Quantitative Precipitation Forecast
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000 FOUS11 KWBC 040848 QPFHSD PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 347 AM EST WED MAR 04 2015 VALID 12Z WED MAR 04 2015 - 12Z SAT MAR 07 2015 DAYS 1 THROUGH 3... ...SOUTHERN PLAINS/LOWER AND MID MS VALLEY/GULF COAST... MOISTURE TRANSPORTED NORTHEAST FROM THE PACIFIC AND WESTERN GULF OF MEXICO IS FUNNELED ALONG A FRONTAL BOUNDARY EXTENDING FROM THE SOUTHERN PLAINS THROUGH THE NORTHERN MID ATLANTIC STATES ON DAY 1. THE FRONT DROPS SOUTH ACROSS THE GULF COAST STATES ON DAY 2...AS COLD AIR POURS SOUTH FROM THE MIDWEST. THE COMBINATION OF MOISTURE AND COLD AIR PRODUCES SNOW...SLEET AND FREEZING RAIN ACROSS THE REGION...MAINLY ON DAYS 2 AND 3. FOR THE MOST PART...THERE WAS GOOD MODEL AGREEMENT WITH RESPECT TO THE OVERALL SYNOPTIC SETUP...THOUGH THERE WERE SOME DIFFERENCES CONCERNING THERMAL FIELDS. THE 00Z GFS APPEARED TO BE TOO FAST WITH LONG WAVE TROUGH DROPPING OUT OF THE NORTHERN PLAINS...WHICH DRIVES THE LOW LEVEL COLD AIR FASTER TO THE SOUTH. THE 00Z NAM/12Z ECMWF WERE CLOSER WITH THE MID LEVEL SOLUTION...AS WELL AS A SLOWER RATE OF COOLING. THUS...THE THERMAL PORTION OF THE FORECAST WAS BASED ON A BLEND OF THE 00Z NAM/12Z ECMWF...AND THE QPF PORTION OF THE FORECAST WAS BASED ON THE LATEST WPC QPF. DAY 1... AS THE DEVELOPING LONG WAVE TROUGH CROSSES THE NORTHERN PLAINS AND GREAT LAKES...IT DRIVES LOW LEVEL COLD AIR SOUTH ACROSS THE SOUTHERN PLAINS AND THE LOWER AND MID MS VALLEY DURING THE FIRST HALF OF DAY 1. THE MID LEVELS ARE SLOWER TO RESPOND...AS THE FLOW BACKS SLOWLY. THIS SCENARIO PROMOTES FREEZING RAIN ACROSS THE SOUTHERN PLAINS FROM CENTRAL TX AND SOUTHEAST OK INTO AR INTO NORTHERN AND CENTRAL LA. THE BEST LIFT ASSOCIATED WITH THE MOISTURE FEED OCCURS ACROSS AR AND NORTHERN LA...WHERE A LARGE AREA OF 0.10 TO 0.25 INCHES OF QPF IS EXPECTED (WHICH WAS SUPPORTED WELL BY THE 00Z NAM). THERE IS A MULTI MODEL SIGNAL FOR 0.25+ INCH ICE AMOUNTS ACROSS EAST CENTRAL AR INTO NORTHWEST MS AND EXTREME SOUTHWEST TN. AS THE MID LEVEL HEIGHTS DROP...THE COLUMN BECOMES COLD ENOUGH TO SUPPORT SNOW FIRST ACROSS CENTRAL TX AND SOUTHEAST OK. IN THIS AREA...QPF SUPPORTS 2 TO 4 INCHES OF SNOWFALL AS THE LAST MID LEVEL IMPULSE TRACKS TO THE NORTHEAST. THE CHANGEOVER OCCURS MORE QUICKLY ACROSS NORTHERN AR AND WESTERN TN...WHERE THE DEEP MOISTURE AND LIFT IN THE DENDRITIC GROWTH ZONE PRODUCES AN AREA OF 4 TO 8 INCHES OF