287
FXUS63 KLSX 271943
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
243 PM CDT Sun Sep 27 2026

.KEY MESSAGES...

- Rainfall arrives for some by Wednesday, becoming more
  widespread Thursday into Friday. Northeast/central MO and
  west-central IL have the best chance (30%) of receiving
  substantial rainfall (2").

- Gradual warming continues, with high temperatures in the 80s
  for most locations Tuesday and Wednesday.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 242 PM CDT Sun Sep 27 2026

Recent mid-level water vapor imagery from GOES-19 reveals a nearly
stationary closed low across New England, ridging centered across
the Great Plains, and troughing approaching the Pacific Coast. This
large-scale pattern places the Mid-Mississippi Valley under
northwesterly flow aloft. Under this regime, lobes of vorticity
continue to slowly traverse southeastward, which has aided in a few
areas of scattered to isolated showers across the area today. These
light showers have been mainly confined to portions of
central/northeast MO along with west-central/southern IL. Most areas
that have remained dry today, under a field of broken 7-10kft
altostatus, have warmed into the lower 70s. While locations that
have experienced brief intermittent showers have stayed slightly
cooler in the mid/upper 60s early this afternoon. A gradual decrease
in coverage of light showers is expected through the evening, with
dry conditions forecast for a majority of the area tonight. There is
an axis of surface high pressure centered near the Great Lakes
Region that will continue to nudge southward into the region,
keeping winds light and variable, especially for low-lying areas.
Locations that clear out or partially clear tonight will have
efficient radiational cooling. Currently, the most widespread
clearing is expected across IL, where lows are forecast to reach the
upper 40s/lower 50s Monday morning. Across MO, cloud cover will be
slower to clear and may not clear for some locations until after
sunrise, resulting in lows that range from the mid to upper 50s for
most. On Monday, a surface high settles into the Ohio River Valley,
reinforcing drier low-level air, resulting in dry conditions with
highs forecast to be in the 70s across the area Monday
afternoon.

&&

.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
Issued at 242 PM CDT Sun Sep 27 2026

The mid-level ridge currently over the Great Plains shifts towards
the Mississippi Valley by Tuesday with the ridge axis eventually
becoming centered across the region. Meanwhile, the aforementioned
longwave trough out west will be propagating into the Desert
Southwest and interacting with the remnants of Hurricane Polo. As
this happens, the return of deep southerly flow encompasses the area
leading to warm/moist advection, mostly from the eastern Pacific.
The most noticeable change to our sensible weather on Tuesday will
be the return of warmer temperatures areawide, with highs expected
to be in the upper 70s/mid 80s, a 10 degree increase from Monday.
Moisture will be a bit slower to return, with the most anomalous
values staying across western MO and the Great Plains Tuesday into
Wednesday. Temperatures by Wednesday afternoon should be warmer for
most the area, as southwesterly low-level flow increases, leading to
highs in the 80s everywhere except northeast MO, where the best rain
chances (30-40%) are forecast. Tuesday and Wednesday will feature an
abundance of rainfall to the west across the Plains, with a steady
eastward progression of the widespread rainfall axis into our area
Wednesday night into Thursday.

Guidance continues to indicate an anomalous axis of moisture (PWATs
>95th percentile) being advected northward from the Pacific, with
general consensus in the greatest moisture arriving Thursday-Friday.
This is when most of the area should experience widespread,
potentially beneficial rainfall. The LREF mean accumulated 24hr
precipitation probabilities for QPF >0.50" continues to reveal 40-
50% values, illustrating a modest chance of substantial rainfall.
Rainfall amounts up to 3"+ are certainly possible for some by next
weekend, per the LREF accumulated precip 90th percentile. With the
greatest mid/upper level synoptic forcing for ascent and highest
column moisture both west of the area, it appears the best chance
for amounts like that should be across the northern/western half of
MO and west-central IL. Rain chances are expected to end for most by
the weekend as long-range guidance indicates a cold front and
trailing surface high sliding south into the region, pushing out the
anomalous moisture. This should also work to bring in cooler more
seasonable air, as LREF IQRs reveal spreads of 5 degrees centered on
values near normal by next weekend.

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1230 PM CDT Sun Sep 27 2026

Isolated to scattered light showers continue to slide southeastward
across portions of central/northeast MO and west-central IL.
Some of these light showers are expected to impact KUIN until
20z with brief MVFR visibilities possible. A majority of the
showers in central MO should stay to the west and south of
KCOU/KJEF, with a low chance (20%) that showers impact the
terminals. Similarly, the St. Louis metro terminals have a low
10-20% chance of sn isolated shower this afternoon. Dry and VFR
flight conditions are expected areawide by later this afternoon
with light and variable winds continuing into Monday.

&&

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

&&

$$

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