PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
429 PM EST WED MAR 04 2009
 
VALID 00Z THU MAR 05 2009 - 00Z SUN MAR 08 2009
 

DAY 1...

WESTERN STATES...

THE HEAVY SNOW THREAT ACRS THE SIERRA IS COMING TO AN END THIS
MORNING AS HT FALLS SHIFT INLAND THRU THE GTBASIN AND WEAKEN THE
UPSLOPE FLOW INTO THE SIERRA.  UPSLOPE WL STRENGTHEN INLAND AHEAD
OF THE INLAND MOVING HT FALLS..WITH HEAVY SNOWS LIKELY FROM THE
WASATCH..NEWD INTO THE TETONS OF FAR ERN ID...NWRN MT AND SCNTRL
MT...ALONG WITH THE SAWTOOTH AND SALMON RIVER MTS OF ID. AT THE
SAME TIME AS THE UPSLOPE FLOW STRENGTHENS ACRS THE ABOVE MENTIONED
REGIONS...MID TO UPR LEVEL HT FALLS AMPLIFYING AS THEY DROP SWD
FROM B.C. INTO NERN WA/NRN ID WL STRENGTHEN THE WLY LOW TO MID
LEVEL FLOW ACRS THE PAC NW INTO THE NRN ROCKIES.  WHILE PW VALUES
WL BE NEAR SEASONAL NORMS...MOISTURE FLUX VALUES WL RISE TO 1 STD
ABOVE THE MEAN THU OVR THE PAC NW AND INTO THE NRN ROCKIES.  THIS
WL SUPPORT HEAVY SNOW TOTALS THRU THE WA/CNTRL TO NRN OR
CASCADES...BLUE MTS...BITTERROOT RANGE AND ROCKIES OF NWRN MT. 
FOR QPF DETAILS...A BLEND OF THE NAM AND ECMWF WAS USED DAY 1 FOR
DETAILS.

NRN PLAINS/UPR MS VALLEY/UPR LAKES...

WEAK SHRTWV ENERGY EJECTING NEWD FROM THE WRN TROF...ACRS THE NRN
PLAINS AND INTO THE UPR MS VALLEY WL STRENGTHEN LARGE SCALE LIFT
ACRS THE NRN PLAINS/UPR MS VALLEY/UPR LAKES
REGION DAY 1.  PREDOMINANT WINTER PTYPE THREAT WL BE FREEZING RAIN
WITH SOUNDINGS FROM THE NAM AND GFS BOTH SHOWING AN ABOVE FREEZING
LAYER ABOVE 900-850MB.  OVERALL...OPERATIONAL AND SREF QPF ARE
SHOWING LIGHT AMTS WITH THE FREEZING RAIN THREAT.  A FEW OF THE
LATEST SREF MEMBERS DID SHOW .25"+ PCPN AMTS IN A NARROW AXIS SW
TO NORTHEAST AXIS ACRS NRN ND.  FOR THIS REASON...A SLIGHT
PROBABILITY OF ICE ACCUMULATIONS OVR .25" WAS INCLUDED ON THE DAY
1 PROBABILITIES ACRS THIS AREA.

DAY 2...

ROCKIES/GTBASIN...

THE STRONG HT FALLS DIVING INTO THE PAC NW DAY 1 WL CONT TO DIVE
SWD DAY 2...AND ALONG WITH HT FALLS EJECTING NEWD FROM CA INTO THE
GTBASIN...WILL AMPLIFY TROFFING FROM THE ROCKIES SWD INTO THE
GTBASIN DURG DAY 2.  WLY UPSLOPE FLOW WL FAVOR ADDITIONAL HEAVY
SNOWS FROM THE OR CASCADES...EWD INTO ALL OF THE NRN ROCKIES OF
ID...MT AND WY.  UPSLOPE FLOW INTO THE WASATCH IS EXPECTED TO
WEAKEN DAY 2 WITH CORRESPONDING LOWER SNOW TOTALS.  UPSLOPE SWLY
FLOW...HOWEVER...WL BE STRENGTHEN INTO THE CO ROCKIES TOWARD THE
END OF DAY 2.  FOR QPF DETAILS...A BLEND OF THE ECMWF...NAM AND
GFS WERE USED.

NORTHEASTERN U.S...

