Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 142355

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
555 PM CST Tue Nov 14 2017

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 319 PM CST Tue Nov 14 2017

Ahead of upper level trough that moves through the CWA later this
evening there remains a slight chance of precip in the western third
of the CWA. As this trough approaches, large scale ascent increases
accompanied by a strengthening LLJ. This will increase the chances
of widespread showers overnight. There is also enough MUCAPE to not
rule out a few thunderstorms. There are expected to be gusty winds
associated with the cold frontal passage late Wed. morning. Gusts
will reach speeds near 20kts.

Front is expected to move out of the CWA by late morning with
clearing skies in its wake. Expect western portions of the forecast
area to warm into the upper 50s, more than those to the east (low to
mid 50s). However, any slowing of the front could lower highs for


.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 319 PM CST Tue Nov 14 2017

(Wednesday Night/Thursday)

Dry/tranquil weather is likely both on Wednesday night and Thursday
with slightly below normal temperatures. Surface high pressure
system will slide east/southeast across the mid/upper Mississippi
Valley on Thursday with return flow beginning to the west of the
high Thursday afternoon.

(Thursday Night - Saturday)

Warm advection begins late Thursday night into Friday so some shower
activity is possible. Temperatures will moderate quite a bit
Thursday night because of increasing southeasterly surface winds and
increasing clouds. Highs on Friday will also be much warmer than
Thursday, but exactly how much warmer will depend largely on
stratus. Models are showing fairly strong signals of extensive
stratus on Thursday, so did not go too aggressive for high
temperatures. Expect a range from the mid 50s to mid 60s from
northeast to southwest across the bi-state region.

All eyes will continue to be on system coming out of the Rockies
Friday night. Still about 6-9 hours of difference in frontal passage
between the GFS/ECMWF and believe a compromise of the two may be the
most likely scenario. This would move the front through mostly late
Friday night which lessens the probability of severe weather.
Dewpoints are also not very impressive, mostly in the mid to upper
50s. In addition, midlevel lapse rates are also nowhere near as
steep as the event earlier this month. All that being said, very
limited instability should preclude organized severe weather, but
always hesitate to totally rule it out with very strong low-layer
and deep-layer shear. A positively titled upper-level shortwave
trough and modest cyclogenesis at the surface also do not support a
widespread cool season convective outbreak of strong/severe

Chances of showers and thunderstorms will move off to the southeast
of the forecast area by late Saturday afternoon, even on the slower
ECMWF model. At least slightly cooler weather is likely in store for
Saturday behind the front. Expecting highs in the upper 40s to low
60s from northwest to southeast.

(Saturday Night - Next Tuesday)

Northwest flow aloft will likely dominate the end of this upcoming
weekend into early next week. The mid-Mississippi Valley will
largely be between the ridge axis across the Rocky Mountains and a
deep closed low across the northeastern CONUS. Should be dry weather
during this timeframe, but uncertainty exists with respect to
temperatures. Exact longitudinal position of upstream ridge axis and
downstream trough axis will be very important, but in general expect
below normal temperatures to moderate back to near normal by early
next week.



.AVIATION...  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 555 PM CST Tue Nov 14 2017

Look for CIGs to lower overnight as rain and clouds thicken ahead
of a surface cold front. Places that are currently MVFR (UIN, COU)
should transition to IFR while places that are currently VFR (STL
Metro sites) should transition to MVFR. This will then continue
until a few hours after cold FROPA when clearing is anticipated.
Cold FROPA is anticipated to be around mid morning Wednesday for
UIN and COU and midday Wednesday for the STL metro sites. Ahead of
the front, expect a moderate southerly flow around 10kts while
behind it winds will become gusty out of the NW to 20-25kts. Winds
should diminish around sunset. Best rain chances will be from late
this evening until cold FROPA.





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