PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 311 AM EST SAT DEC 27 2014 VALID 12Z SAT DEC 27 2014 - 12Z TUE DEC 30 2014 DAYS 1 THROUGH 3... ...PACIFIC NORTHWEST/NORTHERN GREAT BASIN/NORTHERN TO CENTRAL ROCKIES... THE MODELS SHOW A LONGWAVE TROUGH GRADUALLY DRIFTING SOUTH FROM BRITISH COLUMBIA WITH THE 300 MB JET AXIS ON DAY 1 EXTENDING ACROSS WA/OR INLAND INTO ID. UPPER DIVERGENCE MAXIMA IN THE PROXIMITY OF THE JET HELP PRODUCE ASCENT AND COMBINE WITH HIGH AVAILABLE MOISTURE TO MODERATE TO HEAVY SNOW ACROSS THE HIGHER TERRAIN ACROSS THE CASCADES OF WA/OR...THE BLUE MOUNTAINS IN OR...AND THE BITTERROOT AND SAWTOOTH RANGES OF ID. THE COMBINATION OF LIFT AND MOISTURE SUPPORTS AN AXIS OF 8 INCHES OR MORE OF SNOWFALL ACROSS THE WA CASCADES...CONTINUING INLAND TO THE RANGES OF NORTHERN ID. LESS COVERAGE OF HEAVIER AMOUNTS IS EXPECTED OVER THE OR CASCADES TO THE BLUE MNTNS AND RANGES OF SOUTHERN ID. ON DAY 2...THE CONTINUED AMPLIFICATION OF THE WESTERN TROUGH RESULTS IN A STEADY SOUTH MOVEMENT OF THE JET ACROSS ACROSS THE NORTHERN TO CENTRAL GREAT BASIN...WITH SNOW DEVELOPING AS FAR SOUTH AS UT AND CO AS THE 300 MB JET DROPS INTO THESE STATES SUNDAY NIGHT. THE PERSISTENCE OF THE JET OVER OR LEADS TO A CONTINUING THREAT OF SNOWS IN THE OR CASCADES...WITH THE LONGER DURATION HERE LEADING TO HIGHER AMOUNTS THAN OTHER AREAS.STEEP LAPSE RATES AND HIGH RELATIVE HUMIDITY LEAD TO FAVORABLE CONDITIONS FOR SNOW SHOWERS IN THE RANGES OF ID INTO WESTERN MT WITH HIGH SNOW RATIOS RESULTING FROM THE 7-8 DEG C/KM LAPSES FROM 700-500 MB. A BROAD AREA OF UPSLOPE FLOW DEVELOPS ACROSS THE NORTHERN ROCKIES BEHIND THE ARCTIC FRONT. THE ECMWF SHOWS ANOTHER SHORTWAVE ROTATING THROUGH THE BASE OF THE TROUGH ACROSS OR AND ID ON MONDAY...WITH DIFLUENT FLOW IN THE UPPER TROUGH SUPPORTING SPORADIC UPPER DIVERGENCE MAXIMA AND MORE SNOW SHOWERS IN THE OR CASCADES INTO THE RANGES OF ID. THE JET AT THE BASE OF THE TROUGH SUPPORTS MORE SNOW SHOWS ALONG THE JET STREAM CROSSING THE HIGHER ELEVATIONS OF NV AND THEN UT AND CO. AS A LOW AMPLITUDE UPPER TROUGH EXITS FOR THE CENTRAL PLAINS...THE WEST TO EAST JET PROMOTES SNOW SHOWERS EXTENDING EAST ON TO THE PLAINS OF CO AND NORTHWEST KS AND ADJACENT SOUTHWEST NEBRASKA. THE MODELS HAVE COME INTO BETTER AGREEMENT WITH THE GFS/ECMWF/UKMET CLUSTERING WELL WITH THE SOUTH DRIFT TO THE LOW-MID LEVEL FRONT AND CIRCULATION. ...UPPER MS VALLEY/GREAT LAKES... A BAND OF MID LEVEL FRONTOGENESIS STRETCHING SOUTHWEST TO NORTHEAST ACROSS THE UPPER MIDWEST TO THE UPPER GREAT LAKES SHOULD RESULT IN AREAS OF SNOW OVER SOUTHERN MN OVERNIGHT TO CONTINUE MOVING NORTHEAST ACROSS NORTHERN WI AND THE UP OF MI. BROAD SCALE SYNOPTIC ASCENT AIDED BY UPPER DIVERGENCE MAXIMA IN THE LEFT EXIT REGION OF THE 300 MB JET AND A CENTER OF LOW PRESSURE MOVING NORTHEAST ALONG A COLD FRONT WILL EVENTUALLY BE AIDED BY A PERIOD OF ENHANCED MID LEVEL LIFT. POCKETS OF NEGATIVE EQUIVALENT SATURATED POTENTIAL VORTICITY WITHIN A REGION OF FRONTOGENESIS SHOULD LEAD TO SOME LOCALLY HEAVIER SNOWFALL AMOUNTS ACROSS THE UPPER MIDWEST INTO THE WESTERN GREAT LAKES BEFORE THE SYSTEM MOVES NORTH OF THE BORDER AND FRONTOGENESIS WEAKENS SAT NIGHT..THE MODELS HAVE AVERAGE AGREEMENT ON THE AXIS OF SNOW AND AMOUNTS AND A MULTI-MODEL/21Z SREF MEAN CONSENSUS BASED APPROACH WAS TAKEN ALONG WITH CONTINUITY FOR AMOUNTS.
Posted on: Sat, 27 Dec 2014 19:48:59 +0000
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