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000
FXUS63 KLSX 041958
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
158 PM CST Wed Dec 4 2013

.UPDATE:
Issued at 153 PM CST Wed Dec 4 2013

We will be issuing a Winter Storm Watch for the southern and
southeastern third of the CWA and is expected to be in effect from
Noon Thursday through Friday. Counties along I-44 in MO and south
of I-70 in IL will likely be included in this watch. Details are
still being worked out but the Watch will hopefully be out within
the next half hour.

Issued at 1147 AM CST Wed Dec 4 2013

Have tweeked forecasts for this afternoon for a bit more sun in the
warm sector, and also tried to tighten up temperature gradient with
the fropa that will occur over western sections of the CWA this
afternoon. Have also changed precipitation threat to be primarily tied
to post frontal drizzle. Update to zones will be out ASAP.

Truett

&&

.SHORT TERM:  (Through Late This Afternoon)
Issued at 349 AM CST Wed Dec 4 2013

Main concern once again this morning deals with how widespread
and dense the fog will be.  There are not as many observation sites
reporting visibilities below 1SM compared to this time yesterday,
particularly in metro east, though FAM just recently fell to 1/4SM.
For now will mention patchy dense fog in southeast Missouri and
monitor trends over the next few hours for the need of an advisory.

Otherwise, not much change in thinking with respect with the timing
of the cold front that will usher much colder air into our region
for the forseeable future.  It will enter the northeast Missouri
counties by midday and reach metro St. Louis by 6 pm.  Still not
much measurable rain chances with this front as best ascent will
stay north of the area.  Low level moisture will stay around the
area most of the day, so there may still be some patchy drizzle
around, particularly along the front.  Temperatures are at or above
normal highs, so climbing 10-15 degrees today will reach MOS highs.

Britt

.LONG TERM: (Tonight through Tuesday)
Issued at 349 AM CST Wed Dec 4 2013

Primary concern remains wintry precip Thursday through Friday.  The
Arctic airmass which is now oozing south through the Great Plains
will be building across the Mid-Mississippi Valley on Thursday.
After this morning`s mild temperatures, it`s going to be a rude
awakening on Thursday morning with temperatures ranging from the low
to mid 30s across southeast Missouri and southwest Illinois to the
low to mid 20s in northeast Missouri.  All guidance is slowing the
onset of precipitation Thursday afternoon.  It still looks like the
primary forcing will come from the mid and upper levels as a highly
sheared shortwave passes overhead.  The origin of this wave is a
cutoff low now approximately 1000 miles southwest of San Diego and
it looks like it`s finally beginning to merge with the mean longwave
trof digging across the west coast.  Since the only way we really
have to track this low is satellite imagery and (perhaps) aircraft
data, it`s going to be very difficult to predict when the sheared
vortmax will move over the area Thursday.  That being said, models
tend to be too fast when absorbing cutoffs into the mean flow, so a
slower onset of precipitation makes sense.  A slower onset means
that the cold air will have longer to deepen over the area which
means a bit less mixed precip and more snow.  QPF values on most
guidance remain fairly consistent, with the exception of the NAM
which backed way off from yesterday`s runs.  The ECMWF is a high
outlier when compared to the SREF and GFS so have discounted it as
well.  Stuck pretty close to WPC guidance for QPF and played with
snow/water ratios ranging from 12:1 down to 3:1 where sleet is
expected.  There also remains a small area of southeast Missouri
where freezing rain will be a real threat for Thursday afternoon and
evening.  At any rate, 1 to 3 inches of snow with sleet mixed in
looks good for much of the area from east central Missouri into
parts of southwest Illinois, tapering down pretty rapidly further to
the northwest.  Around 1/10 inch of freezing rain looks possible for
parts of Reynolds, Madison, and Iron counties, with decreasing
amounts further north.  This mixed precipitation should turn to all
snow through the night Thursday night.  A second shortwave will move
overhead on Friday bringing another round of snow.  This looks like
another widespread 1 to 2 inches accumulation for east central and
southeast Missouri as well as southwest Illinois.

The remainder of the forecast continues to look very cold.  Guidance
continues to show another round of precip for Sunday/Sunday night as
the western trof begins moving eastward across the Rockies.  There`s
some disagreement with respect to how much precip will fall across
the region as  the resulting low level warm advection ramps up over
the very cold airmass which is locked in over the Midwest.
Thickness progs from both the GFS and ECMWF as well as GFS soundings
indicate the precip won`t be all snow across southern Missouri, so
have changed to a mix of sleet and snow across southern zones on
Sunday for this morning`s package.

Carney

&&

.AVIATION:  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1147 AM CST Wed Dec 4 2013

Stratus in the warm sector has shown a significant improvement
over the past 90 minutes, while post frontal IFR/MVFR is making
slow but steady ewd progress over western Missouri. Based on post-
frontal ob trends this morning guidance looks way too optimistic
once cold air arrives, and have introduced another round of MVFR
and IFR, along with a chance of drizzle, at all TAF sites when fropa
occurs. Based on NAM12 low level rh progs hang onto this for 4-6
hours after fropa and then allowed cigs to increase to around
3kft.

Specifics for KSTL: Latest satellite pics suggest a warm sector
look to the KSTL area with sct-bkn clouds primarily aoa 2kft.
Passage of cold front around 00z will allow a return of MVFR and
ocnl IFR cigs/vsbys with occasional drizzle for the early evening
hours. Based on low level rh progs then allow cigs to climb to
around 3kft by 06z. Precipitation threat with approaching storm
should hold off until Thursday afternoon, and have introduced a
bit of light snow at 21z to highlight this increasing threat.

Truett

&&

.PRELIMINARY POINT TEMPS/POPS:
Saint Louis     65  28  33  19 /  10  10  70  70
Quincy          57  20  28  11 /  10  10  20  20
Columbia        63  21  28  12 /  10  10  40  50
Jefferson City  65  22  30  12 /  10  10  50  50
Salem           64  31  35  21 /  10  10  80  90
Farmington      64  29  33  17 /  10  20  90  90

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...WINTER STORM WATCH from Thursday afternoon through Friday
     afternoon FOR Crawford MO-Iron MO-Jefferson MO-Madison MO-
     Reynolds MO-St. Francois MO-Ste. Genevieve MO-Washington MO.

IL...WINTER STORM WATCH from Thursday afternoon through Friday
     afternoon FOR Clinton IL-Fayette IL-Marion IL-Monroe IL-
     Randolph IL-St. Clair IL-Washington IL.

&&

$$

WFO LSX