685
FXUS63 KLSX 150328
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1028 PM CDT Tue Jul 14 2026

.KEY MESSAGES...

- Seasonable warmth is forecast through the end of the week,
  with rising humidity pushing heat index values near 100 to 105
  degrees Thursday into the weekend.

- Scattered afternoon shower and thunderstorms (30-60%) are
  forecast Wednesday through Friday, with the highest potential
  shifting northward each day. While mostly dry weather is
  expected, storms may produce brief, heavy downpours.

&&

.SHORT TERM /THROUGH WEDNESDAY/...
Issued at 248 PM CDT Tue Jul 14 2026

An anomalously strong and broad mid/upper-level ridge remains parked
across the Upper Midwest along with a persistent surface high across
Iowa. Additionally, cyclonic mid-level remnants from the recent
weekend system continue to slowly retrograde westward across the
Deep South along the southern periphery of this ridge. This synoptic
pattern places our region under light easterly flow that encompasses
the entire vertical column with a majority of the convective
activity displaced off to our south and east. The result for the
area has been seasonable temperatures/humidity and dry conditions as
the airmass upstream (east) being advected into the bi-state region
features typical summertime conditions with no notable forcing
mechanisms. Some high-resolution guidance unveils weak pieces of the
mid-level cyclonic remnants swinging across central AR/far southern
MO later this evening, resulting in isolated to scattered showers
and thunderstorms. With the anomalously strong ridge reinforcing
subsidence across the area, this activity is expected to be very
sparse. Currently, there is only a 10-20% chance for isolated
thunderstorms across Reynolds, Iron, and Madison counties this
evening with increasing chances further south into Arkansas and
Tennessee. Afternoon temperatures across the region are forecast to
reach the upper 80s to near 90 (STL metro) with heat index values
potentially reaching the mid 90s for a few hours this
afternoon/evening. After sunset, surface winds are forecast to go
calm and variable with close proximity to the surface high. This,
coupled with a clear sky may favor the development of patchy fog,
particularly across northeast MO/west-central IL and river valleys,
where surface winds should be more consistently calmer.

Recent model guidance reveals a gradual weakening of the mid/upper-
level ridge commencing later today and continuing through the
remainder of the week. As this evolution happens, this allows the
previously mentioned mid-level cyclonic remnants to approach the
area from the east-southeast by Wednesday. Additionally, as the
surface high slides further east, this introduces southerly flow
leading to increasing warm/moist advection. This is progged by model
guidance to result in PWAT values near 2" across southern MO/IL by
Wednesday, with gradual northward expansion of this column moisture
Thursday into Friday. As moisture and temperatures increase with
southerly flow, heights aloft will be gradually falling as well
thanks to the weakening ridge, where both variables aid in
increasing chances for showers and thunderstorms.

On Wednesday, the best chances (30-60%) for showers and
thunderstorms are forecast to be across southern MO, which is where
greater moisture return and mid-level forcing for ascent will be.
Rich column moisture (PWATs near 2") and a warm cloud layer
>12,000ft will support warm rainfall processes leading to efficient
rainfall production. As a result, the main threat with thunderstorms
will be heavy downpours that may lead to a quick 1-2" of rainfall.
Training of thunderstorms does not appear to be likely as there is
no apparent boundary that convection is forecast to set up on and
effective-layer flow is weak, which will inhibit thunderstorm
organization and maintenance. This activity is expected to be
scattered pop-up thunderstorms that drift westward within the light
easterly flow, but may still be capable of quick heavy rainfall
leading to elevated runoff. As a result, locations that have
experienced recent heavy rainfall and have saturated soils may be
susceptible to pockets of localized flooding.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
Issued at 248 PM CDT Tue Jul 14 2026

By Thursday, model guidance is in good consensus that the mid/upper-
level ridge will become more elongated west to east while continuing
to weaken with the main ridge axis pushing further west. An
evolution like this would allow the cyclonic remnants to slowly turn
to the north before lifting off to the northeast across the Ohio
River valley by Friday. The greatest precipitation chances will be
correlated with the bulk of this mid-level cyclonic vorticity as it
slowly propagates across the area. This means that afternoon/evening
shower and thunderstorm chances expand northward to most of the area
on Thursday and even further northeastward into Illinois on Friday.
Atmospheric conditions remain relatively unchanged as this feature
moves by with anomalously high PWAT values and a warm cloud layer
favoring efficient rainfall production and higher rainfall rates
associated with thunderstorms. As a result, a marginal threat for
pockets of localized flash flooding associated with heavy downpours
from thunderstorms cannot be ruled out Thursday into Friday also.
With continued southerly flow advecting warm/moist air into the area
late this week, humidity and heat index values will continue to
increase as well. Currently, the LREF reveals probabilities for heat
index values >100F reaching up to 30% across parts of the area on
Thursday and Friday.

Guidance indicates that the broad and amplified mid/upper-level
ridge out west continues to retrograde further west as a longwave
trough builds into the northeastern CONUS this weekend. The slow
moving cyclonic vorticity near the area is progged by guidance to be
swept up by this longwave trough with the region under northwesterly
flow aloft. As of now, a majority of long-range guidance keeps the
longwave and its embedded shortwaves off to the northeast this
weekend, keeping any back-door cold fronts away from the area,
locking the area in continued low-level southerly flow. A solution
like this would result in continued warmth over the weekend
potentially including some of the warmer temps/heat index values
over the next week. The latest LREF indicates this well where
probabilities for heat index values >100F jump to 30-60% Saturday
into Monday. However, into early next week, the longwave trough may
begin to have greater influence on our area, potentially leading to
better chances for back-door cold front(s)/precipitation and relief
from the heat. This is represented well by the LREF high temperature
10th-90th percentile spreads that jump from <10F on Sunday to >15F
on Monday, indicating that guidance uncertainty increases. The
result of the increase in spread is due to cooler solutions skewing
the data, where the lower tail (10th percentile) is near
temperatures 5-10 degrees below climatological normals.

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1023 PM CDT Tue Jul 14 2026

VFR conditions are largely expected at local TAF sites during
the 06Z TAF cycle, with a few brief, low-probability
exceptions. Some shallow fog will once again be possible
overnight and early tomorrow morning at local river valley
sites, including SUS/CPS/JEF. While confidence is low regarding
whether the terminals will be directly impacted, visibility
reductions will be possible.

Otherwise, there is a low (20% or less) chance for showers and
thunderstorms late in the afternoon and evening at I-70 corridor
terminals, but this activity is most likely to remain south of
there during this period. Chances for showers and thunderstorms
increase Thursday afternoon, but this will not be within the TAF
forecast window for at least another 12 hours.

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.

&&

$$

SHORT TERM...Peine
LONG TERM...Peine
AVIATION...BRC