000 FOUS11 KWBC 092056 QPFHSD PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD 355 PM EST THU FEB 09 2012 VALID 00Z FRI FEB 10 2012 - 00Z MON FEB 13 2012 DAY 1... ...NORTHERN CASCADES/NORTHERN ROCKIES... THE HIGH AMPLITUDE MID-UPPER RIDGE CENTERED OVER WRN NOAM DRIFTS EAST... WITH A MID-UPPER TROUGH DRIFTING TOWARDS THE COAST. A WEAK SFC LOW MOVES SLOWLY NORTH OFF THE COAST. PACIFIC MOISTURE ADVECTS NORTH ACROSS WESTERN OR/WA OVERNIGHT AND FRI WITH OCCASIONAL 300 MB DIVERGENCE MAXIMA COMING INTO THE CASCADES. THE MODELS CLUSTER IN THE RANGE OF 0.15-0.4 INCHES LIQUID EQUIVALENT...LEADING TO A LOW RISK OF FOUR INCHES OF SNOW. FURTHER INLAND...A BAND OF CONFLUENT FLOW WITH A BACKDOOR COLD FRONT DEVELOPS AS HIGH PRESSURE MOVES EAST ON TO THE PLAINS AND RETURN SOUTHEAST FLOW DEVELOPS. 700 BM CONVERGENCE DEVELOPS FRI AFTERNOON ALONG WITH WEAK 300 MB DIVERGENCE IN THE RIGHT ENTRANCE REGION OF THE UPPER JET. THIS IS EXPECTED TO RESULT IN LIGHT SNOWS DEVELOPING WITH ACCUMS OF 4 INCHES OR MORE ACROSS PORTIONS OF THE NORTHERN ROCKIES. PROBABILITIES WERE EQUALLY WEIGHTED AMONG THE INDIVIDUAL MODELS/SREF MEAN QPF/THERMAL PROFILES. THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT. DAY 2... ...EASTERN NEW ENGLAND/CENTRAL APPALACHIANS... MODELS SHOW THE DEVELOPMENT OF SFC LOW OFF THE MID ATLANTIC COAST AND MOVING NORTHEAST OFF THE NEW ENGLAND COAST SAT...REACHING THE CANADIAN MARITIMES BY SUN MORNING. THE DEVELOPMENT IS COMPLEX WITH MOST OPERATIONAL MODELS SHOWING A DUAL LOW STRUCTURE AND DIFFERENCES BETWEEN MORE DEVELOPMENT/CLOSER TO THE COAST WITH THE NORTHWEST-MOST LOW AND WHETHER THE STRONGER DEVELOPMENT OCCURS WITH THE SOUTHEAST MOST LOW. THE PHASING OF NORTHERN/SOUTHERN STREAM SHORTWAVES AND MAGNITUDE OF THE TROUGH COMING OFF THE EAST COAST WILL HELP DETERMINE WHICH CLUSTER IS MORE CORRECT. PROBABILITIES FOR 4 AND 8 INCHES ARE SHOWN IN CASE THE 12Z-18Z NAM/12Z GEFS MEAN IDEAS OF THE STRONGER/CLOSER IN LOW ARE MORE CORRECT AND MORE SNOW OCCURS ACROSS SOUTHEAST NY/NEW ENGLAND. PRECIP TYPES REMAIN UNCERTAIN AS INDIVIDUAL SOLUTIONS ALSO SHOW POTENTIAL FOR MIXED PRECIPITATION TYPES IN EASTERN LONG ISLAND TO CAPE COD. THE 12Z ECMWF NUDGE OF THE PRIMARY LOW FURTHER OFFSHORE KEEPS RISKS AS MODERATE FOR NOW. LOW LEVEL CONVERGENCE DEVELOPS WITH A COLD FRONT PASSING THROUGH THE OH VLY/CNTRL APPALACHIANS. POST-FRONTAL PERSISTENT MOISTURE COMBINES WITH UPSLOPE NORTHWEST BNDRY LAYER WINDS AND LOCAL 300 MB DIVERGENCE NEAR THE JET