463  
FOUS11 KWBC 060858  
QPFHSD  
 
PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
358 AM EST WED DEC 6 2017  
 
VALID 12Z WED DEC 6 2017 - 12Z SAT DEC 9 2017  
 
 
DAYS 1 THROUGH 3...  
 
***WESTERN TEXAS AND SOUTHEAST NEW MEXICO***  
 
STRONG COLD AIR ADVECTION ACROSS WESTERN TEXAS AND SOUTHERN NEW  
MEXICO BEHIND A STRONG COLD FRONT WILL INTERACT WITH MOISTURE AND  
LIFT ASSOCIATED WITH A POSITIVELY TILTED LONG WAVE TROUGH TO  
PRODUCE AREAS OF SNOW AND SOME MIXED PRECIP, MAINLY DURING THE DAY  
1 PERIOD WEDNESDAY AFTERNOON AND ESPECIALLY WEDNESDAY NIGHT, AND  
ALSO FOR THE BEGINNING OF DAY 2 THURSDAY. A MID-LEVEL SHORTWAVE  
DISTURBANCE WITHIN THIS TROUGH, ALONG WITH RIGHT ENTRANCE JET  
DYNAMICS FROM A SOUTHERN STREAM JET STREAK, WILL PROVIDE A BROAD  
AREA OF SYNOPTIC SCALE LIFT. WITH THE MID-UPPER LEVEL FORCING  
DROPPING SOUTH WITH TIME, IT ALLOWS THE SURFACE TO 700 MB LAYER TO  
COOL AS IT MOISTENS FROM NORTH TO SOUTH ACROSS WESTERN TEXAS AND  
SOUTHEAST NEW MEXICO.  
 
THERE WILL BE A FASTER RATE OF COOLING BELOW 850MB OWING TO THE  
INITIALLY SHALLOW ARCTIC AIRMASS, LEADING TO A SLIGHT INVERSION  
BEFORE THE 850-700MB COOLS BELOW FREEZING LATER IN THE DAY 1  
PERIOD. THEREFORE, A TRANSITION FROM MIXED PRECIPITATION TO SNOW  
IS EXPECTED AS THE LOWER TROPOSPHERE CONTINUES TO BECOME COLDER  
WITH TIME. THE 00Z GFS/12Z ECMWF SUPPORTED THE IDEA OF 2-4 INCHES  
OF SNOWFALL HERE, AND THESE AMOUNTS ARE SUPPORTED BY SOME ECMWF  
ENSEMBLE SPAGHETTI PLOTS INDICATING 4+ INCH SNOWFALL AMOUNTS,  
MAINLY OVER THE HIGHER TERRAIN. A MITIGATING FACTOR IN THE  
SNOWFALL AMOUNTS WILL BE THE INITIALLY WARM GROUND TEMPERATURES  
THAT COULD REDUCE ACCUMULATIONS FROM WHAT THEY OTHERWISE COULD BE.  
 
***GREAT LAKES REGION***  
 
STRONG COLD AIR ADVECTION BEHIND THE DEPARTING STORM SYSTEM AND A  
BUILDING UPPER LEVEL TROUGH WILL RESULT IN MODERATE TO LOCALLY  
HEAVY LAKE EFFECT SNOW THROUGH THURSDAY NIGHT. LAKE SURFACE TO  
850MB TEMPERATURE DIFFERENCES ON THE ORDER OF 15 TO 20 DEGREES C,  
ELEVATED INVERSION HEIGHTS TO NEAR 750MB, AND SUFFICIENT MOISTURE  
THROUGH THE LOWER AND MIDDLE TROPOSPHERE WILL FOSTER A FAVORABLE  
ENVIRONMENT FOR BANDS OF HEAVY SNOW THAT WILL MAINLY BE WEST-EAST  
ORIENTED, AND WSW TO ENE ORIENTED FOR LAKES ERIE AND ONTARIO.  
LOCALIZED SNOWFALL AMOUNTS IN EXCESS OF A FOOT ARE POSSIBLE WITH  
THE MOST PERSISTENT SNOW BANDS WHERE MINIMAL VEERING/BACKING OF  
LOW LEVEL WINDS ARE REALIZED AND WHERE CONVECTIVE ELEMENTS  
DEVELOP. THIS IS MOST LIKELY FOR THE LAKE ERIE AND LAKE ONTARIO  
SNOW BELTS. BY EARLY THURSDAY, THE WINDS ARE EXPECTED TO VEER TO  
A MORE WESTERLY COMPONENT, AND THEREFORE THE BANDS EAST OF LAKE  
ERIE SHIFT FARTHER SOUTH OF THE GREATER BUFFALO AREA. LIKEWISE,  
THE BANDS OFF LAKE ONTARIO SHIFT INTO THE TUG HILL PLATEAU. SNOW  
COVERAGE AND INTENSITY BEGINS TO TAPER OFF BY FRIDAY AS WARM AIR  
ADVECTION AHEAD OF A SHORTWAVE DISRUPTS THE LAKE EFFECT BANDING.  
 
***MID-ATLANTIC REGION***  
 
A BROAD BAROCLINIC ZONE WILL EXIST OFF THE EAST COAST FRIDAY AND  
INTO SATURDAY WITH A COUPLE WAVES OF LOW PRESSURE EXPECTED TO  
DEVELOP ALONG THE SURFACE BOUNDARY. CURRENT INDICATIONS SUGGEST  
THAT THE BULK OF THE MOISTURE SHOULD BE NEAR THE COAST AND  
OFFSHORE. THE UKMET AND CMC ARE PROJECTING MORE OF THIS MOISTURE  
FARTHER INLAND WHERE LOW LEVEL TEMPERATURES ARE MARGINAL FOR LIGHT  
SNOW ACCUMULATIONS, WHEREAS THE GFS AND NAM ARE MORE SUPPRESSED  
AND RESULT IN NO APPRECIABLE WINTER WEATHER. THE GFS HAS TRENDED  
SLIGHTLY FARTHER SOUTH WITH ITS 00Z RUN. THE MOST LIKELY SCENARIO  
AT THIS TIME WOULD BE FOR SOME SLUSHY SNOW ACCUMULATIONS UP TO AN  
INCH OVER PARTS OF THE NORTH CAROLINA AND VIRGINIA PIEDMONT  
REGION, AND ALSO FOR THE DELMARVA PENINSULA. SURFACE TEMPERATURES  
NEAR OR JUST ABOVE FREEZING, RELATIVELY WARM GROUND TEMPERATURES,  
AND THE LACK OF HEAVY PRECIPITATION RATES WILL REDUCE THE  
EFFECTIVENESS OF ANY ACCUMULATIONS.  
 
THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.  
 
D. HAMRICK  
 

 
 
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