000
FOUS11 KWBC 262113
QPFHSD
PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
413 PM EST SUN DEC 26 2010
VALID 00Z MON DEC 27 2010 - 00Z THU DEC 30 2010
DAY 1...
...MID-ATLANTIC/SOUTHERN NEW ENGLAND...
WINTER STORM EXPECTED TO CONTINUE ACROSS TO DUMP HEAVY SNOWFALL
ACROSS NEW ENGLAND AND EASTERN NEW YORK AND NEW JERSEY...
OBSERVATIONS AND RADAR/SATELLITE INDICATE THE EXPECTED INTENSE
OCEAN STORM IS QUICKLY DEVELOPING...WITH MULTIPLE BANDS OF HEAVY
SNOW DEVELOPING IN NJ AND THE ADJACENT COASTAL WATERS. THIS STORM
IS EXPECTED TO OCCLUDE TONIGHT AS IT MOVES SLOWLY NORTHEAST SOUTH
OF LONG ISLAND AND THEN NORTHEAST TOWARDS CAPE COD. THE MODELS
SPLIT ON THE STORM TRACK THEREAFTER..WITH THE NAM SLOWEST AND ON
THE WESTERN SIDE OF THE GUIDANCE ENVELOPE.
THIS BRINGS HEAVY SNOW FURTHER INLAND INTO MAINE. DEEP-LAYERED
FRONTOGENESIS MOVING ACROSS SOUTHERN VT/NH/DOWNEAST MAINE COMBINES
WITH STRONG LOW LEVEL CONVERGENCE IN ONSHORE FLOW TO PRODUCE HEAVY
SNOWS THERE.
THE 12Z GFS WAS ON THE EASTERN SIDE OF THE SUITE OF
MODELS/ENSEMBLE MEMBERS...WITH THE 06Z RUN AND 06-12Z GEFS MEAN
CLUSTERING WITH THE CONSENSUS GROUP. MANUAL PROGS EMPHASIZED THE
00-12Z ECMWF/09Z SREF MEAN/00-12Z UKMET QPF AND THERMAL PROFILES.
A TIGHT QPF GRADIENT LEADS TO CONSIDERABLE VARIABILITY OF SNOW
AMOUNTS...EVEN WITHIN THE HEART OF THE HEAVY SNOW AREA. NAM CROSS
SECTIONS SHOW BANDING POTENTIAL ENHANCED WITH CONTINUING 850-700
MB FRONTOGENESIS AND A THIN LAYER OF NEGATIVE SATURATED EQUIVALENT
POTENTIAL VORTICITY CENTERED AT 700 ACROSS LONG ISLAND/NY CITY
THIS EVENING AND MOVING NORTH ACROSS CT/INTERIOR MA OVERNIGHT. ON
THE EAST SIDE OF THE CIRCULATION...THE NAM/GFS/ECMWF/UKMET SHOW A
700-600 MB DRY SLOT ADVECTING NORTH ACROSS CAPE COD AND SOUTHEAST
MA/EASTERN LONG ISLAND...SO THE BAND OF SNOW IN PLACE THIS EVENING
SHOULD PROGRESS AWAY FROM THESE AREAS AND SNOW INTENSITY SHOULD
TAPER.
...WA/OR/ID/WESTERN MT...
AN UPPER LEVEL TROUGH MOVES ONSHORE FROM THE PACIFIC AND MOVES
STEADILY ACROSS THE PACIFIC NORTHWEST AND NORTHERN ROCKIES. UPPER
DIVERGENCE WITH THE UPPER TROUGH PROVIDES ASCENT FOR THE INITIAL
BURST OF SNOW. AND AMOUNTS APPROACHING 8-12 INCHES ABOVE 4000FT
MSL ALONG THE BLUES AND THE RANGES OF THE IDAHO PANHANDLE...AND
NORTHEAST WASHINGTON. BROADLY DIFLUENT FLOW ALOFT CONTINUES
ACROSS BOTH REGIONS MONDAY WEST COAST AS THE JET INCREASES OVER
THE ADJACENT NORTH PACIFIC...PLACING THE AREAS IN THE FAVORED LEFT
EXIT REGION OF THE UPPER JET FOR ASCENT. MID-LEVEL WARM
ADVECTION/ISENTROPIC LIFT GENERATES ADDITIONAL ASCENT FOR SNOW MON
AFTERNOON IN THE WA/OR CASCADES.
