Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 242145

345 PM CST Tue Feb 24 2015

.SHORT TERM:  (Through Late Tonight)
Issued at 345 PM CST Tue Feb 24 2015

Expect gusty WSW winds to diminish after sunset. An Arctic front
will pass through the region tonight accompanied by a band of
mid-level clouds, however no precipitation is expected. Winds veer
and become northwesterly for a few hours after fropa, then become
LGT/VAR towards daybreak with the passage of the ridge axis.


.LONG TERM:  (Wednesday through Next Tuesday)
Issued at 345 PM CST Tue Feb 24 2015

(Wednesday - Friday)

A longwave upper TROF will persist over eastern North America during
this period, signalling a return to below average temps after our
brief foray towards average today.  Of more important note, the
leading edge of this next intrusion of Arctic air will be
accompanied by a weakening clipper system as it drops southward thru
our region late Wednesday and early Thursday.

The models have been pretty good at advertising light snow accums
for much of the region with plenty of cold air in place and decent
SLRs as a result with what limited QPF that is expected to fall.  A
bit better confidence that the system will still be strong enough to
result in 3" or more snowfalls by late Wednesday night for most of
northeast MO and part of west-central IL to result in a Winter Wx
Advisory being hoisted with this package.  Snowfall amounts decrease
as you head south towards I-70, where less than an inch is forecast
for areas south and east of STL.  Start time for this event is a bit
slower than previous packages:  during the afternoon for northeast
MO and west-central IL, and during the evening for the I-70
corridor, perhaps not until late evening.  Advisory may need to be
ultimately expanded southward a bit but will let mid shift make that
call and still get plenty of lead time.

PoPs were boosted to likely for Thursday morning--a trend that has
been continuing for the past couple of days--as a fresh area of
strong frontogenesis slides down and gives a bit of a boost to the
leftover snow.  Additional amounts will be minimal on Thursday--less
than a half inch.

Very cold temps in place Thursday night with some sub-zero values
forecast once again.  Due to forecast winds, wind chills around 15
below appear likely for parts of the northern CWA.

(Saturday - Next Tuesday)

The models continue to advertise the upper level pattern broadening
out to a more W-SW to E-NE flow across much of the CONUS, resulting
in a great flow for a big, late winter storm, provided the cold air
can hang on over our region.  So with confidence, we can go with
reasonably high PoPs for much of this period, with two distinct
episodes of decent pcpn:  Saturday night thru Sunday night and
another round for Monday night and Tuesday.

The trick is how much confidence can we assign to the vertical temp
profiles and layout over our region.  The 12z models all came in a
bit colder this run, especially for the first round of pcpn over the
weekend.  The snow that occurs for the Wed-Thu system will be key.
If enough falls, it will help steer the cold air to leaving more
slowly and in turn steer the storm tracks a bit more south giving us
a snowier solution.  If the mid-week storm flops, then a warmer
solution will be more favored.  Went with better snow coverage on
pcpn-types for the weekend system but also kept a large area of mix

The second round of pcpn looks warmer overall thru the column and
went straight mix for many areas.  Again, the mid-week storm will
have some say in this, but probably in-so-much as low level cold air
gets locked in or not, but not with the better warm air intrusion
aloft that we are looking at with this round due to more backed flow
aloft.  Can`t emphasize this enough but continue to track updates on
this forecast closely as it could be a prolonged major winter storm
event for a good chunk of our region.



.AVIATION:  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1124 AM CST Tue Feb 24 2015

VFR through the period. Gusty WSW winds will diminish after sunset,
then veer and become northwesterly with the passage of a cold
front tonight. A band of VFR clouds will likely accompany the

Specifics for KSTL: VFR for at least the first 0-6 hours and
probably much longer. Gusty WSW winds will diminish after sunset,
then veer and become northwesterly with the passage of a cold
front tonight. A band of VFR clouds will likely accompany the
front. The greatest uncertainty at KSTL is during the last 3
hours of the 30hr TAF period when a clipper system will be
approaching from the northwest.



MO...WINTER WEATHER ADVISORY from Noon Wednesday to 6 AM CST Thursday
     FOR Knox MO-Lewis MO-Marion MO-Monroe MO-Ralls MO-Shelby MO.

IL...WINTER WEATHER ADVISORY from Noon Wednesday to 6 AM CST Thursday
     FOR Adams IL.



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