Probability Of 4 Inches Of Snow - National USData: MapDiscussion

FOUS11 KWBC 222220
QPFHSD
 
PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
445 PM EST WED FEB 22 2012
 
VALID 00Z THU FEB 23 2012 - 00Z SUN FEB 26 2012
 

...DAY 1...

...NRN PLAINS SE INTO THE MID/UPPER MS VLY...

NRN STREAM CLIPPER LIKE SHORTWAVE WILL DIVE INTO THE FAR NRN
PLAINS BY LATE THU...
AND IN DOING SO WILL AMPLIFY THE LEE SIDE TROUGHING (MORE SRN
STREAM VORTICITY) ACROSS THE CENTRAL US OUT AHEAD OF AN ELONGATED
STRONG PAC JET AXIS. OVER TIME...
THESE TWO FEATURES WILL BECOME MORE IN PHASE (ESP BY DAYS
2/3)...HOWEVER...
IN THE SHORTER RANGE (DAY 1)...THE OPERATIONAL MODELS CONTINUE TO
SHOW DIFFERENCES IN BOTH THE MASS FIELDS AND SENSIBLE WEATHER
(PARTICULARLY WITH THE LOCATION OF THE HEAVIER QPF). HPC HAD
INITIALLY PREFERRED A BLEND OF THE 00Z ECMWF AND 12Z UKMET...
WHICH NOTED MORE RUN TO RUN CONSISTENCY COMPARED TO THE OTHER
MODELS...WHILE ALSO CLOSER TO THE EC ENSEMBLE MEAN. THE 12Z ECMWF
HAS ONLY SOLIDIFIED THIS THINKING...
AS WE CONTINUE TO SEE TWO DISTINCT MODEL CLUSTERS ON DAY 1. THE
GFS AND IN PARTICULAR THE NAM REMAIN FARTHEST N WITH THE SFC LOW
AND THUS PCPN FIELD THROUGH 00Z FRI... WHILE THE 12Z
ECMWF...UKMET...AND EVEN THE CMC ARE WELL CLUSTERED FARTHER SOUTH.
IT SHOULD BE NOTED THAT THE 18Z NAM ALSO SHIFTED FARTHER S WITH
THE HEAVIER QPF. THE UPSHOT...EXPECT AXIS OF MAXIMUM UPPER LVL
DIVERGENCE AND LOW-MID LVL DEFORMATION/FGEN TO BE MAXIMIZED N OF
THE SFC LOW TRACK...WITH SOMEWHAT OF A BROADENING REGION OF DEEP
ISENTROPIC ASCENT (TROWAL) WITH TIME PER THE 280-300K ANALYSIS
(TAPPING GULF MOISTURE LATE IN THE PERIOD). GIVEN THE CHOICE OF
MODEL(S)...HPC FAVORED A SRN TRACK WITH RESPECT TO THE SWATH OF
HEAVIEST SNOWFALL...THOUGH BECAUSE OF THE UNCERTAINTY AMONG THE
SHORT-RANGE OPERATIONAL MODELS AND ENSEMBLES...DID EXPAND AT LEAST
A LOW (10%) RISK WELL N INTO N DAKOTA AND SRN MN. 

...NORTHERN NEW ENGLAND...

A COMBINATION OF DYNAMICAL FORCING ELEMENTS LEFT OVER FROM A
CYCLONE EXITING THE UPPER GREAT LAKES WILL WORK IN CONCERT TONIGHT
OVER SOUTHEASTERN CANADA. THE IMPACT FOR NORTHERN NEW ENGLAND WILL
BE AREAS OF SNOWFALL DEVELOPING NORTH OF A CONSENSUS 12Z
ECMWF/GFS/NAM SFC LOW TRACK...WHICH APPEAR TO BE CLUSTERED FAIRLY
WELL THROUGH 36 HRS (00Z FRI). 