MANUAL PROGS BLENDED THE 09Z SREF MEAN AND 00Z ECMWF QPF AND
THERMAL PROFILES. SNOW TO LIQUID RATIOS DERIVED FROM THE ROEBBER
METHOD RUN THROUGH THE NAM INDICATE RATIOS IN THE LOWER TEENS IN
THE CASCADES INCREASING TO UPPER TEENS IN THE RANGES OF ID.
THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.
DAY 2...
...PACIFIC NORTHWEST/NORTHERN ROCKIES...
A LOW AMPLITUDE MID LEVEL SHORTWAVE IN THE EASTERN PACIFIC RACES
ONSHORE INTO OR. THE GFS AND ECMWF INDICATE THE 700 MB JET IN OR
INCREASES TO NEAR 50 KT. WARM ADVECTION SNOWFALL SHOULD CONTINUE
ACROSS OREGON THE FIRST HALF OF THE PERIOD AND THEN INTO IDAHO.
OROGRAPHIC SNOWS BEHIND THE INITIAL SURGE OF ISENTROPIC LIFT
SHOULD OCCUR ON TUE ALONG THE COASTAL RANGES OF NORTHWEST
CALIFORNIA/SOUTHWEST OREGON AND NORTHERN SIERRA.
THIS IS SUPPORTED BY THE CORE OF THE UPPER LEVEL JET OVER THE
NORTHEAST PACIFIC COMING ONSHORE IN OR AND THEN DRIFTING SOUTH
INTO ADJACENT NORTHERN CA.
MOIST/UNSTABLE ONSHORE FLOW AND STEEP LAPSE RATES COMBINE WITH
SUSTAINED WEST-SOUTHWEST FLOW ALOFT TO SUPPORT MODERATE/HEAVY
SNOWS IN THE OLYMPICS AND CASCADES INLAND INTO THE RANGES OF
ID/NORTHWEST MT. FALLING HEIGHTS ALLOWS FOR LOWERING SNOW LEVELS
AND AN INCREASE IN SNOW IN THE CA PORTION OF THE
CASCADES/SISKIYOUS.
MANUAL GRAPHICS BLENDED THE 09Z SREF MEAN/00-12Z ECMWF QPF AND
THERMAL PROFILES TO ACCOUNT FOR SNOW LEVELS AND SLIGHT VARIATIONS
IN THE TIMING OF THE WAVE.
THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.
DAY 3...
...CALIFORNIA/GREAT BASIN/CENTRAL-NORTHERN ROCKIES...
THE UPPER TROUGH AMPLIFIES AFTER IT MOVES ONSHORE IN CA AND
CROSSES THE GREAT BASIN. WELL DEFINED UPPER DIVERGENCE/LOWER
CONVERGENCE COUPLETS COMBINE WITH UPSLOPE FLOW TO PRODUCE SNOW IN
THE SIERRA NEVADA RANGE IN CA AS THE UPPER JET AXIS COMES ONSHORE
INTO CA 00-12Z WED. THE SNOW EXTEND INLAND INTO THE RANGES OF
NORTHERN NV/WASATCH OF UT/RANGES OF WESTERN CO AS THE JET MAXIMA
PROPAGATES FURTHER INLAND AND UPPER DIVERGENCE MAXIMA CONTINUE
ACROSS EASTERN NV/UT. CONTINUING DIFLUENT FLOW AND STEEP LAPSE
RATES FURTHER NORTH CONTINUE THE SNOW THREAT FROM THE OR CASCADES
INTO THE RANGES OF SOUTHERN ID AND NORTHWEST WY. GFS SOUNDINGS IN
THE OR CASCADES SHOW 850-500 MB LAPSE RATES OF 8 DEG C/KM WITH
DEEP MOISTURE...SUPPORTING CONTINUING SNOW SHOWERS.
THE CONSENSUS HIGHEST AMOUNTS ARE IN THE CA SIERRA...WHERE THE
FORECAST HIGHEST PRECIPITABLE WATER AXIS EXTENDS FROM THE PACIFIC
OCEAN...WITH VALUES OF HALF AN INCH FOR AN EXTENDED PORTION OF
THIS PERIOD. THE HIGH RISK FOR HEAVY SNOW IS SHOWN DUE TO BETTER
THAN NORMAL AGREEMENT IN THE QPF FROM THE GFS/09-15Z SREF
MEAN/00-12Z ECMWF/12Z NAM/CANADIAN GLOBAL/UKMET ON 1-2 INCHES
LIQUID EQUIVALENT PRECIPITATION.
THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.
PETERSEN
$$
