MESOSCALE DISCUSSION 0028
   NWS STORM PREDICTION CENTER NORMAN OK
   0644 PM CST MON JAN 17 2011
   
   AREAS AFFECTED...NRN VA...CNTRL MD...WASHINGTON D.C.
   
   CONCERNING...WINTER MIXED PRECIPITATION 
   
   VALID 180044Z - 180645Z
   
   A SWATH OF PRECIPITATION WILL EXPAND NWD INTO NRN VA/MD AND THE
   WASHINGTON D.C. METRO AREA THIS EVENING...WITH INITIAL P-TYPES OF
   SNOW/SLEET EXPECTED TO TRANSITION TO FREEZING RAIN. LIQUID
   PRECIPITATION RATES OF 0.05-0.10 IN/HR WITH LOCALLY HIGHER AMOUNTS
   SHOULD PERSIST INTO THE EARLY MORNING.
   
   WEAK ASCENT OCCURRING WITHIN A LOW-LEVEL WARM ADVECTION REGIME
   /PRIMARILY CONFINED TO THE 900-800 MB LAYER/ WILL RESULT IN AN
   EXPANSION OF A LARGE SWATH OF PRECIPITATION CURRENTLY OBSERVED OVER
   SERN VA. SFC OBSERVATIONS INDICATE LARGE T-TD SPREADS VARYING
   BETWEEN 10-20 DEG F ACROSS MUCH OF NRN VA AND CNTRL MD...WITH
   TEMPERATURES GENERALLY IN THE LOWER-MID 30S. 00Z IAD RAOB INDICATES
   THE THERMODYNAMIC PROFILE IS ENTIRELY BELOW FREEZING /SUPPORTIVE OF
   SNOW/...AND IS SLIGHTLY COLDER IN THE 900-850 MB LAYER THAN NEARLY
   ALL MODEL POINT FORECAST SOUNDINGS. AS LOW-LEVELS CONTINUE TO
   MOISTEN AND WEAK ISENTROPIC ASCENT OCCURS COINCIDENT WITH
   STRENGTHENING WARM NOSE NEAR 850 MB...THERMODYNAMIC PROFILES SHOULD
   BECOME SUPPORTIVE OF FREEZING RAIN WITH TIME...WITH NEAR-SFC COLD
   LAYER REINFORCED BY WET-BULB COOLING. CURRENT FORECAST SCENARIO IS
   AN INITIAL P-TYPE OF SNOW /OCCASIONALLY MIXED WITH SLEET/
   TRANSITIONING TO PREDOMINANTLY FREEZING RAIN.
   
   ..ROGERS.. 01/18/2011
   
   
   ATTN...WFO...PHI...AKQ...LWX...RNK...
   
   LAT...LON   38697667 38217722 37727767 37497808 37407855 37727895
               38437839 39047758 39347712 39507682 39587660 39487631
               39187617 38697667