Arnold, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

434
FXUS63 KLSX 271057
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
557 AM CDT Sun Jul 27 2025

.KEY MESSAGES...

- Dangerous heat expands areawide today, intensifying Monday and
  Tuesday. The worst impacts will be felt in the greater St. Louis
  metropolitan area where urban heat island influence is
  strongest.

- Thunderstorm chances (20-50%) migrate along and south of I-70
  today, but with weaker forcing and shear there is very little
  concern for severe weather or flash flooding.

- The heat breaks for those north of I-70 on Wednesday, with
  relief felt areawide by Thursday.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 547 AM CDT Sun Jul 27 2025

(Through Late Monday Afternoon)
Issued at 214 AM CDT Sun Jul 27 2025

Subtle mid-level ascent coincident with low-level frontogenesis and
a weak but sufficient low-level jet are stimulating training
thunderstorms across parts of east-central Missouri and west-central
Illinois. As much as 6.00 inches of rain is estimated to have fallen
across parts of Lincoln County, MO where efficient thunderstorms
trained for several hours and resulted in considerable flash
flooding. These thunderstorms are becoming somewhat outflow dominant
and are weakening. This weakening trend will continue as the low-
level jet continues to wane after sunrise. While I don`t expect a
clean break between thunderstorm chances this morning and this
afternoon, the lack of stronger shear and forcing that will be in
place today according to nearly all available guidance will result
in more scattered coverage of the diurnally-driven thunderstorms.
The best chances look to be along the remnant outflow from ongoing
convection, mainly along and around the I-44 corridor during the
afternoon. With precipitable water values still quite high, even for
late July, any convection will threaten heavy rain and localized
flooding. Because there isn`t a really tangible focus for these
storms in the afternoon, any flash flooding will be fairly isolated
in nature. Outside of these thunderstorms, heat index values will
climb into the triple digits areawide, and be highest in the St.
Louis metro area. Rain chances abate after sunset with the upper-
level ridge building quickly overnight. As a result, Monday will be
notably hotter with similar humidity values to today. Dangerous heat
index values will exist areawide, with particular focus again on the
urbanized areas. Parts of far southeast Missouri and southwest
Illinois will see a fairly low (15%) chance for thunderstorms in the
afternoon, but with the very warm air aloft from the ridge in place,
I doubt we will see much convective initiation.

MRB

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Issued at 547 AM CDT Sun Jul 27 2025

(Monday Night through Saturday)
Issued at 214 AM CDT Sun Jul 27 2025

Tuesday looks fairly similar from a heat perspective, with virtually
no rain chances as the upper-level ridge is squarely overhead.
Meanwhile, a longwave trough will amplify over eastern Canada and
push a front south into the Mid-Mississippi Valley. The ridge
will begin to collapse like a house of cards from north to south
on Wednesday, aided by several shortwave impulses amidst the
periphery of the longerwave feature. Showers and thunderstorms are
likely (60- 70%) on Wednesday in the vicinity of the front, and
amidst the favorable antecedent thermodynamic environment for
heavy rain, I can`t rule out a localized flood threat on
Wednesday. There will likely be a stark temperature gradient from
south to north, with cooler air further north and warm/humid
conditions lingering in the south. Depending on how things unfold,
we may need to extend the heat headlines in parts of the area on
Wednesday.

The front advances south, aided partly by synoptic forcing and
partly by convection, clearing the region and finally ending this
oppressive stretch of hot and humid weather. Lower (20-40%) rain
chances linger in parts of the area through the weekend with some
potential for the front to waver back north a bit. That said,
confidence remains high that we will enjoy a few days of below-
normal temperatures before a gradual warmup next week.

MRB

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 547 AM CDT Sun Jul 27 2025

The main impacts to the TAF sites with this issuance will be
ongoing thunderstorm activity for the TAF sites near the St.
Louis metro with brief reductions in visibilities likely over
the next couple hours. A lull in the activity is anticipated
through mid morning before afternoon convection starts to bubble
up once again. The afternoon thunderstorms could impact any of
the TAF sites, but most likely from COU over to STL and points
south. Kept or added PROB30s for each of those sites from
roughly 19Z-23Z during peak heating. Some patchy ground fog may
develop overnight, but confidence was low enough to leave out of
this issuance. Overall winds will be light outside of
thunderstorm activity.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 7 PM CDT Tuesday for MOZ034>036-041-042-
     047>052-059-060-072>074-084-085-099.
     Extreme Heat Warning until 7 PM CDT Tuesday for MOZ061>065-075.
     Heat Advisory from 1 PM this afternoon to 7 PM CDT Tuesday for
     MOZ018-019-026-027.
IL...Heat Advisory until 7 PM CDT Tuesday for ILZ058>060-065-
     097>099.
     Extreme Heat Warning until 7 PM CDT Tuesday for ILZ064-069-070-
     074-079-100>102.
     Heat Advisory from 1 PM this afternoon to 7 PM CDT Tuesday for
     ILZ095-096.

&&

$$

WFO LSX
AVIATION...WFO PAH