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000
FXUS63 KLSX 131219
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
619 AM CST Fri Dec 13 2013

.SHORT TERM:  (Through Tonight)
Issued at 335 AM CST Fri Dec 13 2013

Due to uncertainty and changes in timing and precip type through
tonight, have pushed the advisory back and converted the entire
area to an advisory thru at least early Sat morning.

Differences among mdls thru today have presented an interesting
forecast problem. Mdl guidance has come in warmer than prev
cycles. Based on the latest guidance, believe precip today will
change from ra/fzra to sn. However, there may be periods today,
mainly late this afternoon, where p-type will be tied to precip
intensity. RA shud move into the region mid to late morning and
spread nwd. The mdls suggest this will accompany the wrmfnt as it
lifts nwd thru the area, with temps above freezing with the
precip.

Precip shud change over to SN from N to S this afternoon as the
850 mb low tracks thru the area and CAA begins. The one caveat is
that some mdl guidance suggests the strong WAA ahead of the low
will primarily generate lift and actually cool the atmo quicker
than the going forecast wud suggest. If this occurs, nrn portions
of the CWA could change over to SN by Noon today and easily see
warning criteria amounts. The last area to change over to SN shud
be far ern portions of the CWA, but the entire region shud be SN
by Midnight tonight.

Am also concerned that the strong vort max that is progd to move
just S of the CWA tonight will generate higher snowfall across srn
portions of the CWA overnight. This feature will need to be
monitored today.

.LONG TERM:  (Saturday through Thursday)
Issued at 414 AM CST Fri Dec 13 2013

(Saturday through Sunday)

Latest mdl guidance suggests sn will linger through at least
portions of Sat morning as the system pulls ewd out of the area.
The main upper trof is progd to move thru the CWA during the
afternoon hours on Sat as well, which may produce a brief period
of light sn. However, mdls suggest a lack of moisture when the
trof moves thru, suggesting only flurries. For now, will keep
slight chance POPs going.

As for temps, have trended twd the colder guidance thru Sat, but
then trended twd the warmer guidance for Sun as swly flow returns
to the area.

(Monday through Thursday)

The upper air pattern will transition from NW flow to SW flow next
week as a new long wave trough digs along the west coast. As the
upper pattern adjusts, there should not be any reinforcing shots of
arctic air through Thursday. The surface ridge that builds in behind
today`s system slides to our south with return flow commencing on
Wednesday. This scenario sets the stage for a steady warmup next
week. Another storm system may impact somewhere near the center of
the country late next week or into the weekend. This wave will be
riding along the next arctic cold front. This system is beyond the
fcst period and many uncertainties exist. In advance of this next
system, temperatures should warm to above normal by Wednesday and
Thursday with the ECMWF is advertising +10C 850mb air on Wednesday.
It should be a nice warmup before another significant arctic surge.

2%

&&

.AVIATION:  (For the 12z TAFs through 12z Saturday Morning)
Issued at 553 AM CST Fri Dec 13 2013

Complicated fcst for this period with a complex winter storm
expected to impact the terminals over the next 24 hrs. An
advancing warm front will spread a batch of precip over cntrl and
NE MO this morning. The precip may start off as frz ra if sfc
temps are cold enough but should quickly transition to rain as
temps warm. There may be a brief lull in the precip until the main
precip shield moves into the area from the SW. Most locations will
remain rain until this evening when the precip should make the
change over to snow from NW to SE. The exception is NE MO and w
cntrl IL where the changeover to snow may happen as early as this
aftn. Snow should continue most of the night ending from west to
east around sunrise. MVFR conditions with the rain and IFR with
the snow.

Specifics for KSTL:

Initial surge of precip should arrive around 18Z. Precip should
remain rain until this evening. Snow is expected to continue until
around 12Z. MVFR conditions with rain and IFR with the snow.

2%

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...WINTER WEATHER ADVISORY from 6 PM this evening to 6 AM CST
     Saturday FOR Audrain MO-Boone MO-Callaway MO-Cole MO-
     Crawford MO-Franklin MO-Gasconade MO-Iron MO-Jefferson MO-
     Lincoln MO-Madison MO-Moniteau MO-Montgomery MO-Osage MO-
     Reynolds MO-St. Charles MO-St. Francois MO-St. Louis City
     MO-St. Louis MO-Ste. Genevieve MO-Warren MO-Washington MO.

     WINTER WEATHER ADVISORY from 3 PM this afternoon to 6 AM CST
     Saturday FOR Knox MO-Lewis MO-Marion MO-Monroe MO-Pike MO-
     Ralls MO-Shelby MO.

IL...WINTER WEATHER ADVISORY from 6 PM this evening to 6 AM CST
     Saturday FOR Calhoun IL-Greene IL-Jersey IL-Macoupin IL-
     Madison IL-Monroe IL-Randolph IL-St. Clair IL.

     WINTER WEATHER ADVISORY from 6 PM this evening to Noon CST
     Saturday FOR Bond IL-Clinton IL-Fayette IL-Marion IL-
     Montgomery IL-Washington IL.

     WINTER WEATHER ADVISORY from 3 PM this afternoon to 6 AM CST
     Saturday FOR Adams IL-Brown IL-Pike IL.

&&

$$

WFO LSX