Quantitative Precipitation Forecast
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000 FOUS11 KWBC 232037 QPFHSD PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 336 PM EST SUN NOV 23 2014 VALID 00Z MON NOV 24 2014 - 00Z THU NOV 27 2014 ...PACIFIC NORTHWEST/GREAT BASIN/NORTHERN AND CENTRAL ROCKIES... THE MODELS FCST MOSTLY LIGHT SNOW AMOUNTS ON DAY 1 AS THE MODELS CONTINUE TO SHOW A POWERFUL AND ELONGATED UPPER JET BUILDING DOWNSTREAM OF A HIGH AMPLITUDE RIDGE OVER THE NORTHEAST PACIFIC. THE JET STREAMS FROM WA INTO ID/WESTERN MT/UT WITH SPORADIC UPPER DIVERGENCE MAXIMA COMBINING WITH UPSLOPE CONDITIONS TO PRODUCE SNOW SHOWERS IN THE RANGES FROM WA/ID/WESTERN MY/WESTERN WY/NORTHERN UT/WESTERN CO. ON WED THE DEVELOPMENT OF A LOW LEVEL FRONT OCCURS OVER MT SANDWICHED BETWEEN STRONG HIGH PRESSURE OVER NORTHERN CA/NORTHERN NV AND ANOTHER HIGH OVER WESTERN CANADA. LOW LEVEL FRONTOGENESIS MAXIMA EXTEND ACROSS NORTHERN WA TO NORTHERN ID TO WESTERN MT AND NORTHERN WY. WARM ADVECTION AND 700 MB CONVERGENCE MAXIMA OCCUR AS THE 700 MB JET STREAM ACROSS NORTHERN WA/ID/WESTERN MT...AND THESE FACTORS COMBINE WITH THE FRONTAL LIFT TO PRODUCE SNOWS IN THE FAVORED MOUNTAINS. ON TUE NIGHT-WED...THE STRONG UPPER JET STRADDLES THE US/CANADIAN BORDER NEAR MT/ID/NORTHEAST WA. UPPER DIVERGENCE MAXIMA IN THE RIGHT ENTRANCE REGION ARE LIKELY TO PRODUCE MORE SNOWS IN NORTHERN ID AND WESTERN MT AS THE 700 MB JET NEAR 60 KT FCST BY THE GFS CROSSES THESE AREAS...CONTINUING THE CLUSTERS OF 700 MB CONVERGENCE. THIS COMBINES WITH THE PERSISTENT LOW LEVEL FRONT ACROSS NORTHERN ID AND WESTERN MT IN PRODUCING ASCENT AND RESULTANT SNOWS IN THE REGION. THE UNCERTAINTY RELATES TO THE FRONTAL TIMING AS THE 12Z ECMWF HAS MOVED THE FRONT FASTER ACROSS NORTHERN MT...CUTTING BACK ON POTENTIAL QPF/SNOW AMOUNTS. MANUAL PROGS GAVE MORE WEIGHTING TO THE 12Z ECMWF AND 12Z UKMET...AS THE GFS AMOUNTS ARE ALMOST TWICE AS MUCH AS THE GEFS MEAN AMOUNTS. ...MID-UPPER MS VALLEY AND UPPER MIDWEST/GREAT LAKES... MODELS HAVE MADE ADJUSTMENTS AND NOW CLUSTER PRETTY WELL ON THE PATH AND INTENSITY OF THE LOW DEVELOPING IN THE MID MS VALLEY AND THEN CROSSING THE GREAT LAKES MONDAY MORNING. GIVEN THE NAM MOVED EAST FROM PRIOR RUNS...THIS CUT BACK ON HEAVY SNOW POTENTIAL FOR WESTERN WI. THE MODELS AGREE BETTER ON HIGHER QPF TOTALS AFTER IT BECOMES COLD ENOUGH FOR SNOW IN EASTERN IOWA TO NORTHERN IL/ADJACENT SOUTHERN WI. IN SPITE OF BETTER CYCLONE FORECASTS...THERE ARE STILL DIFFERENCES ON POST-FRONTAL QPF AND DURATION/INTENSITY OF SNOW ONCE THE RAIN TO SNOW TRANSITION OCCURS. LAKE ENHANCEMENT AND LAKE EFFECT ARE ALSO EXPECTED TO BOOST SNOWFALL TOTALS DOWNWIND OF LAKES SUPERIOR AND MICHIGAN. ...NORTHERN PLAINS TO UPPER MS VALLEY TUE NIGHT-WED... AN AREA OF LIGHT SNOW IS FCST ACROSS NORTHEASTERN NORTH DAKOTA AND MN AS THE NEXT 700 MB WAVE MOVES SOUTHEAST FROM THE NORTHERN PLAINS TOWARDS THE MID-UPPER MS VALLEY. MOSTLY LIGHT SNOWS ARE EXPECTED WITH LOCAL ENHANCEMENT POSSIBLE NEAR THE LK SUPERIOR SHORE IN MN..AS UPPER DIVERGENCE MAXIMA OCCUR NEAR THE SLOWLY MOVING UPPER TROUGH AXIS. MANUAL PROGS USED A MULTI-MODEL AND SHORT RANGE ENSEMBLE MEAN BLEND. ...SOUTHERN TO CENTRAL APPALACHIANS/MID ATLANTIC/NORTHEAST.... THE MODELS/ENSEMBLE SHOW THE LIKELIHOOD OF A CYCLONE TO DEVELOP IN THE COASTAL WATERS OFF THE SOUTHEAST COAST AND MOVE ROUGHLY PARALLEL TO THE COAST OF THE MID ATLANTIC AND THEN NEW ENGLAND. TRACK TIMING AND INTENSITY DIFFERENCES RESULT IN DIFFERENCES IN THE SNOW AXIS/ORIENTATION. THOSE SOLUTIONS LIKE THE 12Z UKMET/12Z ECMWF THAT ARE CLOSER TO THE COAST ALLOW WARM AIR TO PENETRATE INLAND WHEREAS THE MORE OFFSHORE CYCLONE TRACKS OF THE 09Z SREF MEAN/12Z GEFS MEAN ALLOW SNOWS IN THE WASHINGTON DC TO PHILADELPHIA AND NEW YORK CITY METRO AREAS. SNOW OCCURS WITHIN THE MID LEVEL FRONTAL CIRCULATION/DEFORMATION ZONE NORTHWEST OF THE CYCLONE. THE HIGH MAGNITUDE OF THE FRONTOGENESIS COULD LEAD TO A NARROW BAND OF HEAVY SNOW WITHIN A COUPLED JET REGION INLAND AWAY FROM THE COAST. THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT DAYS 1-3. PETERSEN $$

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