Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 112012
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
312 PM CDT Tue May 11 2021

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 300 PM CDT Tue May 11 2021

Plenty of cloud cover this afternoon across central and southeast MO
and southwest IL, ahead of a weak shortwave and in a region of
weak mid level frontogenesis. There may be a few light showers
across this area this afternoon into the evening, or at least a
few sprinkles. The precipitation associated with a weakening MCS
moving eastward through western AR and southwest MO should stay
south of our forecast area. Gradual clearing is expected tonight
into early Wednesday morning from north-northeast to south-
southwest as a strong 1030 mb surface ridge centered over MN and
WI builds southward into our area. Unseasonably cold low
temperatures are expected tonight with low level cold air
advection and lowering surface dew points. The 850 mb temperatures
tonight into Wednesday will be anomalously cold for May. Lows
tonight will be about 10 to 15 degrees below normal. Low lying
wind protected locations across parts of northeast MO and west
central and southwest IL may even get some patchy light frost late
tonight and early Wednesday morning. Highs on Wednesday will be
slightly warmer than Tuesday due to plenty of solar insolation
with little cloud cover expected, albeit still below normal.
Unseasonably cold low temperatures can be expected again Wednesday
night as the surface ridge axis moves into our forecast area with
good radiational cooling due to a mostly clear sky and light
surface wind. Patchy light frost may be possible late Wednesday
night.

GKS

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 300 PM CDT Tue May 11 2021

Dry conditions will continue Thursday and Thursday night along
with below normal temperatures with a persistent surface ridge
extending from the Great Lakes area southwest into MO, and
relatively low upper level heights with an upper level trough
centered over the Great Lakes region. A warming trend will begin
Friday and continue through the weekend as the surface ridge
shifts east of the region with surface winds becoming southeasterly.
Upper level heights will be gradually rising as well as an upper
level ridge over the Plains approaches. By Sunday temperatures
should be near normal for mid May. The chance of showers will
return on Friday, mainly across northeast and central MO and west
central IL due to increasing low-mid level warm air advection
ahead of weak northwest flow shortwaves. A warm front will also
lift northward into the forecast area Saturday and Saturday night
with scattered convection expected along and just north of this
front. Beyond the weekend the model solutions diverge with some
of the guidance keeping us in persistent moist southerly low
level flow, while some of the models and ensemble guidance try to
bring a cold front southeastward into our forecast area on Monday
as an upper level trough moves eastward through the Northern
Plains and Great Lakes region. This front will be the focus for
convective initiation on Monday. For now will follow this later
solution of the NBM and GFS, rather than the operational ECMWF model.

GKS

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1144 AM CDT Tue May 11 2021

A strong surface high pressure ridge centered over MN and WI will
build southward into MO and IL. Northeasterly winds this
afternoon and tonight will become east-northeasterly Wednesday
morning. Cloud ceilings early this afternoon in COU and the St
Louis metro area will be around 3000-5000 feet, then slowly rise
through the rest of the afternoon into the early evening hours.
Could not rule out a few sprinkles this afternoon in COU and the
St Louis metro area. The clouds will clear out from the north-
northeast to the south-southwest tonight into early Wednesday
morning.

SPECIFICS FOR KSTL: A strong surface high pressure ridge centered
over MN and WI will build southward into MO and IL. Northeasterly
winds this afternoon and tonight will become east-northeasterly
Wednesday morning. Cloud ceilings early this afternoon will be
around 3000-5000 feet, then slowly rise through the rest of the
afternoon into the early evening hours. Could not rule out a few
sprinkles this afternoon. The clouds will clear out from the
north-northeast to the south-southwest late tonight into early
Wednesday morning.

GKS

&&

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

$$

WFO LSX


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