FOUS11 KWBC 060812
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
311 AM EST TUE JAN 06 2009

VALID 12Z TUE JAN 06 2009 - 12Z FRI JAN 09 2009


DAYS 1 AND 2...

NORTHWEST TO INTERMOUNTAIN...
THE WHOLE PERIOD WILL BE MARKED WITH TREMENDOUS MOISTURE ADVECTION
INTO THE PAC NW WITH UPSLOPE ENHANCED PRECIP TAKING THE FORM OF
SNOW ONLY AT MUCH HIGHER ELEVATIONS DUE TO WAA.  THE WARM FRONT
WILL COME ASHORE EARLY TUESDAY AND WILL MARK THE FULLY MOISTENED
COLUMN AND BEGIN THE ONSLAUGHT.  REGIONALLY SAMPLED MODEL
SOUNDINGS SUGGEST SNOW LEVELS RISING THROUGH THE 5K AND EVEN 6K
LEVELS BY THE END OF DAY 1.  STILL...FOOT+ AMOUNTS WILL BE SEEN
BOTH DAYS ABOVE THOSE LEVELS...AND THE SNOW LEVEL WILL SLOWLY
LOWER ON D3 BRINGING SNOW TO A GREATER AREA AS HEIGHTS LOWER AHEAD
OF THE NEXT WAVE MENTIONED IN D3 SECTION.  THIS DEEP MOISTURE WILL
NOT BE FULLY TAPPED BY THE PACIFIC RANGES...SO EXPECT DOWNSTREAM
SIGNIFICANT SNOW ALL THE WAY TO THE NRN CO/SRN WY ROCKIES
ESPECIALLY ON D1 WITH CLIMATOLOGICALLY FAVORED NW FLOW.  SAWTOOTH
AND BITTERROOTS WILL SEE THE BRUNT OF THIS ACTIVITY THOUGH.

OH VALLEY TO GRT LAKES/NY/NEW ENGLAND...
A SRN STREAM TROF WILL BE MAKING ITS WAY EASTWARD THROUGH THE
PERIOD WITH FAST SOUTHWEST FLOW ALOFT PRECEDING THE SYS.  THIS
CONFIGURATION ALLOWS FOR INCREASING AMOUNTS OF LOW LEVEL MOISTURE
TO MAKE ITS WAY INTO THE TN/OH VALLEY TOWARDS THE APPALACHIANS.
THIS SYSTEM REMAINS MOSTLY A THERMAL CHALLENGE WITH QPF LARGELY
SETTLED UPON BY MODEL CONSENSUS.  00Z RUNS HAVE BACKED OFF QPF
SLIGHTLY...BUT WINTER IMPACTS ARE UNAFFECTED SINCE THIS HAS BEEN
AN ICING/THERMAL ISSUE TO BEGIN WITH.  LATEST THINKING IS THAT
THERE IS A RISK OF FREEZING RAIN TODAY IN A STRIPE ACROSS PARTS OF
ERN IN/OH CONCURRENT WITH THE COMPLEX TERRAIN OF VA/MD/WV.
TONIGHT...THE ACTIVITY SHIFTS TO THE EAST FACE OF THE APPALACHIANS
INTO PA AND INTERIOR MID ATLANTIC REGION...AND EVENTUALLY TO
NY/NEW ENGLAND BY WEDNESDAY.  THE HIGHEST RISK OF 0.25 INCHES OR
MORE OF FREEZING RAIN WAS INDICATED IN THE COMPLEX TERRAIN OF WRN
VA.  SURROUNDING THIS AREA IS A FAIRLY BROAD SLIGHT RISK OF 0.25
INCHES OR MORE OF FREEZING RAIN THAT EXTENDS EASTWARD TOWARDS THE
I95 CORRIDOR.  THE BROAD AREA REFLECTS THE THERMAL UNCERTAINTY
HINGING UPON THE STRENGTH OF THE HI PRES TO THE NORTH.  RANDOMLY
SAMPLED MODEL SOUNDINGS SHOWING A MIX-AND-MATCH OF -4C TO -5C COLD
LAYERS WHICH WOULD INTRODUCE SLEET AND LESSEN THE FZRA. SRN NY IN
PARTICULAR HAS THIS CHALLENGE.  HAVE USED THE MODEL DIAG PREFERRED
EC/GFS/SREF BLEND FOR THIS FORECAST PACKAGE.

BY D3 THE MID LEVEL CIRCULATION WILL HAVE DRIFTED NORTHEAST...WITH
A POSSIBLE REDEVELOPMENT ALONG THE NEW ENGLAND COAST/JUST
OFFSHORE.  THE STRONG SOUTHWEST FLOW ALOFT SHOULD CONTINUE
ADVECTING COPIOUS MOISTURE NORTH ACROSS NEW ENGLAND. DIFFERENCES
IN PHASING AND STRENGTH BETWEEN THE ORIGINAL LOW CROSSING FROM
ONTARIO TO ADJACENT QUEBEC AND THE RATE OF DEVELOPMENT/TIMING OF
THE COASTAL LOW LEAD TO ANOTHER LG SPREAD IN QPF AMOUNTS AND
THERMAL PROFILES D2.  THE GFS/NAM/SREF MEAN ALL SUPPORT CONTINUING
LIGHT FREEZING RAIN IN INTERIOR SOUTHEAST NY AND SOUTHERN NEW
ENGLAND...WITH PREDOMINANTLY SNOW UP FROM THE NY ADIRONDACKS
ACROSS NORTH VT/NH AND ACROSS INTERIOR MAINE.

THE SNOW THREAT NORTH OF THE CRITICAL LINE IS A BIT EASIER TO
PREDICT...AND THE PREFERRED EC/GFS/SREF BLEND GIVES 6-10 INCHES
FROM UPSTATE NY TO NRN ME THROUGH THE TWO DAY PERIOD.

DAY 3...
NORTHEAST...
LK ENHANCED SNOW SHOWERS SHOULD DEVELOP DOWNSTREAM FROM THE LAKES
IN THE WAKE OF THE DEPARTING LOW...AS WELL AS CONTINUE INTO THE
HIGHER ELEVATIONS OF WV/WESTERN MD-PA AND EVEN THROUGH NRN ME
UNTIL THE SYSTEM FULLY DEPARTS BY LATE IN DAY 3.

NORTHERN ROCKIES/CASCADES...
ANOTHER JET MAXIMA WILL MOVE ONSHORE JUST AHEAD OF THE NEXT UPPER
TROF ON THU MORNING.  THE SNOW LEVELS SHOULD DECREASE AS THE WAVE
MOVES INLAND AND THE THICKNESS FIELDS RESPOND TO THE STRONG COLD
ADVECTION.  THE MODELS STILL SHOW STRONG 850-700 MB MOISTURE
FLUXES ACRS WA/OR/ID THRU THE PERIOD...SO HIGH QPF TOTALS ARE
INDICATED ACROSS THE MODEL/ENSEMBLE SUITE. TEMP PROFILES AGAIN
SUPPORT PRIMARILY THE NORTHERN WA CASCADES AND THE RANGES OF ID
AND WEST MT/NORTHWEST WY RECEIVING THE GREATER SNOW AMOUNTS.
MANUAL PROGS BLENDED MAINLY THE ECMWF/GFS FOR PCPN AMOUNTS AND
GFS/SREF THERMAL PROFILES.

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

SOWKO
$$