Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 142122
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
322 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
Wed.

Walsh


.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
thunderstorms.

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.


Gosselin


&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1154 AM CST Tue Nov 14 2017

VFR conditions for all terminals through this evening with the
exception of COU, which will be MVFR. Widespread showers and
possible thunderstorms will move through all terminals this
evening into Wed morning from the northwest. Ceilings and
visibilities will drop with the approach of a cold front putting
all terminals in MVFR conditions. COU and UIN will see IFR
conditions near sunrise tomorrow. These IFR conditions could
affectthe metro but confidence is too low at this point to
include. By mid morning/early afternoon Wed. the front will have
moved through all terminals. Winds will begin to shift from S/SE
today to NW by tomorrow morning with the frontal passage. Winds
will be gusty tomorrow morning with gusts around 20kts.

SPECIFICS FOR KSTL:

STL will be VFR through the early morning with southerly winds. By
8Z widespread showers and possible thunderstorm will move into
the terminal area. As the precip moves in expect lowering ceilings
and visibilities but will remain MVFR for the TAF period. Showers
will move out ahead of cold front that will pass through STL late
Wed. morning. Winds are expected to shift from southerly to
west/northwest as front moves through. Expect gusts of near 20kts
late Wed. morning.

Walsh

&&

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

$$

WFO LSX


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