Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 181223

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
723 AM CDT Mon Sep 18 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 243 AM CDT Mon Sep 18 2017

Main concern the next 24 hours will be the chance for showers and

Jefferson City and Sullivan are reporting reduced visibilities in
fog, but areal extent and density of the fog has been limited by
the clouds overhead. There still may still be some additional fog
develop through sunrise as dewpoint depressions drop, particularly
in low lying areas.

Otherwise, the past few hours have been mainly dry as the cold
front has moved southeast of the area, and there is no low level
jet or upper level disturbance to concentrate forcing over the
area. Later this morning, the front will move back north as a warm
front. The RAP is showing some weak low level convergence along
it. The CAMS are consistent with showing the potential for
scattered showers and thunderstorms developing over southeast
Missouri into southwest Illinois by late this morning into early
this afternoon. This chance will spread north and increase with
the approach of a shortwave trough by late in the afternoon and
early evening. The chance will slowly decrease to slight chances
overnight as the front moves off to the north and the upper
trough moves east leaving the area under weak forcing.

Highs this afternoon will be near or above normal depending on
the timing of showers and thunderstorms this afternoon. Lows
tonight will be above normal as the front will be north of the
area and dewpoints are expected to be above the normal lows.


.LONG TERM...  (Tuesday through Sunday)
Issued at 243 AM CDT Mon Sep 18 2017

The large-scale upper air pattern will become highly amplified
during the extended forecast period. A strong upper ridge across
the eastern half of the CONUS will bring unseasonably warm
temperatures from Tuesday through Sunday along with very few
opportunities for widespread precipitation.

The arrival of a large mid/upper low across the northwestern
CONUS will induce surface cyclogenesis across the northern plains
and into the Canadian Rockies as well as downstream height rises
across the eastern half of the CONUS. An strong PV anomaly will
then follow a similar track onshore, quickly ejecting the initial
disturbance northeastward through Canada while reinforcing and
deepening a longwave trough over the western half of the CONUS.
Subsequent downstream height rises will allow an elongated upper
ridge axis to stretch from Mexico through the central CONUS and
into southeastern Canada. Some models show a slight variation of
this pattern and depict a closed upper high over the Great Lakes
and a separate closed high over Mexico. The overall pattern is the
same and there is excellent agreement between the ECMWF, GFS, and
GEM regarding the placement and evolution of the overall pattern
and features which would affect MO/IL. Rather less agreement is
noted for a couple of tropical cyclones over the Atlantic Ocean,
and part of the disagreement can be traced to the models which
depict a height weakness between two separate upper highs instead
of a solid ridge axis.

The active frontal boundaries will remain north and west of the
LSX CWA for most periods in the extended forecast, although there
are a few periods when a frontal boundary sneaks close enough to
the CWA to justify 20-30% PoPs due to H85 moisture convergence and
increasing instability due to steady S/SW flow.

High temperatures will be in the mid/upper 80s to around 90
degrees, and these values are 10-15 degrees above average for
September. Record highs for St. Louis, Columbia, and Quincy for
September 19-24 are generally in the mid/upper 90s or around 100.
While it is possible that a stray low-90s record high could fall
during this time, the current forecast high temperatures are a
few degrees short of most of the records at most locations.



.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 722 AM CDT Mon Sep 18 2017

Some patchy fog remains over the area, but terminals should be
clear of fog by 13Z. Otherwise dry weather is expected through the
late morning before scattered thunderstorms develop during the
early afternoon and continue into the evening. Any thunderstorms
that do develop may produce brief MVFR or possible IFR ceilings
and visibilities. Winds are expected to be light through the

SPECIFICS FOR KSTL: Mainly dry and VFR conditions are expected
through early afternoon. Then scattered thunderstorms will develop
around the terminal during the afternoon and evening hours. Any
thunderstorms that do develop may produce brief MVFR or possible
IFR ceilings and visibilities. Winds are expected to be light
through the period.





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