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Mesoscale Discussion 122
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   MESOSCALE DISCUSSION 0122
   NWS STORM PREDICTION CENTER NORMAN OK
   1154 PM CST WED MAR 04 2015

   AREAS AFFECTED...PORTIONS OF MIDDLE/SWRN TN...NWRN AL...CNTRL/NRN
   MS...SRN AR...NRN LA...NERN TX

   CONCERNING...WINTER MIXED PRECIPITATION 

   VALID 050554Z - 051200Z

   SUMMARY...WINTER MIXED PRECIPITATION WILL CONTINUE TO OVERSPREAD A
   BROAD REGION FROM NERN TX ENEWD TOWARD THE ARKLAMISS DELTA REGION
   AND NEWD INTO THE TN VALLEY AREA. FREEZING RAIN AND SLEET ARE
   EXPECTED TO OCCUR.

   DISCUSSION...THE INTERACTION OF A MIDLEVEL VORT MAX EMERGING OVER
   THE SRN PLAINS WITH THE CYCLONIC CIRCULATION SURROUNDING A SHORTWAVE
   TROUGH FARTHER N IS SUPPORTING THE EWD SPREAD OF LARGE-SCALE ASCENT
   FROM PARTS OF CNTRL TX TO THE ARKLATEX REGION -- PER RECENT WATER
   VAPOR LOOPS. PRECIPITATION...INCLUDING THAT ASSOCIATED WITH
   LIGHTNING-PRODUCING CONVECTION...CONTINUES FORMING WITHIN THIS
   REGIME...WITH AREAS OF EARLIER-DEVELOPED PRECIPITATION SPREADING EWD
   TOWARD PARTS OF E-CNTRL TX. ALL OF THIS ACTIVITY IS EVOLVING WITHIN
   AN ANAFRONTAL FLOW REGIME...WITH CONTINUED EWD SPREAD OF
   PRECIPITATION ANTICIPATED DURING THE OVERNIGHT HOURS.

   WHILE SFC TEMPERATURES ARE PRESENTLY ABOVE 0C IN MANY
   LOCATIONS...AMPLE COLD ADVECTION...COUPLED WITH NOCTURNAL COOLING
   AND AN INFLUX OF DRIER AIR FROM THE N WITH RESULTANT WET-BULB
   COOLING...WILL ALL CONTRIBUTE TOWARD PROMINENT SFC TEMPERATURE
   REDUCTIONS DURING THE OVERNIGHT HOURS. THE 0C ISOTHERM PRESENTLY
   EXTENDS FROM MIDDLE TN TOWARD SRN AR TO NEAR WACO TX...AND IS
   FORECAST TO CONTINUE ADVANCING ON A SWD/SEWD TRAJECTORY. GIVEN THE
   WEAKLY SLOPING NATURE OF THE BAROCLINIC ZONE NWD/NWWD...RESIDUAL
   WARM AIR ABOVE THE SFC SAMPLED BY THE 00Z SHV AND BNA RAOBS WILL
   RESULT IN PARTIAL TO COMPLETE MELTING OF DESCENDING HYDROMETEORS.

   SLEET AND FREEZING RAIN SHOULD BECOME INCREASINGLY DOMINANT ACROSS
   THE AREA THROUGH THE NIGHT OWING TO REFREEZING WITHIN THE
   SFC-BASED...SUB-FREEZING...POST-FRONTAL COLD LAYER. THIS WILL LIKELY
   EXTEND SWD TO PORTIONS OF THE INTERSTATE 20 CORRIDOR...ESPECIALLY
   AFTER 08Z. FREEZING RAIN RATES OF 0.05 INCH TO LOCALLY 0.10 INCH PER
   HOUR WILL BE POSSIBLE...HIGHEST IN AREAS OF CONVECTIVELY ENHANCED
   PRECIPITATION. SLEET ACCUMULATION RATES OF ONE-TENTH TO ONE-QUARTER
   INCH PER HOUR WILL ALSO BE POSSIBLE. SLEET IS EXPECTED TO BE
   RELATIVELY MORE COMMON ACROSS PARTS OF MIDDLE TN INTO SWRN
   TN...WHERE COOLING OF THE ELEVATED WARM LAYER WILL HAVE A TENDENCY
   TO REDUCE MELTING OF DESCENDING HYDROMETEORS.

   COMPARISONS WITH RECENT SFC OBSERVATIONS REVEAL MODEST WARM BIASES
   IN SFC TEMPERATURE FORECASTS FROM RECENT RUNS OF THE RAP AND NAM.
   THIS IS PARTICULARLY THE CASE INTO THE ARKLAMISS DELTA REGION AND
   VICINITY WHERE LOCAL-FLOW CHANNELING ACCENTUATES NEAR-SFC COLD
   ADVECTION IN VALLEY-PARALLEL...NLY FLOW. THIS SUBSTANTIATES THE SWD
   EXTENT OF THE DISCUSSION AREA. WHILE WARM TEMPERATURES ACROSS SOME
   OF THESE AREAS DURING THE PRECEDING DAY -- ESPECIALLY IN PARTS OF
   LA/MS -- MAY INHIBIT INITIAL ICE ACCUMULATIONS OWING TO A WARMER
   GROUND...ICE ACCUMULATIONS WILL BE OF PARTICULAR CONCERN ON ELEVATED
   SURFACES AND METAL OBJECTS.

   ..COHEN.. 03/05/2015


   ATTN...WFO...OHX...BMX...HUN...MEG...JAN...LZK...SHV...

   LAT...LON   32729078 32199285 32189454 32609469 33509241 34499070
               35798820 36588633 35868573 34468768 32729078 

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