CORE TO PRODUCE SOME LOCALLY HVY SNOWS ON SAT ALONG THE WRN SLOPES OF THE WV MTNS. ...RANGES OF WYOMING/ADJACENT CO-SOUTH CENTRAL MT.... THE FRONTAL ZONE PERSISTS OVER THE CENTRAL AND NORTHERN ROCKIES AS THE HIGH BUILDS SOUTHEAST ACROSS THE PLAINS STATES. THE INITIAL 300 MB JET MAXIMA EXTENDS NORTHWEST TO SOUTHEAST ACROSS WY AND THEN PROPAGATES SOUTHEAST INTO THE PLAINS...WITH AN UPPER LEVEL RIDGE BUILDING EAST ACROSS THE ROCKIES. LIKEWISE...THE INITIAL BANDS OF 700 MB CONVERGENCE EXTENDING NORTHWEST-SOUTHEAST ACROSS THE STATE LINGER FOR SEVERAL HOURS AND THEN WEAKEN WITH TIME. THE HIGH QPF AMOUNTS ARE FOCUSED ON THE RANGES OF NORTHWEST AND SOUTHEAST WY...WHERE THE HIGHER PROBABILITIES ARE SHOWN. THE LIMITED DURATION OF THE EVENT KEEPS PROBABILITIES FROM REACHING A HIGH LEVEL. THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT. DAY 3... ...CENTRAL-SOUTHERN GREAT BASIN/ROCKIES... A MID-UPPER TROUGH AND SFC LOW DRIFT EAST ACROSS THE GREAT BASIN...WITH THE UPPER LEVEL JET MAXIMA CROSSING AZ/NM. UPPER DIVERGENCE MAXIMA WITHIN DIFLUENT FLOW ALOFT CROSS THE RANGES OF UT/CO/NORTHERN AZ-NORTHERN NM. MOISTURE WILL BE LIMITED WITH THIS SYSTEM...WITH AREAL PRECIPITABLE WATER VALUES AVERAGING NEAR A QUARTER INCH. HOWEVER...700 MB CONVERGENCE IN ADVANCE OF THE FRONTAL BOUNDARY ACROSS UT/CO/NORTHERN AZ/NORTHER NM AIDS OROGRAPHICS IN PRODUCING ASCENT. THIS RESULTS IN SNOW THREAT AREAS IN PARTS OF THE WASATCH AND SAN JUAN MTNS. THE GFS SHOWS FASTER INLAND MOVEMENT OF THE TROUGH AND THUS WAS GIVEN LESS WEIGHT. THE 00-12Z ECMWF WAS CONSISTENTLY SLOWER AND CLUSTERS WELL WITH THE TIMING OF THE 09Z SREF MEAN/12Z GEFS MEAN/12Z UKMET...SO THESE SOLUTIONS WERE GIVEN GREATER WEIGHTING. THE NAM HAS THE HIGHEST AMOUNTS DUE TO STRONG SUSTAINED 700 MB CONVERGENCE COMING INTO THE SAN JUANS. ...GREAT LAKES... THE MODELS FCST LOW LEVEL CONFLUENT FLOW WITH A BNDRY LAYER WIND MAXIMA EXTENDING FROM LK SUPERIOR ACROSS NORTHERN LK HURON ND THEN DOWNSTREAM INTO LK ONTARIO/ADJACENT PORTIONS OF NEW YORK. AN ENHANCED BAND OF HIGHER RH FLOWS IN CONJUNCTION WITH THE LOW LEVEL JET...SUPPORTING A PERIOD OF LK EFFECT SNOWS. WITH THE LOW LEVEL RIDGE BUILDING FROM THE UPPER MS VALLEY TOWARDS THE UPPER LAKES ON SUN...THE COVERAGE/INTENSITY WILL WIND DOWN. AS A RESULT...PART OF THE FORECAST UNCERTAINTY WILL BE EVENT DURATION. THE SHORT DURATION OF FAVROABLE DEEP MOISTURE LEAVES PROBABILITIIES AS LOW. THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT. PETERSEN $$