Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 262139
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
339 PM CST Fri Dec 26 2014

.SHORT TERM: (Through Sunday Night)
Issued at 329 PM CST Fri Dec 26 2014

Low pressure will develop along the cold front currently over
northwest Missouri in response to shortwave trough ejecting out of
the base of mean trough over the western CONUS tonight.  This low
will move northeast into the Great Lakes by tomorrow at the same
time that the shortwave moves into the the Upper Midwest.  NAM and
GFS show that the cold front will enter the northwest part of the
CWA late tonight and exit the CWA by tomorrow afternoon.  While
precipitation is currently limited on radar over the central CONUS,
expect rain chances to increase rapidly this evening as low level
moisture convergence increases ahead of shortwave trough. Latest
runs of the HRRR as well as the NSSL-WRF have depicted rain
developing throughout the evening hours with it becoming more
concentrated over southeast Missouri into southern Illinois after
midnight. Have continued going forecast trend of increasing
chance PoPs through the evening hours and with likely PoPs over
the southeast half of the CWA after midnight. Then concentrated
likely PoPs tomorrow morning over the southeast half of the CWA
when the GFS depicts increasing mid level frontogenesis associated
with the shortwave trough. Kept a chance of rain or snow going
over the far southeast part of the CWA tomorrow evening before
drier air and subsidence works into the area on Saturday evening.
Sunday and Sunday evening still looks dry at this point.

Kept with falling temperatures will frontal passage tomorrow.
Otherwise MOS guidance are in decent agreement and looks
reasonable.

Britt

.LONG TERM: (Monday through Next Friday)
Issued at 329 PM CST Fri Dec 26 2014

There is relatively good agreement in the global models in that a
positively tilted mean trough will stay established through at
least the first half of next week from central Canada into the
western CONUS. This will keep a relatively fast zonal flow over
MO/IL through most of the period. A large area of surface high
pressure will move under the zonal flow with no real notable
shortwaves troughs that would offer any precipitation chances at
least through Thursday. Model discrepancies show up by Friday as
the operational GFS is faster than it`s own ensemble mean in
ejecting the main upper low out of the southwest toward the
Midwest. While this is faster than the rest of the models, even
the ECMWF is showing precipitation developing over the area in a
broad warm air advection regime ahead of it bringing the upper low
through the area on Saturday. So with this said will go ahead and
introduce snow with some rain to the forecast on Friday.

Cold front will pass through the area on Monday and temperatures
will fall below normal on Tuesday and Wednesday before returning to
near normal for the start of a new year late in the week.

Britt

&&

.AVIATION:  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1203 PM CST Fri Dec 26 2014

MVFR SC deck over far southern Missouri to slowly develop and
slide north this afternoon as low level moisture is on the
increase ahead of next weather system. But movement/development is
a bit slower than originally thought, so backed off at taf sites
til early this evening. Otherwise, winds to remain from the south
til frontal passage which will be between 12z-15z Saturday, then winds
to veer to the west around 15kts. Precipitation will be limited
with it mainly occurring with frontal passage, except over
central, northeast MO, west central IL. Kept mention of light rain
for KCOU after 03z Saturday and around 06z for KUIN. Cigs to lower
but remain low end mvfr with frontal passage.

Specifics for KSTL:
MVFR SC deck over far southern Missouri to slowly develop and
slide north this afternoon as low level moisture is on the
increase ahead of next weather system. But movement/development is
a bit slower than originally thought, so backed off at KSTL til
after 00z Saturday. Otherwise, winds to remain from the south til frontal
passage which will be around 15z Saturday, then winds to veer to
the west around 15kts. Precipitation will be limited with it mainly
occurring with frontal passage. Cigs to lower but remain low end
mvfr with frontal passage.

Byrd

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX






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