Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 242017
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
317 PM CDT SAT SEP 24 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 316 PM CDT Sat Sep 24 2016

Getting a little more diurnal cumulus clouds today than previous
days with slightly more favorable low level moisture and
instability. Widely scattered showers/storms have also recently
developed this afternoon which will likely impact parts of northeast
and central MO.  This cumulus field along with the showers/storms
should dissipate early this evening due to the loss of daytime
heating/surface instability. Showers and storms should move back
into northeast MO late tonight ahead of an approaching upper level
trough and associated cold front. Unseasonably warm and muggy
conditions can be expected tonight with high surface dew points for
late September. Lows tonight will be about 15 degrees above normal
for late September. There may also be patchy fog late tonight across
parts of southeast MO and southwest IL where the least cloud cover
and lightest surface winds are forecast.

GKS

.LONG TERM...  (Sunday through Next Saturday)
Issued at 316 PM CDT Sat Sep 24 2016

The synoptic pattern at the start of the long term period will
feature a longwave trough across portions of the Central Plains,
with downstream ridging across the Midwest.  This trough will split
into two branches during the day on Sunday, with one part becoming a
cutoff low across the Desert Southwest, and another more progressive
branch digging into the Northeast by early next week.  The latter
will help push a strong cold front through the region on Sunday,
which will bring much cooler and drier weather to the region for
much of next week.

Main focus of the period will be the frontal passage Sunday evening.
Ahead of the front, temperatures will rise into the upper 80s to
near 90 once again across eastern MO and western IL.  However, cloud
cover will quickly begin to filter in from the west through the
daylight hours ahead of the upper-level trough and surface cold
front.  How quickly these clouds fill in will likely affect our
convective coverage Sunday afternoon/evening.  If we are sunny
through much of the morning, then convective coverage will likely be
a bit higher than the current 60% chances.  However if clouds move
in quicker and limit destabilization, then precipitation coverage
along/ahead of the front will likely remain more scattered in
nature. Given the cooler mid-level temps remaining well off to the
northwest with the core of the trough, mid-level lapse rates will be
weak. Additionally, the better kinematics will be displaced behind
the frontal surface, so severe weather is not anticipated.  Main
threat will be locally heavy rainfall.

The front will push through Sunday evening, bringing the coolest
temperatures thus far in this young meteorological fall season.
Highs Monday through Wednesday will be confined to the lower 70s
(some areas may not even get out of the 60s Monday afternoon).
Overnight lows will dip into the upper 40s and lower 50s!

A slight moderating trend is expected late next week as the Canadian
surface ridge shifts off to the east.  Look for highs to recover
back into the mid/upper 70s with overnight lows in the low/mid 50s.
Dry weather looks to persist through the upcoming workweek.

KD

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1109 AM CDT Sat Sep 24 2016

Just scattered diurnal cumulus clouds this afternoon along with
some patchy mid level clouds. Mainly southeast surface winds this
afternoon and tonight with surface ridge extending from MI
southwest into southeastern MO and a cold front approaching from
the Plains. Mid-high level cloudiness will be increasing tonight
ahead of an upper level distubance. Showers and storms may move
into UIN and COU Sunday morning just ahead of the approaching cold
front and into STL Sunday afternoon. For now will just include
VCTS in the tafs during these time periods. The surface wind will
veer around to a southwest direction Sunday morning.

SPECIFICS FOR KSTL: Just scattered diurnal cumulus clouds this
afternoon along with some patchy mid level clouds. Mainly
southeast surface winds this afternoon and tonight with surface
ridge extending from MI southwest into southeastern MO and a cold
front approaching from the Plains. Mid-high level cloudiness will
be increasing late tonight and Sunday morning ahead of an upper
level distubance. Showers and storms may move into STL Sunday
afternoon just ahead of the approaching cold front. For now will
just include VCTS in the STL taf Sunday afternoon. The surface
wind will veer around to a southwest direction Sunday.

GKS

&&

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

$$

WFO LSX



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