Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 210442

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1142 PM CDT Wed Sep 20 2017

Issued at 921 PM CDT Wed Sep 20 2017

Cold frontal boundary has become quasi-stationary just to the
northwest of the CWA as of 0200 UTC. Am not expecting much if any
additional push to the southeast overnight tonight and short-term
model guidance continues to suggest the front will largely wash
out late tonight into early Thursday morning. Main change to the
forecast for tonight was to focus PoPs more toward the front
tonight, trimming some of the schc and chc PoPs from the
southeast. Otherwise, previous forecast largely looks to be in
good shape with a mostly clear night outside of the convective
blowoff across portions of northeast Missouri predominantly. Look
for a very mild night for mid/late September with low temperatures
only getting down into the upper 60s to mid 70s. Will continue to
feel more like the middle of summer than the beginning of autumn,
at least for the next few days.



.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 312 PM CDT Wed Sep 20 2017

The main forecast issue tonight is the threat of showers and
thunderstorms. A northeast-southwest oriented cold front is creeping
eastward across IA and northwest MO this afternoon. Ahead of the
front is a band of extensive cu. Spotty tstorms have begun to
develop across northeast IA into western WI, but further southwest
along the front there haven`t been any signs of vertical development
in the cu field. The latest mesoanalysis data shows that the cap is
weakening into southwest IA and extreme northwest MO, so I would
expect additional development in this area during the later part of
the afternoon. Further east into the LSX CWA there is a lack of cu
and the air mass remains capped. The front isn`t expected to make
much more progress either before stalling by early evening. Given
these aforementioned factors, the present thinking is that storms
will have to wait until after dark to move into the CWA or they
will develop across northeast MO this evening in response to lift
via the southwesterly LLJ. The southwesterly LLJ and stalling
boundary would suggest a better threat of more persistent storms
and potential southwestward backbuilding across northwest MO. Some
of the guidance suggests that storms that do manage to impact
northeast MO and west central IL this evening will eventually die
out later tonight with more preferential forcing for storms
further west. Overnight temps look rather mild for late September.

I have kept some low pops in northeast/north central MO thru mid
morning on Thursday to account for any residual shower/storm activity.
The main story however will be well above average temps and hot
temperatures. While the cold front/boundary will never make it
into the CWA, it will lift northward and a broad warm sector
marked by moist and very warm temps will exist. This along with
the strengthening ridge aloft will support a continuation of high
temps in the low- mid 90s with heat indices averaging near 100
degrees on Thursday afternoon.


.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 312 PM CDT Wed Sep 20 2017

The unseasonably hot and humid conditions will continue into the
upcoming weekend as the upper level ridge holds its position across
Missouri and Illinois. Afternoon high temperatures in the lower to
middle 90s will combine with dewpoints in the 60s/70s to produce
afternoon and early evening heat index values around 100 degrees.
May need to hoist a Heat Advisory for the St. Louis Metro for
longevity (4 days of heat index values above 100 degrees) if
Saturday forecast remains on track. Will let midnight shift get
another run of model guidance in before hoisting an advisory.

Ridge finally shows some signs of breaking down/shifting east by
early next week as a deep trough of low pressure approaches the
region from the west. Have a few periods of slight chance to chance
PoPs but threat of a widespread meaningful rain event look slim at
this point. Temperatures should cool some Monday and Tuesday with
highs in the middle to upper 80s, with the real cold front bringing
temperatures back to seasonal normals on Wednesday of next week.



.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1140 PM CDT Wed Sep 20 2017

Dry/VFR conditions expected through the valid TAF period.
Convection is expected to stay northwest of the terminals along a
stalled frontal boundary. Clear to mostly clear sky is forecast
along with south/southeast winds around 10 knots.


Dry/VFR through Thursday night. Clear or mostly clear sky
expected along with a southerly wind around 10 knots.



Issued at 146 PM CDT Tue Sep 19 2017

Record High Temperatures Through Saturday

              KSTL  KCOU  KUIN
Sept 20     96 (1940)   98 (1893)   95 (1940)
Sept 21     97 (1881)  100 (1893)   95 (1940)
Sept 22     95 (1956)   97 (1937)   96 (1937)
Sept 23     94 (1891)   94 (2007)   95 (1937)




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