Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 042142

Area Forecast Discussion
National Weather Service Saint Louis MO
342 PM CST Mon Dec 4 2017

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 205 PM CST Mon Dec 4 2017

An ongoing pattern shift aloft will bring a variety of active
weather to MO/IL tonight and tomorrow. A strong PV anomaly has
induced surface cyclogenesis across the upper Midwest, and the
system`s attendant cold front was moving southeastward through
the central CONUS this afternoon. The upper air pattern will
continue evolving tonight and tomorrow such that a longwave trough
deepens over the eastern half of the CONUS.

Ongoing gusty winds across parts of MO/IL will continue from the
south to southwest this aftn/eve until a strong cold front has
moved through the region. Thereafter, winds will remain gusty
overnight and tomorrow, but winds will be from the
west-northwest instead of the south or southwest. Unusually deep
mixing is expected on Tuesday, especially north of I-70, and a
Wind Advisory may be needed there given the strong wind speeds
aloft and the small separation angle between the low-level wind
direction and the winds above the top of the boundary layer.

The aforementioned strong cold front will also support a narrow
line of thunderstorms which will accompany the boundary as it
moves across the region. Scattered thunderstorms had already
developed in NW MO along the front early this afternoon and are
expected to quickly congeal into a thin line which will then
rapidly move across the area in tandem with the front. Ample 0-6km
shear, sufficient instability, steep H7-H5 lapse rates, and large
scale ascent are favorable for severe thunderstorms. In
particular, the strong wind field aloft will further support
damaging wind gusts at the surface via downward momentum transfer
within thunderstorm downdrafts. Most of the precipitation should
exit the CWA by 06z/midnight.

This front will also end the recent stretch of unusually warm
weather. Temperatures will fall from near-record highs in the
60s/70s today into the 20s/30s tonight. Highs on Tuesday are only
expected to reach the 40s.


.LONG TERM...  (Tuesday Night through Next Monday)
Issued at 205 PM CST Mon Dec 4 2017

The upper air pattern will feature a deep longwave trough over
the eastern half of the CONUS with multiple embedded PV anomalies
rotating through the trough. Any of these features could support
light precipitation through next Monday, but the best chance will
be on Friday night when the strongest of these features drops
nearly due southward from MB into the central CONUS. Light
snowfall accumulations (perhaps less than one-half inch) are
possible across parts of the area on Friday night.

Temperatures will remain near or below normal through the first
half of the weekend. At least one reinforcing cold front is
expected to reinvigorate the starkly colder air mass which will
have settled across the central CONUS tonight, but there is poor
model agreement attm due to inconsistent handling of several
individual PV anomalies embedded within the broader longwave



.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1134 AM CST Mon Dec 4 2017

Gusty south to southwest winds will continue this afternoon ahead
of a strong cold front. A thin line of SHRA/TSRA is expected to
develop along the front late this afternoon and quickly progress
eastward through the terminals between 22-04z. Winds will change
direction after fropa and become west-northwesterly to
northwesterly but will remain gusty through the remainder of the
valid TAF period. Initially MVFR cigs at TAF issuance at COU and
UIN will likely remain between 025-030 until after fropa.



Issued at 324 AM CST Sat Dec 2 2017

Record High Temperatures for Monday December 4th
St. Louis  74 in 2001
Columbia   70 in 1933
Quincy     69 in 1998




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