Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 260530
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1130 PM CST Tue Nov 25 2014

.UPDATE:
Issued at 921 PM CST Tue Nov 25 2014

Temperatures were falling a little quicker than previously
forecast over a portion of the area this evening due to good
radiational cooling. The temperature may actually become steady or
even rise slightly late tonight as clouds spread into the area
ahead of an approaching northwest flow shortwave and associated
weak surface low over the northern Plains, and as a low-mid level
warm air advection regime shifts southeastward into our forecast
area, while south-southeasterly surface winds gradually increase.
Looking at the latest model guidance light snow should spread
southeastward into portions of northeast and central MO around
12z Wednesday.

GKS

&&

.SHORT TERM: (Through Wednesday Night)
Issued at 348 PM CST Tue Nov 25 2014

(Tonight through Wednesday night)

Main focus will be the potential for some accumulating snow over the
region during the day on Wednesday.  Low amplitude trough currently
entering the northern CONUS will dive southeastward and strengthen
over the Plains tonight.  The GFS has shown decent run-to-run
continuity with this feature and the ECMWF has come into agreement
with it`s solution that large scale ascent will become more
widespread as the trough moves into central and northeast Missouri
toward daybreak.  At this time it appears this ascent will be
concentrated along and west of the Mississippi River between 12-18Z
and then shift east into Illinois during the afternoon.  Most of the
forecast soundings suggest this will be snow during the morning
hours when the low levels will be the coldest and the ascent will
be the strongest. By nightfall, the upper trough will be moving
quickly east of the area which will cause subsidence to set by
evening. Have just slight chances of snow going during the evening
hours over Illinois.

MOS guidance temperatures are in good agreement during this period
and generally followed.  High temperatures tomorrow will be warm
enough to cause the snow to mix with or change over to rain which
will cut back on total snowfall amounts, particularly along and
south of I-70. Did not change going snow amounts too much. Still
appears that northeast Missouri will get around 1 inch of snowfall
with lesser amounts to the south centered along the Mississippi
River.

Britt

.LONG TERM:  (Wednesday through Next Tuesday)
Issued at 348 PM CST Tue Nov 25 2014

(Thursday and Friday)

Upper pattern will gradually flatten out and temperatures will begin
to warm back up during this period as the models are in good
agreement that surface high will pass through the area on Thursday
night.  By Friday, south to southwesterly winds will advect warmer
air into the area which bring temperatures back to normal.

(Saturday through next Tuesday)

GFS and ECMWF still agree that upper pattern will be zonal over the
weekend with a cold front moving south across the CWA on Saturday
night and Sunday. Still looks like chances for precipitation will
increase along the the front by late in the weekend as both models
show a shortwave wave trough moving through the area on Sunday
night. Then model solutions become more divergent by early next
week as the GFS is slightly more southwesterly aloft with an
approaching storm system whereas the ECMWF stays more zonal.
Temperatures ahead of the front on Saturday still look like they
will be about 10 degree above normal with 850mb temps around +10C.
Then temperatures will be closer to normal behind the front as
coldest air will not be able to come down because of the zonal
flow.

Britt

&&

.AVIATION:  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1125 PM CST Tue Nov 25 2014

VFR flight conditions and dry weather will persist overnight with
clouds increasing and thickening across the region. A northwest
flow storm system is still on track to impact the area on
Wednesday with precipitation spreading in KCOU around 14z and KUIN
and the St. Louis area terminals between 16-17z. Temperature
structure at the onset of the precipitation suggests all snow at
KCOU and KUIN with predominately MVFR flight conditions, while
there is more uncertainty in the ptype at and KSTL/KSUS/KCPS with
snow at the onset and then mixing with rain. Flight conditions
could lower into the IFR category during the precipitation.
Precipitation should taper to very light snow/flurries by mid-
afternoon to evening with a northwest wind shift as the system
passes.

Specifics for KSTL:

VFR flight conditions and dry weather will persist overnight through
mid morning on Wednesday with clouds increasing and thickening. A
northwest flow storm system will impact the area on Wednesday with
precipitation spreading into KSTL between 16-17z. There is some
uncertainty in the ptype, however the current thinking is that we
will see snow at the onset and then mixing with rain in the afternoon.
Flight conditions could lower into the IFR category during the
precipitation. Precipitation should taper to very light snow/flurries
by early evening with a northwest wind shift as the system passes.

Glass
&&

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

$$

WFO LSX





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