Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 220442

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1142 PM CDT Fri Jul 21 2017

Issued at 947 PM CDT Fri Jul 21 2017

Yet another warm evening with thunderstorms staying well north of
the area across northern Illinois. These storms are staying on
the northern side of the large upper high centered over the
Central Plains. CAMS have been consistent in showing additional
thunderstorms developing along an east-west stationary front over
Iowa-Illinois overnight, and that a few thunderstorms may move
into far northeast Missouri and west central Illinois toward
morning. Other than this chance, still expect dry weather
overnight with mostly clear skies. Temperatures are running about
the same as they were at this time this time last night. Have
only made minor adjustments to going lows.



.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 06z TAFs through 06z Saturday Night)
Issued at 1140 PM CDT Fri Jul 21 2017

Expect dry and VFR conditions at the terminals through midday on
Saturday. There is a slight chance that a thunderstorm could
affect KUIN around 12Z, but the chance is too low to include in
the TAF at this time. Then a cold front will begin moving south
across the area bringing isolated thunderstorms during the late
afternoon and evening hours. Coverage will be too isolated to
include in the TAFs at this time. MVFR/possible IFR ceilings and
visibilities are possible with any thunderstorms. A low level jet
will also cause LLWS at KUIN and KCOU between 07-13Z.


Mainly dry and VFR conditions are expected through the TAF
period. The only exception will be on Saturday evening when a
cold front will move through the area from north to the south
bringing isolated thunderstorms. Winds will be west to
southwesterly through Saturday afternoon before they veer
northwesterly behind the front on Saturday evening.



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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