SNOWFALL. THESE AMOUNTS ARE WELL SUPPORTED BY BOTH THE 21Z SREF AND THE LATEST ECMWF ENSEMBLE OUTPUT. DAY 2... SHALLOW COLD AIR DRIVES SOUTH TO THE LOWER MS VALLEY AND GULF COAST DURING DAY 2. THE MID LEVELS REMAIN TOO WARM TO SUPPORT MUCH IN THE WAY OF SLEET ACROSS LA/MS/AL/NORTH GA...BUT THE QPF COULD SUPPORT LOCAL 0.10 INCH ICE AMOUNTS STRETCHING FROM PORTIONS CENTRAL MS ACROSS THE MOUNTAINS OF NORTH GA. ...OH VALLEY/MID ATLANTIC/SOUTHERN NEW ENGLAND... MOISTURE FUNNELED ALONG A FRONTAL BOUNDARY STREAKING THROUGH THE OH VALLEY INTO THE MID ATLANTIC AND SOUTHERN NEW ENGLAND AS COLD AIR DROPS INTO PLACE LATE ON DAY 1 AND DURING DAY 2. THE COMBINATION OF MOISTURE AND COLD AIR DRIVING SOUTH RESULTS IN THE POTENTIAL FOR HEAVY SNOW ACROSS THE OH VALLEY AND MID ATLANTIC STATES DURING THIS TIME PERIOD. WHILE THERE WAS GENERALLY GOOD MODEL AGREEMENT WITH RESPECT TO THE OVERALL SYNOPTIC SETUP...THERE WERE SOME SUBTLE BUT IMPORTANT THERMAL FIELD DIFFERENCES LATE ON DAY 1 AND EARLY ON DAY 2...ESPECIALLY ACROSS THE MID ATLANTIC. THE 00Z GFS IS A BIT FASTER WITH THE COLD AIR IN THE LOWER AND MID LEVELS...RESULTING IN FASTER PRECIPITATION CHANGE TO SNOW IN THE MID ATLANTIC. THE 00Z NAM/12Z ECMWF SOLUTION IS A TAD SLOWER...WHICH SLOWS THE TRANSITION FROM SLEET TO SNOW ACROSS THE MID ATLANTIC LATE ON DAY 1. THEREFORE...THE THERMAL PORTION OF THE FORECAST WAS BASED ON THE 00Z NAM/12Z ECMWF...WHILE THE QPF PORTION OF THE FORECAST WAS BASED ON THE LATEST WPC QPF. DAY 1... MOISTURE STREAMING ACROSS THE TN VALLEY PRODUCES RAIN ACROSS THIS REGION DURING THE FIRST PART OF DAY 1...AS THE COLUMN IS NOT YET COLD ENOUGH FOR SNOW AND SLEET. LOW LEVEL COLD AIR DRIVING SOUTH PRODUCES A FREEZING RAIN PROFILE ACROSS PORTIONS OF WESTERN TN AND SOUTHWEST KY AFTER 05/00Z...AS TEMPERATURE CHANGES IN THE MID LEVEL LAG. THERE IS MULTI MODEL SUPPORT FOR 0.10 TO 0.25 INCHES OF ICE BETWEEN 05/00Z AND 05/06Z...AFTER WHICH TIME THE MID LEVEL COOL ENOUGH FOR A CHANGE TO SLEET. FURTHER NORTH...LOWERING MID LEVEL HEIGHTS ASSOCIATED WITH THE LONG WAVE TROUGH DEVELOPING OVER THE NORTHERN PLAINS AND GREAT LAKES STARTS COOLING THE MID LEVELS FROM NORTH TO SOUTH ACROSS THE OH VALLEY. USING THE BLEND OF THE 00Z NAM/12Z ECMWF AS A BASIS...THE CHANGE FROM SLEET TO SNOW OCCURS ACROSS THE OH VALLEY BEFORE 05/06Z...AS THE BEST MID LEVEL FRONTOGENETIC LIFT OCCURS FROM KY INT NORTHERN WV. MODEL SOUNDINGS ALONG THIS AXIS ALSO SHOW MOISTURE AND LIFT IN THE DENDRITIC