until 9 PM CDT Saturday for ILZ058>060-064-065-
     070-098-099.
     Extreme Heat Warning until 9 PM CDT Saturday for ILZ069-074-
     079-100>102.

&&

$$

SHORT TERM...Asel/MRB
LONG TERM...Asel/MRB
AVIATION...Britt