171
FXUS63 KLSX 131855
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
155 PM CDT Sun Jul 13 2025

.KEY MESSAGES...

- The threat for locally heavy rainfall and flooding continues
  into early this evening. A few strong storms could produce
  isolated damaging winds as well.

- Typical Summer weather continues this week. Temperatures will
  remain within a few degrees of seasonal normals with an almost
  daily chance for afternoon and early evening thunderstorms.

&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 152 PM CDT Sun Jul 13 2025

A long wave mid/upper level trough extends from the Upper Midwest
southwest into the southern Plains this afternoon .  This trough,
along with a quasi-stationary front extending from near Moberly
eastward into central Illinois is producing a broad area of showers
and and thunderstorms this afternoon.  The threat for locally heavy
rain continues for the rest of the afternoon into the early evening
as P-wat values are in excess of 2 inches and the movement of some
of these storms is relatively slow.  There`s also an isolated severe
threat with the storms for the rest of the afternoon into early
evening.  RAP is forecasting MLCAPE between 1500-2000 J/Kg across
southwest and south central Illinois through sunset with 0-6km shear
between 20-30kts.  This is enough for marginally severe multi-cell
clusters with an isolated damaging wind threat.  Soundings are not
particularly supportive of large hail showing deep moisture and only
200-500 J/Kg of hail CAPE, However, if a storm can start rotating,
the enhancement to the updraft could produce some isolated 1 inch
hail as well. Thunderstorms should diminish with loss of daytime
heating this evening.

Attention turns to fog potential tonight.  There should be plenty of
ground and low level moisture available after this afternoon and
evening`s rain.  Much of the high cloud cover is expected to clear
out tonight, and light winds should promote fog development.  There
may be more low stratus than fog if the sky doesn`t clear as much as
expected, and this is more likely across parts of south central and
southwest Illinois.  Elsewhere, areas of fog are likely, with patchy
dense fog possible.  Not sure how widespread the truly dense fog
(visibility 1/4 mile or less) will be, but a Dense Fog Advisory may
be needed before sunrise on Monday morning for parts of the area.
Monday`s weather looks relatively quiet compared to today`s with a
lingering chance for afternoon thunderstorms, mainly across the
eastern Ozarks and far southwest Illinois.  Afternoon MLCAPE values
once rise once again into the 1500-2000 J/Kg range, but here is far
less deep-layer shear at only around 5kts, so severe storms are not
expected at this time.

Carney

&&

.LONG TERM...  (Monday Night through Next Sunday)
Issued at 152 PM CDT Sun Jul 13 2025

The flow aloft will become quasi-zonal over the Mid Mississippi
Valley for much of the work- week. Global deterministic guidance
shows a few vorticity maxima moving through the region in the
zonal flow during this period. The most prominent of these is the
remnants of the trough which is now extending southwest from the
Midwest into the southeast Plains. This vortmax is forecast to
lift northeast from Oklahoma/western Arkansas through Missouri and
Illinois on Tuesday. The resulting mid-level dynamical lift will
probably give us the best chance for widespread rain across the
forecast area for Tuesday through next weekend. Mean to 75th
percentile P-wat remain relatively high in the 1.8-2.0 inch range,
so locally heavy rainfall will continue to be a possibility as
this vortmax moves northeast across the area. Mean 500mb heights
bottom out Tuesday night and then begin rising Wednesday as the
upper ridge over the Southeast U.S. spreads westward into the
Lower and Mid Mississippi Valley. As mentioned above, there are
subtle indications of weak short waves moving across the Mid
Mississippi Valley during the week in the deterministic GFS and
ECMWF, though these are little more than brief shifts in the flow
from slightly west-southwest to slightly west- northwest. An
actual synoptic scale short wave trough moves across the Upper
Midwest/southern Canada late in the week which pushes a weak cold
front into southern Iowa or northern Missouri by Thursday
night/Friday according to the deterministic guidance. If the front
actually makes it that far south it will be a focusing mechanism
for convection on Friday, most likely across northern Missouri
into west central Illinois. It`s questionable though how far south
the front will actually move due to the ridge building from the
Southeast U.S. The deterministic GFS and ECMWF hint at another
short wave moving across the Mid Mississippi Valley on Saturday or
Sunday, but there are timing and strength differences between the
two. The wave barely registers on global ensembles which only
show a general shift from weak zonal flow to weak west-northwest
flow so the precipitation forecast for the weekend is fairly low
confidence. Temperatures through the period should continue to be
within a few degrees of normal, with highs largely controlled by
afternoon convection.


Carney

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1242 PM CDT Sun Jul 13 2025

Scattered showers and thunderstorms will intermittently reduce
the visibility to 2SM or less in moderate to heavy rain this
afternoon. VFR visibilities will be associated with a broader
area of stratiform rain stretching across central and northeast
Missouri into central Illinois...however there may be occasional
ceilings down to 2000ft or lower. Rain and thunderstorms should
end from northwest to southeast from late afternoon into the
middle of the evening. A period of VFR will likely follow the end
of the rain, but light winds and a clearing sky tonight will
promote fog development. Widespread IFR visibilities likely, and
areas of visibilities below 1SM are possible. Fog will lift and
dissipate after 14Z Monday.

Carney

&&

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

$$

WFO LSX