Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 200857
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
357 AM CDT Wed Sep 20 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 337 AM CDT Wed Sep 20 2017

Main concern today is very warm temperatures and heat index
values. Temperatures are already running well above normal for
mid-late September early this morning with 2am temperatures in the
lower-mid 70s. With the warm start today, this should allow
temperatures to climb quickly above normal given that skies will
be mainly sunny other than some clouds entering northeast Missouri
this afternoon. Forecast soundings are showing mixing up to 850mb
this afternoon with values at this level forecast to be around
20C. This supports the forecast MOS values in the lower to middle
90s. Combine these with dewpoints rising into the lower 70s today
will cause heat index values to climb to around 100 degrees across
much of the area. Low tonight will once again only fall off into
the upper 60s and lower 70s as dewpoints will be higher than the
normal lows.

Fog has not been a problem so far tonight as we mixed out
yesterday afternoon and there is a wider spread in dewpoint
depressions than there was yesterday. This should limit fog
development the rest of the morning. Expect mainly dry conditions
through early afternoon before a cold front currently over the
Upper Midwest and Central Plains moves southeast. Scattered
thunderstorms are expected to develop along this front and move
into central and northeast Missouri as well as west central
Illinois tonight. This front is expected to stall late tonight
and move back north as a warm front, so thunderstorms are not
expected to progress very far south into the CWA. A few strong
storms will be possible over northeast Missouri this evening
given MLCAPES will be near 2000 J/kg with marginal wind shear.

Britt

.LONG TERM...  (Thursday through Tuesday)
Issued at 337 AM CDT Wed Sep 20 2017

The next several days look unseasonably warm with little
opportunity for widespread precipitation.

An elongated upper ridge axis will continue developing across the
eastern CONUS, stretching from Mexico through southern Quebec at
times. Models continue to show very good agreement regarding the
large scale pattern over the central CONUS. Subsidence beneath the
upper ridge combined with southerly to southwesterly flow will
produce unseasonably warm temperatures through the weekend and
into early next week. High temperatures will range from the
mid/upper 80s to the low/mid 90s over the next several days,
which is around 10-15+ degrees warmer than average. Increasing
humidity will push heat index values into the upper 90s to near
100 degrees on Thu and Fri.

At this time, forecast conditions do not meet the criteria for a
duration-based Heat Advisory (max heat index of 100-104 degrees F
on 4+ consecutive days), therefore no heat headlines will be
issued with this package. A Heat Advisory may be needed with a
future forecast package if higher forecast temperatures or
humidity values result in higher heat index values.

There is very little opportunity for precipitation until the
middle of next week. One or more frontal boundaries may get close
enough to the CWA to bring a slight chance of rain to parts of
central or northeastern MO, but most locations will probably
remain dry for several days. Precipitation chances increase during
the middle of next week due to an approaching cold front and
mid/upper trough axis.

Kanofsky

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1045 PM CDT Tue Sep 19 2017

Southeasterly surface winds will continue late tonight. Only some
high level clouds are expected tonight into Wednesday morning.
There may be few-scattered diurnal cumulus clouds along with high
level cloudiness Wednesday afternoon, mainly in UIN ahead of an
approaching cold front. South-southwesterly surface winds can be
expected on Wednesday.

SPECIFICS FOR KSTL: Southeasterly surface winds will continue
late tonight with south-southwesterly surface winds on
Wednesday. Only some high level cloudiness is expected through
the forecast period.

GKS

&&

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

Record High Temperatures Through Saturday

              KSTL  KCOU  KUIN
Sept 20     96 (1940)   98 (1893)   95 (1940)
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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