Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 252120
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
320 PM CST Wed Feb 25 2015

.SHORT TERM:  (Through Late Tonight)
Issued at 302 PM CST Wed Feb 25 2015

A weakening clipper system will bring light snow to most of the
CWA tonight and tomorrow morning. This forecast package generally
followed model depictions of H9-H8 frontogenesis fields for timing
and placement of the highest PoPs, a change which delays pcpn
onset at most locations until after 26/00z and extends the
occurrence of light snowfall across the greater St. Louis metro
area through the Thursday morning commute. Accumulations of 1-3"
still look reasonable areawide with higher amounts (2-4") in
northeast MO and extreme west central IL, lower amounts (<1")
across southeastern MO and southwest IL, and about an inch around
the St. Louis metro area. Since drivers in the St. Louis metro
area normally have a difficult commute when light snow falls
during rush hour, we have issued an SPS to advise area residents
of tomorrow morning`s forecast.

We discussed whether to extend the existing Winter Weather
Advisory farther south and east to include the metro area between
09z-15z, but chose to hold off for now due to concerns that the
weakening clipper system might weaken even more than what is
currently forecast, which would leave very little (if any)
snowfall across the headline extension. The evening shift and mid
shift will re-evaluate the need for additional headlines based on
future trends.

Kanofsky

.LONG TERM:  (Thursday through Next Wednesday)
Issued at 302 PM CST Wed Feb 25 2015

The clipper system will be followed by an Arctic high pressure
center which builds across the central CONUS on Thursday/Thursday
night. The coldest temps on Thursday night/early Friday morning
should occur across the northwestern CWA due to proximity to the
ridge axis along with the fresh snow cover. The high pressure
center then shifts eastward on Friday and Saturday. Return flow
begins on Saturday around the back side of the high pressure
center.

Kanofsky

Atmospheric responses to a large low pressure system over the
southwestern CONUS and a trough over the northern plains (strong
WAA and frontogenesis) could bring a wintry mix of all ptypes to
the area on Saturday night and then again for Monday through
Tuesday. but confidence in ptypes, timing, duration, amounts,
transitions, and PoPs remains very low. Kept likely PoPs on days 6
& 7 for collaboration with surrounding offices. Anyone who has
travel plans or other weather-dependent plans early next week are
advised to continue monitoring the forecast over the next few
days.

42

&&

.AVIATION:  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1140 AM CST Wed Feb 25 2015

Surface low pressure system will move from southwest Iowa to south
east Missouri over the next 24 hours, bringing a round of snow to
the region and another arctic blast from the north. Timing of
precipitation continues to slip as the system is relatively slow
for a clipper. Appears snow will finally move into northern
Missouri and west central Illinois shortly after 00Z and then
spread south across the eastern half of Missouri and western
Illinois overnight into Thursday morning. Total accumulation will
range from less than an inch south of St. Louis to as much as 3 or
4 inches across far northeast Missouri. A southeast wind ahead of
the low pressure system will become northwest in its wake and
increase as colder air spills into the Midwest. Expect VFR
conditions to quickly deteriorate to IFR as snow begins and only
improve after the snow diminishes Thursday morning from north to
south.

Specifics for KSTL:
VFR conditions are expected until overnight when snow arrives from
the north. Conditions will quickly go down to IFR levels and
likely stay there through mid morning on Thursday. Snow will
diminish Thursday afternoon to flurries and northwest wind will
increase and become gusty. Total accumulation of 1 to 2 inches is
expected at this time at KSTL.

CVKING

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...WINTER WEATHER ADVISORY until 9 AM CST Thursday FOR Knox MO-
     Lewis MO-Marion MO-Monroe MO-Ralls MO-Shelby MO.

IL...WINTER WEATHER ADVISORY until 9 AM CST Thursday FOR Adams IL-
     Brown IL-Pike IL.

&&

$$

WFO LSX






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