Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 051637
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1037 AM CST Thu Dec 5 2013
Issued at 1023 AM CST Thu Dec 5 2013
Forecast on track with radar echoes increasing across the southern
CWA. Latest ACARS soundings and spotter reports depict an ever cooling
atmosphere with freezing rain now limited to the far southern/southeastern
counties of the CWA. Next wave of precipitation taking shape to
our southwest will overspread the southern half of the CWA this
afternoon with sleet and snow becoming the primary precipitation
types. Some worry that sleet accumulation could reach warning
criteria (half inch) in advisory counties. Will monitor and make
any changes to warnings/advisories by 4 pm issuance.
Latest model data suggests a bit of a northern trend in snowfall
for Fridays wave. Still digging through this data though and will
update this afternoon as needed.
.SHORT TERM: (Today through Friday)
Issued at 334 AM CST Thu Dec 5 2013
Focus remains on wintry precip today through Friday as to shortwaves
moves over the Arctic airmass building across the region. The first
shortwave is right over the gulf of California at this time and will
be moving rapidly northeast toward our area through the morning.
Unfortunately it looks like it won`t quite make it into U.S.
airspace so we can sample it with radiosondes for the 12Z models.
Here`s hoping the Mexican National Meteorological Service can get
some sondes into it this morning...please...por favor.
This morning`s update is basically a refinement of earlier
forecasts. The northern edge of the precipitation looks to be very
sharp for this event. This is due at least partially to the very
dry air that is pushing into the region with the Arctic high. The
precipitation will come primarily in two waves. The first arriving
late this morning or early afternoon as the shortwave arrives.
Latest guidance is now showing some low to mid level frontogenesis
along the thermal gradient draped across the area from around 850mb
up to 700mb. This should enhance precipitation allowing for greater
liquid equivalents today and this evening. While the precip will
likely taper off tonight, it probably won`t end entirely across the
eastern Ozarks. The second wave will move in Friday morning as
another slug of vorticity is ejected northeast out of the base of
the deep longwave trof over the western U.S. This should be all
snow, and there is now an 850mb reflection of the wave which ripples
up from Arkansas over the Boot Heel of Missouri causing significant
moisture convergence along the baroclinic zone over the southern
portion of the CWFA. Have boosted snowfall totals down there
accordingly, but again the north-south gradient of accumulation will
be very tight.
Expect the gradient to stretch from near Osage Beach east-northeast
across the St. Louis Metro area up to near Litchfield Illinois.
Across the gradient snowfall amounts will go from 0 to 1-3 inches of
snow with a little sleet mixed in. Further southeast from near Cuba
to around Vandalia expect 3-5 inches of sleet and snow, with around
6 inches of snow stretching from Salem Illinois down through Sparta
and Farmington. Across our far southern counties it looks
particularly messy with freezing rain and sleet at the outset,
mixing and changing to snow tonight. Look for about 2/10 of an inch
of ice with 5-7 inches of sleet and snow. Have laid out advisories
and warnings accordingly.
.LONG TERM: (Saturday through Wednesday)
Issued at 405 AM CST Thu Dec 5 2013
Cold air will settle across the area Saturday as a 1040+ mb high
pushes southeast from the Great Plains. Temperatures will bottom
out in the single digits and low teens across the area. Afternoon
highs will be 20-25 degrees below normal in the low to mid 20s.
This will set the stage for another shot of winter weather late
Saturday night into Sunday. The upper pattern begins to shift
Saturday night with the longwave currently over the western U.S.
moving across the Rockies. Low level warm advection ramps up as
the flow at 850mb turns to the south-southwest. Forecast soundings
indicate primarily snow along and north of the I-70 corridor with
snow perhaps mixing with sleet or maybe even freezing rain in
southern zones. Partial thickness is also agreeable with these
precip types. Model QPF is light, generally 1-2 tenths of an inch,
but is very widespread and not focused in any real discernible
band. Have kept pops nearly identical to previous forecasts with
generally high chance across the entire area.
Another slug of Arctic air will push down from the Northern Plains
into the Mid-Mississippi Valley Sunday night and Monday. The mean
upper level trof is forecast to dig over the eastern 1/2 of North
America Tuesday into Wednesday which will lock this cold airmass
into place. Temperatures will therefore remain as much as 25
degrees below normal into midweek.
.AVIATION: (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1023 AM CST Thu Dec 5 2013
Monitoring latest radar and model trends for snow potential this
afternoon/evening at terminals. It appears that St. Louis metro
sites will be on the northern edge of the snow shield with the
heaviest precipitation falling just south. The next round of snow
is expected to occur late tonight and into Friday. A few inches of
snow is possible. Ceilings and visibility will likely fall to
MVFR, if not IFR category with periods of snow.
Specifics for KSTL:
Light snow should build across the terminal this afternoon and
continue into early this evening. A break in the snow will likely
occur until early Friday morning when another round of light snow
occurs. Light snow accumulation is possible.
MO...WINTER WEATHER ADVISORY until 6 PM CST Friday FOR Crawford MO-
Jefferson MO-Washington MO.
WINTER STORM WARNING until 6 PM CST Friday FOR Iron MO-Madison
MO-Reynolds MO-St. Francois MO-Ste. Genevieve MO.
IL...WINTER WEATHER ADVISORY until 6 PM CST Friday FOR Bond IL-
Clinton IL-Fayette IL-Monroe IL-St. Clair IL.
WINTER STORM WARNING until 6 PM CST Friday FOR Marion IL-
Randolph IL-Washington IL.