Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 231805

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
105 PM CDT Sun Jul 23 2017

Issued at 1237 PM CDT Sun Jul 23 2017

It appears that temperatures will be slightly cooler today across
parts of southwestern Illinois, which will keep the heat index
values lower than previously anticipated. Therefore, the Excessive
Heat Warning has been cancelled for portions of southwestern



.SHORT TERM...  (Through Late Tonight)
Issued at 407 AM CDT Sun Jul 23 2017

MCS continues to track eastward across forecast area early this
morning. Main convection exiting MO counties and into southwestern
IL counties. Will see lingering showers and storms today, along and
south of frontal boundary. So kept slight chance pops along and
south of Interstate 70. Another issue later today, will be a
secondary frontal boundary sliding southeast across region this
afternoon through tonight with isolated showers and storms possible.
This boundary will be the main front that will bring cooler, drier
air, so will still be dealing with excessive heat today for areas
along and south of Interstate 70, so no changes to ongoing heat
headlines. By tonight, dewpts to finally dip down into the 60s with
low temps in the mid 60s to low 70s.


.LONG TERM...  (Monday through Saturday)
Issued at 407 AM CDT Sun Jul 23 2017

The second cold front is expected to already be exiting our region
to the south on Monday morning as an area of high pressure edges us
to the northeast for both Monday and Tuesday.  This should result in
Monday being close to seasonal averages with Tuesday just slightly
above average:  an improvement nonetheless on what we had just
experienced with extreme heat the past week.

The upper RIDGE still looks on schedule to return to our region late
Tuesday and continue into Wednesday, with the extreme heat
holding off however until the surface setup becomes more favorable.
This is expected to happen on Wednesday with passage of a warm
front thru our region Tuesday night and southerly flow taking
hold.  Preferred max temps along the lines of EC MOS, which yields
values near 100 for STL metro and mid 90s for most other areas.
Combined with the humidity, heat index values should peak in the 105
to 110 range for many areas.

Fortunately, it appears this latest heat event will be short-lived,
with mid level flow becoming NW already by Thursday in response to a
retrogression of the upper RIDGE westward, and if anything, becomes
more amplified with time giving us a more northerly flow aloft
heading into the upcoming weekend.  This will have the effect of
swinging the door wide open for any cold fronts that want to drop
down and one indeed will waste no time dropping thru our region on
Thursday.  This looks to be our best, most widespread, pcpn event
for this period and have high chance and likely PoPs to cover and
these will go even higher as confidence on timing increases.

Depending on the precise timing of the cold front Thursday and its
associated pcpn, may see in a few areas one more day of high heat
and humidity.

Heading into next weekend, it looks to be a nice setup for some
below average temps and dry wx with nearly northerly flow aloft and
northeast surface winds.  A rare treat if it is realized.



.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1237 PM CDT Sun Jul 23 2017

VFR conditions are expected for the first 0-6 hours and likely for
the majority of the TAF period. Widely scattered showers and
thunderstorms are expected to develop near a cold front as it
moves through Missouri and Illinois, but it is not clear whether
these will affect the terminals. Initially light and variable or
southwesterly winds will become northerly behind the front.



MO...Excessive Heat Warning until 8 PM CDT this evening for Boone MO-
     Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Lincoln MO-Madison MO-Moniteau MO-
     Montgomery MO-Osage MO-Reynolds MO-Saint Charles MO-Saint
     Francois MO-Saint Louis City MO-Saint Louis MO-Sainte
     Genevieve MO-Warren MO-Washington MO.

IL...Excessive Heat Warning until 8 PM CDT this evening for Madison
     IL-Monroe IL-Saint Clair IL.



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