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Area forecast discussion 
National Weather Service St Louis MO
316 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 Illinois.
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. 850 mb
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 County Warning Area (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 18z tafs through 18z Thursday afternoon)
issued at 1239 PM CDT Wednesday Aug 20 2014

Tricky aviation forecast this afternoon...primarily along and east
of the Mississippi. Outflow boundary from this morning's
thunderstorms over Illinois is approaching the stl metropolitan area at
this time. Wind will shift to a northeasterly direction as the
boundary passes. This will cause a runway swap for the metropolitan
terminals. Expect outflow boundary to stall over eastern Missouri
at some point, though exactly where is uncertain. Next problem will
be the probability that thunderstorms will form along the boundary
this afternoon. At this time, expect there will be isolated storms
along the boundary, however am not confident enough there will be
great enough coverage to include in tafs at this time. Any storms
that do form should dissipate within an hour or two of sunset this
evening. There will be another chance for storms overnight
tonight, primarily over central Illinois as the low level jet
pushes more warm/moist air into the region. Expect VFR flight
conditions through the period outside of thunderstorms.

Specifics for kstl:

Outflow boundary from this morning's thunderstorms over Illinois
is approaching the terminal at this time. Expect this will force a
runway swap. Expect outflow boundary to stall over eastern
Missouri at some point, though exactly where is uncertain. Next
problem will be the probability that thunderstorms will form along
the boundary this afternoon. At this time, expect there will be
isolated storms along the boundary, however am not confident
enough there will be great enough coverage to mention
thunderstorms in the Lambert taf. Any storms that do form should
dissipate within an hour or two of sunset this evening. Expect VFR
flight conditions tonight with more thunderstorms developing well
northeast of the terminal in central Illinois.

Carney

&&

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.

Illinois...heat advisory until 7 PM CDT Sunday for Bond Illinois-Calhoun Illinois-
Clinton Illinois-Greene Illinois-Jersey Illinois-Macoupin Illinois-Marion Illinois-Monroe
Illinois-Randolph Illinois-Washington Illinois.

Excessive heat warning until 7 PM CDT Sunday for Madison IL-St.
Clair Illinois.

&&

$$

Weather forecast office lsx

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