GROWTH ZONE...WHICH SHOULD RESULT IN HEAVY SNOW BETWEEN 05/06Z AND 15/12Z IN THIS AREA. THERE IS SUPPORT FROM BOTH THE LATEST ECMWF ENSEMBLE OUTPUT AND THE 21Z SREF FOR 12+ INCH AMOUNTS STRETCHING FROM NORTHERN KY INTO NORTHERN WV AND SOUTHWEST PA. THESE AMOUNTS ARE PART OF A LONGER AXIS OF LIFT AND MOISTURE OF WHICH SUPPORTS 6 TO 10 INCHES OF SNOW EXTENDING FROM SOUTHEAST MO THROUGH NORTHWEST TN...MUCH OF KY...SOUTHERN OH AND NORTHERN WV. THE CHANGE FROM SNOW TO SLEET OCCURS BETWEEN 15/06Z AND 15/12Z ACROSS THE SOUTHERN AND CENTRAL PA...NORTHERN MD AND NORTHERN VA...MUCH OF NJ AND SOUTHERN NEW ENGLAND (FROM NORTH TO SOUTH). THE 00Z GFS IS A BIT FASTER THAN THE 00Z NAM/12Z ECMWF BRINGING THE LOW LEVEL BOUNDARY (AND LOWER AND MID LEVEL COOLING)...WHICH ACCELERATES THE PROCESS INTO NORTHERN MD/DE. AT THIS POINT...THAT LOOKS TOO FAST...AND THE 4 TO 6 INCH AXIS WAS PLACED OVER PA/NJ. THIS TREND IS BETTER DEPICTED BY THE ECMWF ENSEMBLE OUTPUT THAN THE 21Z SREF OUTPUT. DAY 2... THE BEST MID LEVEL FRONTOGENETIC FORCING NORTH OF THE SURFACE BOUNDARY STRETCHES FROM NORTHERN VA THROUGH CENTRAL AND SOUTHERN NJ TO A POSITION SOUTH OF LONG ISLAND. ALONG THIS AXIS...MODEL SOUNDINGS SHOWED THE LIFT AND MOISTURE CONTINUING TO STREAM NORTHEAST ALONG THE BOUNDARY FROM THE WESTERN GULF OF MEXICO IN THE DENDRITIC GROWTH ZONE BETWEEN 0612Z AND 06/18Z. THIS SUPPORTS A LONG AXIS OF 6 TO 8 INCHES OF SNOW IN THIS CORRIDOR...WHICH INCLUDED KDCA/KBWI THROUGH KILG/KACY. THESE AMOUNTS ARE SUPPORTED BY THE LATEST ECMWF ENSEMBLE OUTPUT...WHICH SHOWED AN 8+ INCH STRIPE ALONG THIS AXIS. THE 21Z SREF AXIS WAS A TAD FURTHER NORTH...BUT SHOWED SIMILAR AMOUNTS. FURTHER SOUTH...THE COLD AIR DRIVES SOUTH WITH THE LOW LEVEL BOUNDARY...BUT THE SLOWLY BACKING MID LEVEL KEEPS THE LEVEL BETWEEN 800 MB AND 70 MB TOO WARM FOR SNOW. THE TRANSITION FROM FREEZING RAIN TO SLEET OCCURS ACROSS SOUTHEAST VA ACROSS MUCH OF NC. MUCH OF THE QPF DURING THE SUBFREEZING PORTION OF DAY 2 OCCURS AS FREEZING RAIN...WITH LOCAL 0.10 TO 0.25 INCH ICE AMOUNTS POSSIBLE OVER EAST CENTRAL AND FAR WESTERN NC. DAY 3... AS THE MOISTURE PULLS AWAY FROM THE MID ATLANTIC COAST AT THE START OF DAY 3...LOW LEVEL COLD AIR REMAINS WEDGED IN ALONG EASTERN NC. THE MID LEVEL REMAINS WARM...SO THERE IS NO ICE IN THE DENDRITIC GROWTH ZONE FOR SNOW OR EVEN SLEET. ICE AMOUNTS BETWEEN 0.01 AND 0.05 INCHES ARE EXPECTED ALONG COASTAL NC. HAYES $$

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