Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 261718

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1218 PM CDT Tue Sep 26 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 406 AM CDT Tue Sep 26 2017

An upper level trough over the northern Plains will move eastward
into the northern portion of the Great Lakes region tonight, and
send a cold front currently extending from northwestern WI southwest
into central OK southeastward through our forecast area today and
this evening.  Scattered showers and a few thunderstorms can be
expected along and just behind this front today into this evening.
Going with mainly just slight chance pops across northeast and
central MO and west central IL with the best upper level forcing
remaining north of our forecast area and low level moisture limited
as the Gulf is cut off. The HRRR model keeps most of the
precipitation associated with the front north and west of our
forecast area this morning with widely scattered convection
developing this afternoon, enhanced by diurnal heating and
instability.  It appears that the coverage of the precipitation will
become more isolated later this evening with any remaining showers
of only light intensity.  Unseasonably warm temperatures are
expected again today, especially across east central and southeast
MO and southwest IL ahead of the cold front.  Cooler temperatures,
closer to seasonal normals can be expected across parts of northeast
MO today, and northeast and central MO and west central IL tonight.


.LONG TERM...  (Wednesday through Monday)
Issued at 406 AM CDT Tue Sep 26 2017

A few post-frontal showers are possible on Wednesday, mainly across
portions of central and southeastern Missouri. Otherwise, much
cooler temperatures will be the main story. Near to slightly below
normal temperatures, with unfortunately a continued dry trend, is
likely then for the rest of the work week and this upcoming weekend.
Expect high temperatures mainly in the 70s Thursday through Sunday,
with lows each morning in the 40s and 50s. Model guidance for
temperatures seemed reasonable for the most part, but did lower
minimum temperatures for Saturday and Sunday morning a couple of
degrees, focusing predominantly on the favored terrain of the
eastern Ozarks. A moderating trend will begin on Monday as 850-hPa
temperatures rebound into the mid teens. A weak Pacific cold front
will also approach portions of northeastern Missouri on Monday
bringing a chance of showers.



.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1218 PM CDT Tue Sep 26 2017

VFR conditions will prevail at the TAF sites for much of the valid
period. Cold FROPA expected at or by the beginning of the valid
period at KCOU, by mid-afternoon at KUIN, and by early evening for
STL metro sites, with surface winds becoming NW or N. The best
pcpn chances, such as they are, will be within a few hours of
FROPA, but look too small to include at STL metro TAFs at this
time. A large swath of MVFR CIGs exist currently to the N and W of
the TAF sites but should become low- end VFR by the time it moves
in to our area and will attempt to become MVFR category again
during the night. Current indications are that this will not occur
until it is mostly to the S and E of the STL metro sites, but did
retain at KCPS with this set. Look for clearing to occur from N to
S on Wednesday.

SPECIFICS FOR KSTL: VFR conditions expected thru the valid period,
although there will be a window where it could get close between
09-15z. Dry weather is also expected, but there is also a window
where a VC may be justified, between 23-03z. Will need to watch
development closely to see for the need. Otherwise, SW surface
winds to veer NW or N with cold FROPA around 23-24z.





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