MESOSCALE DISCUSSION 2197
NWS STORM PREDICTION CENTER NORMAN OK
1213 PM CST FRI DEC 04 2009
AREAS AFFECTED…S/CNTRL LA/MS
CONCERNING…HEAVY SNOW
VALID 041813Z – 042215Z
CLIMATOLOGICALLY RARE SNOW EVENT WILL CONTINUE TO DEVELOP EWD
THROUGH THE EVENING…WITH RAIN TRANSITIONING TO SNOW OVER PARTS
SRN/CNTRL LA AND MS. RATES NEAR .5 TO 1 INCH AN HOUR ARE POSSIBLE.
A BROAD UPPER TROUGH ACROSS THE CNTRL CONUS WILL CONTINUE TO
PROGRESS EWD…WITH NLY/NWLY FLOW ALOFT DRIVING A CONTINENTAL POLAR
AIRMASS SEWD TO THE GULF COAST AS THIS OCCURS. CONFLUENT FLOW ALOFT
AROUND THE BACKSIDE OF THE UPPER TROUGH IS LEADING TO SURFACE
PRESSURE RISES ACROSS MUCH OF TX/LA. THIS WILL ALLOW A SURFACE HIGH
CENTERED ROUGHLY NEAR N CNTRL TX TO DEVELOP SEWD…WITH STRONG COLD
AIR ADVECTION LEADING TO FALLING SURFACE TEMPERATURES THROUGH THE
AFTERNOON/EVENING. 18Z LCH SOUNDING INDICATES A SHALLOW LAYER OF
SUBSATURATED AIR BELOW 900 MB WHERE WET-BULB PROCESSES WILL LIKELY
CONTRIBUTE TO FALLING SURFACE TEMPERATURES. WET-BULB EFFECTS/COLD
AIR ADVECTION HAVE BEEN CONTRIBUTING TO A FASTER TRANSITION FROM
RAIN TO SNOW THAN MODEL FORECASTS INDICATED ACROSS SERN TX/SWRN LA.
MOISTURE TRANSPORT NEAR THE COAST WITH PWAT VALUES NEAR .75 TO 1
INCH WILL CONTRIBUTE TO INCREASED PRECIPITATION RATES/EFFICIENCY.
ALTHOUGH GROUND TEMPERATURES ARE ABOVE FREEZING…STEADY SNOWFALL
WILL LIKELY LEAD TO ACCUMULATION.
..HURLBUT.. 12/04/2009
…NOTICE…
SPC WINTER WEATHER MESOSCALE DISCUSSIONS WILL BE ISSUED ON A
SCHEDULED BASIS FROM DECEMBER 1 THROUGH FEBRUARY 28…FOUR
TIMES A DAY…AROUND 00 UTC…06 UTC…12 UTC…AND 18 UTC
FOR SIGNIFICANT SHORT RANGE WINTER WEATHER EVENTS.
ATTN…WFO…MOB…JAN…LIX…LCH…SHV…
