Area Forecast Discussion
Issued by NWS St. Louis, MO

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000
FXUS63 KLSX 202352
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
652 PM CDT Wed Aug 20 2014

.SHORT TERM:  (Through Late Tonight)
Issued at 312 PM CDT Wed Aug 20 2014

Isolated SH/TS are possible tonight across parts of extreme
northeastern MO through west central and southwest IL.
Precipitation should tend to lift northeastward with time.

Kanofsky

.LONG TERM:  (Thursday through Next Wednesday)
Issued at 312 PM CDT Wed Aug 20 2014

The primary forecast concern over the next several days is the
increasing heat and humidity. An upper level ridge will build
across the region and remain in place through the weekend. H85
temperatures are forecast to rise to 20-24 deg C through early
next week, which translates to highs in the mid to upper 90s.
South to southwest surface winds will continue to transport a
moist air mass into the region, keeping dew points in the mid 60s
to mid 70s.

The combination of hot temperatures and a humid air mass will
produce heat index values over 100 deg F for 4 days over most of
the LSX CWA, and some areas should reach values of 105-107 at
times. The going Heat Advisory and Excessive Heat Warning both
still look reasonable given concerns about dew points mixing out
near the Ozarks. There is some concern that the Heat Advisory may
need to be extended farther north because forecast heat index
values are just shy of the 4-day duration criteria across the
northern CWA (the difference is within the margin of error for
T/Td forecasting) and this has been a fairly mild summer overall.
After coordination with neighboring offices, we will hold off on
expanding the Heat Advisory northward and let the next shift take
another look.

This has been a fairly mild summer so far, and most people have not
yet become acclimated to extended periods of hot/humid weather. Heat
safety tips are available here: www.nws.noaa.gov/om/heat/index.shtml

The 12z model runs depict significant differences regarding how the
ridge breaks down next week in response to a strong trough moving
into the northwestern US. At this time, it looks like the next
opportunity for widespread organized rainfall would be associated
with that pattern transition during middle/late next week.

Kanofsky

&&

.AVIATION:  (For the 00z TAFs through 00z Thursday Evening)
Issued at 647 PM CDT Wed Aug 20 2014

Mainly VFR conditions are expected through the valid TAF period with
winds during the day tomorrow veering to the southwest. Exception
could be showers/storms through early this evening which are
currently entering east-central Missouri. Atmosphere will be quite
unstable out ahead of this activity but current radar imagery is
depicting outflow that is outracing the leading edge of convection
leading to weakening. Not sure if any of this activity will make
it to the metro TAF sites...but did add a VCTS group through 0200
UTC this evening to account for this possibility at KSUS and KSTL.


Specifics for KSTL:

Mainly VFR conditions are expected through the valid TAF period with
winds during the day tomorrow veering to the southwest and backing
to the south tomorrow evening. Exception could be showers/storms
through early this evening which are currently entering east-
central Missouri. Atmosphere will be quite unstable out ahead of
this activity but current radar imagery is depicting outflow that
is outracing the leading edge of convection leading to weakening.
Not sure if any of this activity will make it Lambert...but did add a
VCTS group through 0200 UTC this evening to account for this
possibility.


Gosselin

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...HEAT ADVISORY until 7 PM CDT Sunday FOR Audrain MO-Boone MO-
     Callaway MO-Cole MO-Franklin MO-Gasconade MO-Jefferson MO-
     Lincoln MO-Madison MO-Moniteau MO-Montgomery MO-Osage MO-
     St. Charles MO-St. Francois MO-Ste. Genevieve MO-Warren MO.

     EXCESSIVE HEAT WARNING until 7 PM CDT Sunday FOR St. Louis City
     MO-St. Louis MO.

IL...HEAT ADVISORY until 7 PM CDT Sunday FOR Bond IL-Calhoun IL-
     Clinton IL-Greene IL-Jersey IL-Macoupin IL-Marion IL-Monroe
     IL-Randolph IL-Washington IL.

     EXCESSIVE HEAT WARNING until 7 PM CDT Sunday FOR Madison IL-St.
     Clair IL.

&&

$$

WFO LSX






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