Area Forecast Discussion

845
FXUS63 KLSX 190911
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
311 AM CST Wed Nov 19 2025

.KEY MESSAGES...

- Widespread rainfall is expected to arrive Thursday morning, extending
  through early Saturday. The pattern remains active with another
  round of rain possible early next week.

- Temperatures will be near to above normal through the next
  several days.


&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 309 AM CST Wed Nov 19 2025

Surface high pressure is centered over WI early this morning with
cyclonic flow resulting in dry northerlies that maintain surface
dewpoints in the 40s over much of the area. IR satellite shows low
clouds/fog slow building west of the MS River early this morning.
Surface temperatures were in the mid-40s to around 50 degrees behind
a cold front that has now slipped into the Tennessee Valley as of
03z this morning.

(09z-15z This Morning)

The initial issue early this morning will be fog and low clouds.
High clouds run along a slowly advancing wing of warm air advection
that extends southward over western MO. A narrow area of clearing is
sandwiched between the upper cloud cover to the west and low
clouds/fog that continue to build westward from IL into MO. Latest
microphysics products show much of the CWA blanketed by low clouds
and fog, as upper level westerlies draw relatively mild, moist air
over a shallow, dense cold layer to the east. Watching nighttime
products in motion show the low clouds/fog slowing building
west/southwest, as opposed to being advected with light/calm
surface flow. Fog is expected to erode by early to mid-morning.

(15z This Morning-00z Friday)

High clouds will continue to stream in on westerlies aloft, while
shallow saturation will support low clouds through much of the day
today. After passage of a cold front yesterday, clouds will help
keep temperatures in the 50s over most of the area. Low to mid-60s
will be the exception over far southern sections of the CWA as the
surface high pulls east and return flow precedes the approaching
system.

Ridging will very slowly shift eastward through the day Thursday,
as the lead upper low rounds the base of a longwave trough and
takes on a negative tilt over eastern NM and western TX.
Southwesterly flow at the western side of the mid-level ridge
advects moisture northward from the western Gulf into the Plains
and eventually the Mid-Mississippi Valley. While long range
guidance has come into better agreement over the last couple of
days, there are a couple of signals in the medium range (RAP/NAM)
guidance that shed light on progressively later start (Thursday AM
vs. Wednesday PM). First, dewpoints in the upper 30s to mid-40s
will be in place at the surface as dewpoints in the 60 over
eastern TX attempt to make a northward surge. Second, a stalled
boundary is expected to extend east to west somewhere between
northern AR and southern MO. And third, guidance has shown an
elongated pocket of dry air between 925- 850MB, which get ingested
into the southwesterly flow and could impact start times Thursday
morning. The back end of the HRRR is beginning to capture this
with widespread rain developing as moisture overruns the surface
boundary. The 00z HREF 1-hour timing tool (for measurable precip.)
shows rainfall making it into the southern end of the CWA after
midnight Wednesday night and northward near I-70 around 17z. That
is about as far as the initial push makes it with rainfall become
isolated/scattered as you progress north from there. Widespread
rainfall is most favored south of I-70 Thursday, creating a sharp
N/S gradient in rainfall amounts.


Maples

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 309 AM CST Wed Nov 19 2025

Mid and upper level ridging will encompass the Gulf and extend
northward over the Lower Mississippi Valley early Thursday. A pair
of upper level lows will rotate over the southwestern U.S. into the
central U.S. from mid to late-week. The lead system over AZ/NM, near
the Mexico/U.S. border, will be he first to bring widespread,
beneficial rainfall early Thursday through early Saturday. The
second system straddles the Pacific Coastline until the end of the
week. It then moves eastward to bring additional rain chances to the
region early next week.

(00z Friday-12z Saturday)

The upper low begins to shift northeast into the Plains late
Thursday into Friday. This lifts the surface boundary back to the
north as a warm front, spreading precipitation further north than
Thursday. This is when rain likely becomes most widespread with
moisture, forcing mechanism (front), and ascent with vorticity
nearly continuously hurled northeast from the upper low and around
the northern side of the Gulf ridge. While Thursday doesn`t have
much potential for thunderstorms being north of the boundary, the
northward move in the warm front may bring some instability with it
as Mucape values reach 300-500 J/kg under a cold. Even this amount of
instability under a cold upper air mass (-17C at 500MB) could
provide the temperature differential for a few thunderstorms (TBD).
PWATs of 1-1.5" surge northward Thursday night into Friday, as well.
Should convective trends show any promise, showers and a few
thunderstorms will be efficient rain-makers.

While there is some variability between guidance with respect to the
surface boundary position, LPMM suggests a strips of higher totals
will fall along this boundary with as much as 1.75" of QPF by 00z
Friday (6 p.m. Thursday). As stated above, rainfall become more
widespread late Thursday into Friday. The northward push in this
part of the system gets everyone involved with the rain. 50% of the
LREF members support 1+ inch of rainfall along and south of I-70
with probabilities for 2+ inches only at 15-20% for same areas. This
information, along with LPMM, and LREF members, provide higher
confidence that this will not be a flash flood case, but more so a
long, light to occasionally steady precipitation.

(12z Saturday-Tuesday)

They system pulls east into the Ohio Valley Saturday morning.
Surface high pressure pushes very dry air in from the north, brings
rain to an end from west to east. Verdantly, the weekend is looking
dry at this point as surface high pressure drops in behind a cold
front. A similar scenario may play out next week as the next system
kicks out of the southwest U.S. and into the Plains. It looks like
another soaking rain could affect the region. Ensemble spread isn`t
terribly large (7-10 degrees) toward later part of the forecast with
an evident downward trend in temperatures.

This will be the time to keep an eye on the pattern to 1) start
thinking about holiday travel 2) watch for a cold outbreak resulting
from a potential Sudden Stratospheric Warming (SSW) event beginning
just beyond the end of the forecast period. This rare event may just
be materializing as SLU`s CIPS Hazard/Threat guidance is showing a
much colder trend for late November. For now, though, temperatures
remain above normal through much of the next week.


Maples

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1029 PM CST Tue Nov 18 2025

Extensive area of low clouds with ceilings between 700-0200 feet
is expected to move southwest into the the terminals between
06-08Z and remain over the area into Wednesday afternoon. Do
expect the ceilings to gradually climb back to VFR between 19-21Z
on Wednesday afternoon. There will be some potential for MVFR
visibilities between 09-15Z. Otherwise, winds are expected to be
light through the period.

Britt

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion