Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 241742

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1242 PM CDT Sun Sep 24 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 335 AM CDT Sun Sep 24 2017

The persistent upper level ridge has shifted northeastward early
this morning and was now centered over the OH Valley region.  Our
forecast area continues to be on the western periphery of a large
surface ridge centered over the OH Valley region. Weather conditions
today and tonight will be close to persistence with only subtle
differences.  With the upper level ridge further northeast of the
forecast area, we will likely have a little more diurnal cumulus
clouds this afternoon with a better possibility of isolated
afternoon convection. The most likely area for afternoon
convection should be across southeast MO, eastern Ozarks where
there will be a little more favorable low-mid level moisture and
instability. For now will just include mainly slight chance pops
for showers and thunderstorms across central and southeast MO this
afternoon. Highs today will be well above seasonal normals and
similar to yesterday, maybe a degree or two cooler. Will be close
to record highs for STL and possibly also UIN as well.


.LONG TERM...  (Monday through Saturday)
Issued at 335 AM CDT Sun Sep 24 2017

The synoptic pattern at the start of the long term period will
continue to feature a very amplified regime, with a deep trough
across the western CONUS and a downstream ridge located over the
central/eastern United States. The trough will attempt to shift to
the east through the period, but the strength of the ridge will help
to deamplify it and deflect it more into the north-central CONUS.
The end result will be a fairly benign long term period.

The approach of the trough will help to push a surface cold front
into the region Tuesday and especially into Wednesday.  Just enough
moisture return ahead of this boundary may be enough to kick off a
shower/storm Monday into Tuesday mainly over central MO.  However,
think most areas remain dry with continued hot temperatures as highs
remain in the upper 80s/low 90s.

The front should pass through the area by Wednesday afternoon.
Several factors will be working against any precip along the front
which include limited convergence, a lack of upper-level forcing for
ascent (the front will be running into the upper-level ridge), and
fairly meager deep-layer moisture with southeastward extent.
Therefore, have kept the Wednesday forecast dry.

The passage of the front will usher in much cooler temperatures by
the end of the week. Highs Thursday through Saturday will only climb
into the 70s. Overnight lows will become rather chilly by Friday
night as a secondary front ushers in readings in the 40s/low 50s!



.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1229 PM CDT Sun Sep 24 2017

High pressure over the region will persist through Monday
afternoon. Scattered clouds with VFR bases over the area will
give way to clear skies tonight. Scattered showers are possible
mainly south of the terminals over the Ozarks. A brief period of
fog is possible at KSUS Monday morning, but confidence is too low
to put into TAF at this time.

High pressure over the region will persist through Monday
afternoon. Scattered clouds with VFR bases over the area this
afternoon and Monday will give way to clear skies tonight.



Record Max Temperatures Through Tuesday

              KSTL  KCOU  KUIN
Sept 24     94 (1891)   95 (1891)   94 (1935)
Sept 25     92 (1900)   94 (1891)   93 (1920)
Sept 26     94 (1897)   96 (1897)   91 (1956)




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