Area Forecast Discussion
Issued by NWS St. Louis, MO

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000
FXUS63 KLSX 210421
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1121 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 06z TAFs through 06z Thursday Night)
Issued at 1043 PM CDT Wed Aug 20 2014

Complex of showers and thunderstorms which brought locally heavy
rain and strong wind gusts is currently moving into south-central
Illinois away from TAF sites. Rest of the tonight and tomorrow
period will likely be dry...though secondary warm front which
helped trigger organized convection earlier is still forecast to
eventually move back north and east tonight. Also cannot rule out
some diurnal airmass type thunderstorms tomorrow as capping does
not look too strong. Will keep TAFs though dry for now due to a
low confidence in either one of these scenarios actually
occurring. Other issue in the short term is fog potential as skies
are already clearing out behind the MCS in Illinois. Will have to
monitor this close through the night as combination of clear skies
and high dewpoints may cause fog development overnight
tonight...especially at KSUS and KCPS which received some heavy
rainfall earlier this evening. One factor that may preclude fog
development is winds look to stay up aoa 5 knots.


Specifics for KSTL:

Rest of the tonight and tomorrow period will likely be
dry at the terminal...though secondary warm front which helped trigger
organized convection earlier is still forecast to eventually move
back north and east tonight. Also cannot rule out some diurnal
airmass type thunderstorms tomorrow as capping does not look too
strong. Will keep TAF though dry for now due to a low confidence
in either one of these scenarios actually occurring. Other
potential in the short term is fog...but believe winds should stay
up enough so will not include in TAF for now but will monitor
overnight as it is a possibility due to clearing skies and very
high dewpoints/low level moisture.


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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