THE HT FALLS MOVG INTO THE UPR MS VALLEY AT THE END OF DAY 1 ARE
EXPECTED TO CONT TO AMPLIFY DAY 2 AS THEY PUSH NEWD THRU ONTARIO
AND QB.  LATEST GUIDANCE IS CONSISTENT WITH PAST RUNS IN SHOWING
STRENGTHENING WAA ON THE SE SIDE OF THIS SYSTEM SUPPORTING A SNOW
TO SLEET/FZ RAIN/RAIN TRANSITION ACRS NRN NY INTO NRN NEW ENGLAND.
 THE OVERALL QUICK MOVEMENT OF THE SFC LOW ACRS SERN CAN SHOULD
KEEP PCPN AMTS FROM BEING HEAVY.  OPERATIONAL AND SREF GUIDANCE IN
AGRMNT WITH AREAL AVG .10-.25" PCPN AMTS..WITH 2"+ SNOWFALL AMTS
DEPICTED FAR NRN NY AND ACRS NRN NEW ENGLAND.  UNCERTAIN HOW FAR
TO THE SOUTH THERE WL BE ANY ICING DAY 2.  MODEL SOUNDING SUGGEST
SFC COLD AIR MAY HANG ON EARLY FRI TO SUPPORT SOME ICING FROM THE
POCONOS..CATSKILLS. HOWEVER..WITH THE SFC HIGH MOVG
OFFSHORE...ICING ACRS THESE AREAS SHOULD HAVE LOW IMPACT.

DAY 3...

GTBASIN INTO THE CNTRL ROCKIES...

THERE IS FAIRLY GOOD MODEL AGRMNT WITH THE EVOLUTION OF THE LARGE
SCALE FLOW ACRS THE GTBASIN REGION DAY 3.  FURTHER AMPLIFICATION
TO THE TROF MOVG EWD THRU THE GTBASIN EXPECTED...WITH THE TROF
MOVG INTO THE CNTRL ROCKIES BY THE END OF DAY 3.  ADDITIONAL HEAVY
SNOWS LIKELY ACRS WRN CO WHERE FAVORABLE SWLY LOW TO MID LEVEL
FLOW WL PERSIST...ESPECIALLY DURG THE FIRST HALF OF DAY 3.  WLY
FLOW INTO THE WASATCH WL RE-STRENGTHEN SLIGHTLY DAY 3 FROM DAY 2
VALUES...WITH ADDITIONAL LIGHT TO MDT AMTS.

CNTRL TO NRN PLAINS TOWARD THE UPR MS VALLEY/UPR LAKES...

THE AMPLIFYING MID TO UPR LOW OVR THE GTBASIN/CNTRL ROCKIES WL
HELP INDUCE CYCLOGENESIS ALONG THE ARCTIC FRONTAL BNDRY LATE FRI
AFTERNOON INVOF ERN CO/WRN KS..WITH THIS LOW MOVG QUICKLY ENEWD
TOWARD THE UPR MS VALLEY DURG SAT.  WELL DEFINED LIFT AHEAD OF
THIS WAVE AND NORTH OF THE ARCTIC FRONT WL SUPPORT AN AXIS OF
OVERRUNNING PCPN FROM INVOF SERN WY...ENEWD INTO PORTIONS OF THE
CNTRL TO NRN PLAINS AND UPR MS VALLEY.  A BLEND OF THE OPERATIONAL
AND SREF GUIDANCE WAS USED...WITH MDT TO POSSIBLY LOCALLY HEAVY
SNOWS FROM SERN WY INTO NWRN NE/SWRN SD DEPICTED DAY 3.  

MODEL PTYPES SHOW A QUICKER TRANSITION FROM SNOW TO FREEZING RAIN
OVR THE UPR MS VALLEY AND INTO THE UPR LAKES.  SNOW TOTALS HERE
EXPECTED TO BE LIGHTER THAN UPSTREAM OVR THE CNTRL TO NRN
PLAINS...WITH LIGHT ICING POSSIBLE.

PAC NW...

THE NEXT DIGGING HT FALLS TO HEAD SEWD FROM WRN CAN AND TOWARD THE
MEAN WRN TROF WL PUSH THE ASSOC SFCFNT SEWD INTO THE PAC NW SAT
AFTERNOON.  STRENGTHENING UPSLOPE FLOW INTO THE WA CASCADES WL
SUPPORT MOSTLY LIGHT SNOWFALL AMTS LATE DAY 3.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT

ORAVEC