PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
427 PM EST SUN FEB 08 2009
 
VALID 00Z MON FEB 09 2009 - 00Z THU FEB 12 2009
 

DAY 1...

NRN PLAINS...
THE NAM AND GFS/ECMWF REMAIN AT ODDS OVER HOW LONG COLD AIR WILL
REMAIN IN PLACE OVER THE ERN DAKOTAS AND PARTS OF NRN MN...WHICH
HAS A LARGE IMPACT ON THE THREAT OF FREEZING RAIN POTENTIAL. THE
08/00Z ECMWF AND THE 08/12Z GFS TENDED TO WARM THE SFC UP QUICKER
THAN THE NAM.  AS A RESULT...THE AMOUNT OF FREEZING RAIN WOULD
TEND TO BE LESS FOR THE EC/GFS.  GIVEN THE MAGNITUDE OF COLD AIR
IN PLACE RIGHT AT THE START OF THE PERIOD...WE WOULD BE INCLINED
TO KEEP A RISK OF FREEZING RAIN GOING IN THE DAKOTAS DESPITE OUR
OVERALL PREFERENCE OF THE GFS AND ECMWF.  THEREFORE...WE OPTED FOR
A MDT RISK OF 0.25 INCHES OF FREEZING PCPN IN ERN NODAK...WITH A
SLIGHT RISK EXTENDING EASTWARD INTO NORTHERN/CENTRAL MN DUE TO THE
WARM ADVECTION AND INCREASING MOISTURE ALOFT.  ON THE NORTH SIDE
OF THE DEVELOPING SYS...WE ALSO INDICATED A SLIGHT RISK OF 4 INCH
SNOWFALL AMOUNTS OVER N CNTRL ND WHERE MESOSCALE FORCING OCCURS IN
THE COLD SECTOR.  

CA TO THE SRN/CNTRL ROCKIES...

ONE MID/UPR LEVEL WAVE INITIALLY OVER THE SOUTHWESTERN STATES WILL
BE CROSSING THE ROCKIES AND EJECTING OVER THE ADJACENT PLAINS
WHILE A SECOND TROF DIGGING ALONG THE WEST COAST HELPS ANCHOR LOW
HEIGHTS OVER THE REGION.  GIVEN LOW HEIGHT/THICKNESS AND
INCREASINGLY STRONG SUPPORT ALOFT...A BROAD AREA 4 INCH AND 8 INCH
SNOWFALLS WERE INDICATED WHILE KEEPING THE BEST RISK OF A FOOT OR
MORE OF SNOW IN THE TOPOGRAPHICALLY FAVORED AREAS OF THE SOUTHERN
SIERRA AND THE NORTHERN MOGOLLON RIM.

DAY 2 AND 3...

NRN PLAINS...
12Z GUIDANCE GUIDANCE TENDED TO DIVERGE A BIT AFTER GETTING INTO
BETTER AGREEMENT WITH THE TRACK AND SPEED OF THE CIRCULATION
LIFTING NWD THRU THE REGION ON THE 00Z MODEL PRODUCTION CYCLE. 
LOWERING HEIGHTS ASSOCD WITH THE APPROACHING TROUGH SHOULD ALLOW
FOR SUFFICIENT DYNAMIC COOLING FOR PCPN TO CHANGEOVER TO SNOW
ALONG THE WRN SIDE OF THE INTENSIFYING LOW LEVEL SYSTEM.  AXIS OF
POTENTIALLY HVY SNOW IS EXPECTED TO ALIGN BASICALLY NORTH/SOUTH
ACROSS ND AS SFC LOW REACHES FROM ERN SD INTO NRN MN.  8 INCH PLUS
SNOW AMTS ARE POSSIBLE IN THIS BAND WITH TIMING OF PCPN CHANGE
AROUND 00Z/10 CRITICAL TO SNOW TOTALS.  CIRC IS FCST TO FILL AND
DRIFT NWD INTO WRN ONTARIO TUES NIGHT WITH DIMINISHING SNOW OVER
THE DAKOTAS.

SRN CA TO SRN ROCKIES...
PROGRESSIVE TROUGH WITH MUCH LOWER HEIGHTS THAN THE PREVIOUS
SYSTEM WILL BRING A BATCH OF SNOW THRU THE MTNS OF SRN CA
ESPECIALLY EARLY IN THE DAY ON TUESDAY.  EMBEDDED ENERGY WILL THEN
CROSS AZ WITH A PERIOD OF HVY SNOW LIKELY ACROSS THE MOGOLLON RIM
AND PORTIONS OF UT BEFORE AFFECTING THE MTNS OF CO/NM TUESDAY INTO
WEDNESDAY.  TRACK OF THE SYSTEM WILL PROVIDE HVST SNOW ACROSS THE
AZ MTNS WITH MODELS HIGHLIGHTING CNTRL/NRN AZ WITH LOCALIZED 1
FOOT SNOW AMTS THRU THE FAVORED TERRAIN.

WA/OR...NRN CA...
NEXT IN A SERIES OF PAC SYSTEMS WILL REACH INTO THE PAC NW AND NRN
CA DURING TUES.  A POTENTIALLY STRONG LOW LEVEL CIRC MAY PROVIDE
AN INFLUX OF PAC MOISTURE FOCUSING INTO OREGON AND NRN CA WITH LOW
SNOW LEVELS.  FOLLOWED A BLEND OF THE REASONABLY SIMILAR 00Z ECMWF
AND GFS WITH THIS FEATURE.

PLAINS TO GREAT LAKES ON DAY 3...
THE GFS AND THE ECMWF BOTH DEVELOP SFC LOW PRESURE OVER KS...AND
THEN SCOOTS THE CENTER OF LOW PRESSURE NORTHEAST TOWARDS THE
SOUTHERN LOWER MI ON WEDNESDAY.  MANUAL QPF FROM HPC SHOWS
POTENTIAL FOR A COUPLE INCHES OF SNOW ALONG THE PATH.  WE OPTED TO
OUTLOOK A SLIGHT RISK IN THIS AREA GIVEN 

BANN