809
FXUS63 KLSX 130834
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
334 AM CDT Sun Jul 13 2025

.KEY MESSAGES...

- Temperatures will be cooler with scattered to numerous thunderstorms
  this afternoon and evening, mainly along and south of I-70.
  Locally heavy rainfall may result in isolated flooding.

- Thunderstorm chances will ebb and flow through the course of the
  upcoming week, peaking each afternoon and evening.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 332 AM CDT Sun Jul 13 2025

Regional analysis shows a surface trough situated over southern IL
into southeastern MO. The trough precedes a cold front (moreso
wavering stationary boundary) extending from the Great Lakes through
northern IL and northern sections of MO. Water vapor imagery shows
moisture streaming northeast into the mid-Mississippi Valley ahead
of an upper level trough, the base of which extends southward
through Texas.

These features will be the key players in today`s forecast as an
upper level shortwave rotates over eastern OK and ejects lobes of
vorticity northeastward into MO/IL. The trough remains pronounced at
least down through 700mb, depicting the deep mid/upper ascent that
will progress east-northeast overhead late this morning through this
evening. Mid-level southwesterlies will continue to draw moisture
northward out of the western Gulf as anti-cyclonic flow rotates
around a mid-level ridge centered over the northern Gulf.

A weak surface inflection/low is progged to track east-northeast
through southern sections of MO and into IL from late this morning
through this evening. RAP guidance shows two relative maxima of
MLCAPE developing ahead of these features this afternoon. One is in
a narrow corridor along the boundary over northern MO into west-
central IL with values peaking at 1000-1500 J/kg. The second is a
broader pool of buoyancy running from southeastern/east-central
MO through southern IL with values reaching 1500-2000 J/kg. Mass
convergence fields stand out along the I-44 corridor in MO
eastward along and south of I-70 in IL, but a lesser pronounced
area of convergence runs along the boundary to the north. As the
upper wave and weak surface low progress east, modest instability
will give rise to showers and thunderstorms in a broad, moisture-
rich airmass, where dewpoints remain in the low to mid-70s with
PWATs reaching an impressive 1.9-2.25".

Fortunately, cooler temperatures are expected today with extensive
cloud cover west of the Mississippi River. Mid-level lapse rates
remain <6C through the afternoon with soundings showing saturation
through much of the modeled environment. Though a localized gust of
wind isn`t out of the question, it looks to be the exception rather
than the rule. With deep layer shear around 25 knots, scattered to
numerous thunderstorms are expected to develop as ascent increases
overhead. With anomalously high PWATs, thunderstorms will be very
efficient and producing locally heavy rainfall. HREF ensembles
show 60% of the members suggesting 1" or more of rainfall running
from southeast MO through southwest IL and 40% of the members
favoring 2" or more. LPMM data suggests amounts may reach 4-5" on
a localized scale, mainly along and east of the Mississippi River
into southwestern IL. Considering the performance of last
evening`s thunderstorms, this doesn`t sound terribly off. The
difference today will be thunderstorm movement, as individual
cells track eastward at 30-40 mph, leaving less residence time
over a specific location. Areas that stand a chance to reach these
amounts will be those affected by multiple rounds in a few hours
time. Localized flooding is possible, but given the localized
nature, a Flash Flood Watch was not warranted.

Thunderstorms are expected to weaken this evening as the upper
trough departs to the east and instability wanes. Drier air is
pulled southward in the wake of the departing trough with dewpoints
dropping into the 60s along and north of I-70. Upper vorticity over
eastern OK kicks east along the MO/AR border Monday with much of the
deeper moisture south of I-70. Scattered thunderstorms will largely
be confined to this region with dry conditions favored to the north.


Maples

&&

.LONG TERM...  (Monday Night through Saturday)
Issued at 332 AM CDT Sun Jul 13 2025

Guidance is in fairly good agreement through the early portion of
the long term period, but quickly diverges from midweek on.

A vertically stacked system slingshots out of central Texas as
ridging over the southeastern U.S. begins to show signs of building
westward through midweek. The stalled surface boundary/trough to the
south lifts back to the north as a warm front late Monday night into
Tuesday. Though guidance differs in the strength of this system, the
general track/timing has come into better alignment, taking it
overhead Tuesday afternoon/evening. This draws warm, moist air
northward with scattered showers and thunderstorms Tuesday afternoon
and evening during diurnal peak.

From midweek onward, specifics are less certain as key, small scale
features become more difficult to resolve. A big part of this is how
global guidance handles the strength and westward extent of the
southeastern ridge, while zonal flow is maintained aloft over the
northern U.S. As multiple waves continue to track west to east
through the zonal pattern to the north, the ridge to the southeast
ejects weaker systems northward around its western periphery.
Guidance diverges in time, especially later in the week as a cold
front attempts to drop south behind a shortwave to the north. The
strength/placement of the ridge will determine whether the front
even makes it into the area or stalls out somewhere over the
region. The large spread between NBM interquartiles persists with
temperature spread of 7-10 degrees from Thursday through the end
of the period. What looks somewhat more certain is that the
diurnal ebb and flow pattern for thunderstorms will continue.


Maples

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1044 PM CDT Sat Jul 12 2025

VFR conditions are expected at the start of the period, but some
patchy fog will be possible in a few locations due to lingering
humidity and recent rain. However, the persistence of high level
cloud cover may limit coverage, and confidence is low that
significant visibility reductions will occur at local terminals.
Showers and a few thunderstorms will be possible early this
morning at COU/JEF, and at St. Louis area terminals by mid
morning. Shower and thunderstorm chances will continue through the
mid to late afternoon, and will diminish during the evening.
Bursts of heavy rain, lightning, and occasional erratic gusty
winds are all possible if a storm moves directly over the
terminal, along with brief ceiling/visibility category
reductions.

Showers will diminish in the evening, but low ceilings and fog
will be possible overnight. Most of this is likely to occur after
06Z, and confidence is low regarding the extent of impacts at this
time.

BRC

&&

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

$$

WFO LSX