BULK OF THE COLDER LOW-LEVEL AIRMASS REMAINS LOCKED IN ACROSS THE
ST LAWRENCE VALLEY AND ALONG THE NY/VT AND NRN MAINE BORDER TODAY
AHEAD OF A PV ANOMALY ALOFT AND SURFACE WAVE MIGRATING SLOWLY
EASTWARD IN CONCERT ACROSS SOUTH CENTRAL ONTARIO INTO THE ST
LAWRENCE VALLEY. BROAD...DIFFUSE TROWAL ORIENTED WEST TO EAST OVER
QUEBEC AND ITS QUASI-STATIONARY POSITION INVOF NORTHERN NEW
ENGLAND PIVOTS COUNTERCLOCKWISE AS SURFACE WAVE MIGRATES EASTWARD
AND H85-H7 FRONTOGENESIS AXIS TIED TO THE SURFACE WAVE AND ITS
WARM SECTOR INVADES NEW ENGLAND FROM THE SOUTH AND SOUTHWEST. THE
RESULTING FACTORS WILL GENERATE A BROAD AREA OF MID-LEVEL MOISTURE
AND AREAS OF BANDED SNOWFALL ACROSS THE NORTHERN GREEN MOUNTAINS
OF VERMONT AND NORTHERN WHITE MOUNTAINS. THE SURFACE WAVE IS
EXPECTED TO TRACK ACROSS SOUTHERN MAINE ON THURSDAY. ALL
TOTALED...LOW TO MOD PROBABILITIES OF 4+ INCHES WILL EXIST OVER
PTNS OF NRN NY AND ACROSS VT...NRN NH...AND THE NRN HALF OF MAINE.
PROBABILITIES OF 8+ INCHES WILL REMAIN LOW ON DAY ONE...AND
CONFINED OVER NRN ME WHERE THE COLDER LLVL AIRMASS WOULD PROMOTE
MORE SNOWFALL (AT HIGHER SNOW TO LIQUID RATIOS) VS. ANY MIXED PCPN.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT ON
DAY 1.

  
...DAYS 2 AND 3...

...UPPER MIDWEST INTO NEW ENGLAND...

AFOREMENTIONED NRN/SRN STREAM TROUGHS WILL CONTINUE TO GRADUALLY
PHASE FRI AND SAT...
WITH CYCLOGENESIS GETTING GOING ON SAT AS THE LEAD TROUGH (BROADER
SRN STREAM WAVE) REACHES THE ATLC COAST. AGAIN...FAVORED A
ECMWF/UKMET CLUSTER WITH THE MASS FIELDS AND SENSIBLE
WEATHER...WHICH AGAIN HAS THE AXIS OF HEAVIEST PCPN FARTHER S
COMPARED TO THE GFS/NAM/SREF CLUSTER. 24-HR PROBABILITIES OF 4-8+
INCHES OF SNOW WILL EXPAND WITH TIME...N OF THE PROJECTED SFC LOW
TRACK...GIVEN THE DEEPENING UPPER SYSTEM...
INCREASED LOW-MID LVL FGEN...INCREASED GULF MSTR TRANSPORT
(STRONGER/DEEPER MOIST ISENTROPIC ASCENT) N TO NW INTO EXPANDING
CCB AND DEVELOPING TROWAL. GIVEN THE FVRBL (STRENGTHENING)
DYNAMICAL SETUP WITH TIME...AND EXPANDING/STRENGTHENING REGION OF
POTENTIAL VORTICITY PER THE ECMWF PV1.5 PROGS...PROBABILITIES
EXIST FOR A 4-8+ INCH SNOWFALL ACROSS SRN LWR ME ENE...NW 2/3 OF
NY...AND NRN NEW ENGLAND. PROBABILITIES FOR A FOOT OR MORE OF SNOW
WILL EXIST ACROSS FAR NRN NEW ENGLAND (NRN ME). 

...PACIFIC NORTHWEST...

CONSENSUS APPROACH FOR FRIDAY INTO SAT AS DEEP AND INTENSE SURFACE
WAVE TRACKS EASTWARD INTO SOUTHERN BC AND THE PACIFIC NORTHWEST.
DEEP-LAYERED FRONTOGENESIS RESULTS FROM A BROAD LOW-LEVEL WARM
FRONT MIGRATING WEST TO EAST FROM THE CANADIAN BORDER INTO
PORTIONS OF SOUTHERN OREGON...ATOP A COLD AIRMASS IN PLACE
ALONG/EAST OF THE CASCADE SPINE. THE SYSTEM IS QUICKLY FOLLOWED BY
A RATHER DEEP AND UNSTABLE MARITIME AIRMASS AND PRONOUNCED COLD
FRONT THAT SURGES AS FAR SOUTH AS NORTHERN CALIFORNIA AND IS WELL
EAST OF THE 115W LONGITUDE BY SATURDAY MORNING. HIGHER SNOWFALL
AMOUNTS ARE PROGGED NEAREST THE PRIMARY SURFACE LOW MOVING THROUGH
PUGET SOUND AND OLYMPICS...WITH SNOW LEVELS PLUNGING TO NEAR SEA
LEVEL. ACROSS OREGON AND NORTHERN CALIFORNIA...ALONG THE LEADING
EDGE AND TAIL END OF THE FRONT RESPECTIVELY...SNOW LEVELS FALL TO
1000-1500FT MSL ALONG THE COASTAL RANGES AND VALLEY FLOORS INLAND. 

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT ON
DAY 3.

HURLEY
$$