Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 250527

1127 PM CST Tue Feb 24 2015

Issued at 920 PM CST Tue Feb 24 2015

Most of the lower level clouds have dissipated or advected east-
southeast of our forecast area this evening. Mid-high level
cloudiness will advect southeastward into our region towards
morning ahead of the approaching storm system in the northern
Plains. Temperatures have been slow to fall so far this evening,
but should see a greater drop later this evening into the
overnight hours due to a clear sky and light surface winds with
weak surface ridging across our area. Low temperatures should be
about 10 degrees below seasonal normals for late February.



.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 06z TAFs through 06z Wednesday Night)
Issued at 1120 PM CST Tue Feb 24 2015

VFR stratus that was over terminals earlier this evening has
advected southeastward and dissipated with time. Clear skies for
most of the rest of tonight is expected before midlevel clouds
advect into region from the northwest ahead of clipper-like
disturbance. Ceilings should lower through the day on Wednesday as
top-down saturation occurs. Timing of light snow looks to be a bit
slower than previous TAF package...starting at KUIN around 2200
UTC 25 February...KCOU 0300 UTC 26 February and KSTL 0700 UTC 26
February. Light snow should begin at KSUS and KCPS just after this
valid TAF package. Snow is expected to intensify a bit at KUIN
with IFR visbys/ceilings becoming more likely by late Wednesday

Specifics for KSTL:

Clear skies for most of the rest of tonight is expected before
midlevel clouds advect into region from the northwest. Ceilings
should lower through the day/evening on Wednesday. Timing of
light snow looks to be about 3 or 4 hours slower than previous TAF
package after analyzing latest model guidance. Now have light snow
beginning at KSTL at 0700 UTC 26 February. Snow may
intensify a bit between 0900 and 1200 UTC Thursday with IFR
visbys/ceilings possible...though since it was at the very tail
end of this valid TAF package elected not to put in those lower
visbys/ceilings quite yet and will let mid shift evaluate things further.



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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