Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 212311

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
611 PM CDT Fri Jul 21 2017

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 320 PM CDT Fri Jul 21 2017

Hot and humid conditions will remain in place tonight and Saturday.
Temperatures on Saturday will likely be similar to and possibly a
few degrees warmer than Friday for most locations. The exception
might be across northeast MO and west central IL. There is greater
uncertainty regarding temperatures over that part of the CWA because
remnant cloud cover from anticipated nocturnal thunderstorm
complexes over IA/IL may reduce insolation for part of the day.

A weak boundary over IA is expected to sink southward into MO/IL on
Saturday, but the baroclinic zone itself will likely remain over IA
until later this weekend (see discussion below). Scattered SHRA/TSRA
are expected to develop invof the boundary on Saturday afternoon and
evening. The air mass will be very unstable, and a few strong to
severe thunderstorms will be possible.


.LONG TERM...  (Saturday Night through Next Friday)
Issued at 320 PM CDT Fri Jul 21 2017

Extreme heat is to continue into Sunday for metro area and south.

The upper level ridge that has been positioned over the CWA will
finally begin to give way as first a pressure trough Sat night into
Sunday and then a cold front Sunday night pass through the area. The
passage of the trough brings chance POPs Sat night but should be
dried out by 12Z Sun. This will allow for a decent amount of diurnal
heating which will bring temps back into the mid/upper 90s with heat
index values of 105+ for the metro and south. Therefore we have
upgraded to an excessive heat warning because the area has had 105+
heat index for the past four days. The cold front that makes its way
into the CWA Sun night will bring a NW flow and cooler temps for Mon
and Tue. Temps for Mon and Tue should be near seasonal normals.

A new ridge looks to build in by Tue night. Temps will once again
work their way into the mid 90s as surface flow will become
southerly. Upped the Pops for Tue as cutoff low from the ECMWF looks
to be a little less of an outlier. There is better support from the
GFS than previously but location still is uncertain.

Ridge begins to erode as trough drops down from the Great Lakes
and brings the eastern half of the CWA chance Pops on Friday.



.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 600 PM CDT Fri Jul 21 2017

Mainly dry and VFR conditions are expected through midday on
Saturday. Then a cold front will begin moving south across the
area bringing isolated thunderstorms along it. MVFR/possible IFR
ceilings and visibilities are possible with any thunderstorms. A
low level jet will also cause LLWS at KUIN and KCOU late tonight.

SPECIFICS FOR KSTL: Mainly dry and VFR conditions are expected
through the TAF period. The only exception will be late Saturday
afternoon and evening when a cold front will move through the area
from north to the south bringing isolated thunderstorms.



MO...Excessive Heat Warning until 8 PM CDT Sunday for Crawford MO-
     Franklin MO-Gasconade MO-Iron MO-Jefferson MO-Madison MO-
     Reynolds MO-Saint Charles MO-Saint Francois MO-Saint Louis
     City MO-Saint Louis MO-Sainte Genevieve MO-Washington MO.

     Excessive Heat Warning until 8 PM CDT Saturday for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Knox MO-Lewis MO-Lincoln MO-
     Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike
     MO-Ralls MO-Shelby MO-Warren MO.

IL...Excessive Heat Warning until 8 PM CDT Sunday for Clinton IL-
     Madison IL-Marion IL-Monroe IL-Randolph IL-Saint Clair IL-
     Washington IL.

     Excessive Heat Warning until 8 PM CDT Saturday for Adams IL-Bond
     IL-Brown IL-Calhoun IL-Fayette IL-Greene IL-Jersey IL-
     Macoupin IL-Montgomery IL-Pike IL.



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