641
FXUS63 KLSX 050951
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
451 AM CDT Sun Apr 5 2026

.KEY MESSAGES...

- Seasonably cool temperatures will persist through Tuesday
  including a 40 to 70 percent chance of a freeze in northeastern
  MO and west-central IL Tuesday morning.

- There is a 40 to 70 percent chance of light rain changing to
  light snow Monday night/Tuesday morning in northeastern MO, but
  confidence is not high that there will be any accumulations or
  impacts.

- Temperatures will return to above average Wednesday onward with
  increasing chances of showers and thunderstorms through the end
  of the week.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 251 AM CDT Sun Apr 5 2026

Low-level CAA behind Saturday`s morning front continues to usher in
a seasonably cool airmass into the Mid-Mississippi River Valley with
temperatures beginning to fall into the 30s F before sunrise.
However, northwesterly winds are expected to remain just strong
enough to prevent more than patchy frost development where
temperatures reach the mid-30s F. Gradually tapering CAA and strong
insolation aside from some early day diurnal cumulus or
stratocumulus will favor seasonably cool high temperatures in the
mid-50s to around 60 F.

Slightly warmer temperatures are expected tonight (mid-30s to low-
40s F) as low-level flow becomes westerly ahead of a cold front
passing southward through the CWA Monday morning in response to
shortwave troughs rounding upper-level longwave troughing in the
Great Lakes. Although the front is progged to be south of the CWA
before peak heating, post-frontal CAA will lag the front with high
temperatures in the 60s F along/south of I-70 and 50s F to the
north. Clouds will also overspread the CWA from northwest to
southeast Monday afternoon and evening as mid-level frontogenesis
increases, but HREF and global ensemble membership indicate only a
20 percent chance of (light) measurable rainfall in northeastern MO
before sunset.

Pfahler

&&

.LONG TERM...  (Monday Night through Saturday)
Issued at 251 AM CDT Sun Apr 5 2026

Low-level CAA will further amplify Monday night as an anomalously
strong (99th climatological percentile) surface anticyclone
traverses the Midwest and Great Lakes, providing the coldest
temperatures of the forecast period. NBM probabilities of sub-32 F
temperatures are 40 to 70 percent across northeastern MO and west-
central IL Tuesday morning, which threatens damage to susceptible
vegetation. With cloud cover, this freeze will be predominantly
driven by CAA rather than the more common radiational cooling. Model
guidance continues to signal a quasi-stationary band of
frontogenetical precipitation developing somewhere between I-70 and
northern MO Monday night/Tuesday morning, with ensemble model 24-
hour probabilities of measurable precipitation now 40 to 80 percent
in those areas. With the cold airmass and potential wet-bulbing, 40
to 70 percent of membership also has light rain transitioning to
light snow in northeastern MO. With forecast soundings indicating
strongest ascent below the DGZ (poor SLRs), only marginally
supportive temperatures, and a dependency on precipitation rate;
confidence is not particularly high in accumulations or impacts at
this time. Not only that, but these types of setups are notoriously
difficult to predict including location and QPF.

On Tuesday, upper-level flow will begin transitioning from
northwesterly or cyclonic toward quasi-zonal, leading to the
aforementioned frontogenetical band of precipitation lifting
northward during the morning as a warm front advances into the CWA.
With an associated low-level thermal gradient extending across the
CWA and less clouds to the south, there will be a spread in high
temperatures from north to south. This spread ranges from the 40s F
in northeastern MO/west-central IL and 60s F in southeastern MO--
regardless, these will be the coolest daytime temperatures of the
period for nearly the entire CWA.

With the upper-level quasi-zonal flow, low-level flow will finally
be able to return to southerly including WAA as one or more
shortwave troughs pass. This evolution equates to temperatures
quickly warming to above average on Wednesday, but spread in the NBM
temperature distribution increases through the remainder of the week
into the upcoming weekend as a front oscillates across the Midwest
and Mid-Mississippi River Valley. Ensemble model membership also
suggests that this front and passing shortwave troughs will lead to
showers at times and eventually thunderstorms as deeper Gulf of
Mexico moisture arrives by the weekend. Additionally, extended
analog guidance (e.g., SLU CIPS) advertises a wet and active pattern
this weekend continuing into early next week.

Pfahler

&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 450 AM CDT Sun Apr 5 2026

Dry and VFR flight conditions will continue through the TAF period
with northwesterly winds gusting 18 to 25 kt at times today, before
slacken and becoming westerly to southwesterly this evening. A dry
cold front will advance southward through the region Monday morning,
veering winds northwesterly.

Pfahler

&&

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

$$

WFO LSX