Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 211744
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1244 PM CDT Thu Sep 21 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 351 AM CDT Thu Sep 21 2017

Main concern today deals with another very warm and potentially
another record breaker at St. Louis and Quincy.

The stationary front early this morning was staying just north
and west of the area stretching from northwest Missouri into east
central Iowa. Current temperatures are closer to normal highs for
this time of year, and they won`t fall too much more before
sunrise. Like yesterday, we will have a very warm start and near
full sunshine with mixing up to 850mb where values will be around
20C, so highs should be close to the same as yesterday afternoon.
Dewpoints are already around 70 degrees, and they should mix like
they did yesterday, so would expect heat index readings to climb
to around 100 degrees, particularly around the St. Louis metro
area. Current forecast is for a tie on the record high in St.
Louis and just shy of the record high at Quincy. Columbia`s
record high today is 100 which will be difficult to reach.

Went dry and mostly clear through tonight as the upper high is
building over Missouri and Illinois. Some of the CAMS are showing
diurnal thunderstorms developing over the Ozarks this afternoon,
but forecast soundings are showing capping under the ridge over
the eastern Ozarks, so kept with the dry forecast.

Britt

.LONG TERM...  (Friday through Wednesday)
Issued at 351 AM CDT Thu Sep 21 2017

Model guidance remains in good agreement that the next several
days will be unseasonably warm (humid at times) and with little
opportunity for widespread precipitation.

The unseasonable warmth is being caused by a strong mid/upper
ridge which will continue building across the eastern half of the
CONUS over the next few days. As long as the ridge remains in
place, temperatures across MO/IL will be unseasonably warm and
there will be little chance of widespread precipitation, although
a few diurnal SHRA/TSRA cannot be entirely ruled out, especially
over the higher terrain of the eastern Ozarks. The ridge is
expected to remain in place through the weekend and then begin
shifting eastward early next week. By the middle of next week, the
ridge will have shifted sufficiently far enough eastward that a
cold front over the plains will be able to sag southeastward into
MO/IL ahead of an approaching longwave trough. Along with the
cooler post-frontal temperatures, lift ahead of the trough in the
presence of the boundary may be sufficient to produce SHRA/TSRA
across the area on Tue/Wed.

Although temperatures will remain unseasonably warm through the
weekend, the surface wind field at times is more southeasterly
than southwesterly based on the position of the surface high
pressure center over the Great Lakes and Ohio Valley. This means
that there will not be a continuous flow of humid GOMEX air into
the region. The heat index values should remain in the upper 90s
on Sat/Sun due to the lower dew points even prior to any diurnal
mixing. Forecast conditions do not meet the criteria for a
duration-based Heat Advisory (max heat index of 100-104 deg F on
4+ consecutive days). The need for heat headlines will be
continually re-assessed on subsequent shifts.

Kanofsky

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1244 PM CDT Thu Sep 21 2017

VFR conditions, dry wx, and light southerly surface winds will
prevail at the TAF sites for much of the valid period. Exceptions
to the wind will be areas for much of the STL metro and UIN which
are behind a convective outflow boundary with easterly winds and
this will persist for a few hours more before finally reverting
back to more southerly. Another potential issue is VSBY reductions
to fog at SUS and CPS and have included a minor reduction for now
until a better handle can be obtained for the 00z set, such as
observed crossover temps.

TES

&&

.CLIMATE...
Issued at 146 PM CDT Tue Sep 19 2017

Record High Temperatures Through Saturday

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


&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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