Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 302129
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
329 PM CST Fri Jan 30 2015

.SHORT TERM:  (Through Late Tonight)
Issued at 328 PM CST Fri Jan 30 2015

Tranquil weather and seasonably cold weather conditions expected
tonight.  Extensive high clouds currently located across the Plains
in advance of the western U.S. trof are filtering into the area this
afternoon. These clouds will continue to increase and thicken
tonight while the surface high currently centered through the CWA
slowly retreats eastward.

Glass

.LONG TERM:  (Saturday through Next Friday)
Issued at 328 PM CST Fri Jan 30 2015

Clouds along with rain will spread eastward through much of our
forecast area, particularly north and west of STL on Saturday as a
southwesterly low level jet brings increasing low level warm air
advection and moisture into our ahead ahead of an approaching storm
system.  It appears that the precipitation type will be mainly
liquid rain on Saturday, although there is a possibility of a brief
period of snow/sleet across northeast and central MO if the
precipitation makes it into this area already by early to mid
morning when the surface layer will still be quite cold.  There will
also be some evaporational cooling at the onset of the precipitation
as well.  Precipitation should spread throughout the entire forecast
area Saturday night and intensify with the approach of a strong
northern stream shortwave and as a surface low develops over
southwestern MO.  The models also depict upper level divergence,
particularly across northeast MO and west central IL in the right
entrance region of an upper level jet streak over the Great Lakes
region.  The rain will gradually change over to snow, mainly across
northeast MO and west central IL Saturday night as a cold front sags
southward into this area and pseudo adiabatic cooling increases due
to stronger upward vertical motion.  The surface low will track
eastward through southern portions of MO and IL on Sunday, while the
850 mb low moves eastward through northern MO and central IL.  The
precipitation will continue on Sunday with a gradual changeover to
snow across most of the rest of the forecast area as colder air
filters into the region behind the surface/850 mb low and causes the
freezing level to drop.  Surface temperatures across much of our
area late Sunday morning and afternoon may be nearly steady or
slowly fall.  The latest model runs have trended further north
with the storm system and are now slower with the change over to
snow across central and southeast MO as well as southwest IL.  This
will lead to less snow accumulation across this area, but am still
expecting significant snowfall across northeast MO and west central
IL where the greatest forcing is expected along with the quickest
changeover to snow.  Although it is still a little early to pin down
exact snowfall totals as the models continue to vary from run to
run, but preliminary forecasted amounts will range from 6 to 8
inches across parts of northeast MO and west central IL down to
around 1 to 3 inches across parts of central MO and southwest IL to
less than 1 inch further south over southeast MO.  With forecasted
snowfall totals reaching and slightly exceeding 6 inches over the
extreme northern portion of our forecast area will issue a winter
storm watch for this area for Saturday night and Sunday.  There may
be some lingering light snow in the deformation zone mainly across
northeast MO and west central into southwest IL Sunday evening, but
it should be tapering off with time Sunday night.  Strong and gusty
winds may produce some blowing and drifting snow across the northern
portion of our forecast area Sunday afternoon and evening.  Much
colder temperatures are expected Sunday night behind the storm
system as the models drop the -12 degree C 850 mb isotherm south of
STL Sunday night.

A cold day is on tap for Monday with below normal temperatures as a
large surface high settles into the area in the wake of the
departing vigorous upper trof and attendant cold front. The flow
aloft then becomes broadly cyclonic after this vigorous upper trof
departs the region and this will allow for several lower amplitude
disturbances to traverse the area within the flow aloft, one of
which will bring a cold front midweek. We should see a brief
moderation in temperatures Tuesday into early Wednesday with the
next strong cold front pushing through the region during the day
Wednesday. I am not expecting much if anything in the way of
precipitation but there is some post-frontal frontogenetical forcing
that is worthy of a low pop. Cold weather with below average
temperatures will prevail through the week`s end as high pressure
dominates.

GKS/Glass

&&

.AVIATION:  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1114 AM CST Fri Jan 30 2015

VFR flight conditions will prevail for the valid forecast period.
High clouds will steadily increase across the region this
afternoon and tonight as a large surface high slowly retreats
eastward. Mid and eventually lower clouds will begin spreading
into the area on Saturday morning. The onset of precipitation with
the next storm system is a bit tricky in timing. At most
terminals the onset will be after the valid TAF forecast period,
however light rain will probably develop at KCOU between 15-18z
Sat 31 Jan.

Specifics for KSTL:

VFR flight conditions will prevail for the valid forecast period.
High clouds will steadily increase this afternoon and tonight as
a large surface high slowly retreats eastward. Mid clouds will
begin spreading into the area on Saturday morning with lower clouds
on Saturday afternoon. The onset of precipitation with the next
storm system is a bit tricky, but current thoughts are light rain
will probably develop at KSTL around 21z Sat 31 Jan.

Glass

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...WINTER STORM WATCH from Saturday evening through Sunday
     afternoon FOR Knox MO-Lewis MO-Marion MO-Shelby MO.

IL...WINTER STORM WATCH from Saturday evening through Sunday
     afternoon FOR Adams IL-Brown IL-Pike IL.

&&

$$

WFO LSX






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