Area Forecast Discussion
Issued by NWS St. Louis, MO

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000
FXUS63 KLSX 252353
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
553 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 00z TAFs through 00z Thursday Evening)
Issued at 531 PM CST Wed Feb 25 2015

Prev TAFs still appear to be on track. However, some minor changes
in timing were made given latest trends and mdl guidance. Still
expect winds to pick up behind the system with SN. Precip type may
briefly be IP at onset of precip due to wet bulb effects, but this
shud be very short lived before turning to all SN. Believe max SN
accumulations will be at UIN and STL metro sites. Check with the
latest public forecast for SN amounts.

Tilly

&&

.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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