Quantitative Precipitation Forecast
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000 FOUS11 KWBC 010753 QPFHSD PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 352 AM EDT SAT NOV 01 2014 VALID 12Z SAT NOV 01 2014 - 12Z TUE NOV 04 2014 DAYS 1-3... ...INTERMOUNTAIN WEST/NORTHERN AND CENTRAL ROCKIES... A DEEP UPPER TROUGH IS FCST TO BE ONSHORE IN CA THIS MORNING. A 700 MB LOW DOWNSTREAM NEAR THE NV/OR BORDER AT 12Z THIS MORNING MOVES NORTHEAST ACROSS ID AND WESTERN MT TONIGHT. MODEST COUPLING OF NORTHERN AND SOUTHERN STREAM JET STREAKS LEADS TO UPPER DIVERGENCE MAXIMA /MID LEVEL DEFORMATION BEING MAXIMIZED OVER CENTRAL ID TO WESTERN MT. ON SUNDAY THE PRIMARY 700 MB LOW MOVES OUT OF MT INTO THE CANADIAN PRAIRIES. THE DEPARTURE OF THIS LOW LEADS TO A DIMINISHING THREAT ACROSS THE NORTHERN ROCKIES ON MON. DIFFERENCES IN TIMING OF THE LOW CONTINUE TO DRIVE HOW LONG THE SYSTEM REMAINS A THREAT TO PRODUCE SNOW AS THE TREND FOR LATER IN THE PERIOD IS FOR DRYING AND THE END OF THE EVENT. THE THREAT FOR HEAVY SNOW SHIFTS FURTHER SOUTH INTO THE CENTRAL ROCKIES AS THE SOUTHERN STREAM WAVE IS FORECAST TO LIFT THROUGH THE FOUR-CORNERS REGION. LEFT-EXIT REGION UPPER JET DYNAMICS ALONG WITH LOW-MID LEVEL FRONTOGENESIS AND OROGRAPHIC EFFECTS ARE EXPECTED TO CONTRIBUTE TO HEAVIER AMOUNTS ACROSS THE RANGES OF WESTERN CO. MANUAL PROGS BLENDED CONTINUITY AND A CONSENSUS OF THE 00Z MODELS AND 21Z SREF MEAN QPF/TEMP PROFILES. THE TIMING OF THE DEPARTURE OF THE UPPER TROUGH WILL DETERMINE THE EXTENT OF LINGERING SNOW SHOWERS EARLY ON DAY 3 (MON). THE UPPER TROUGH PASSAGE LEADS TO CESSATION OF RISING MOTION AND TRANSITION TO SINKING MOTION AND DRYING AIRMASS...SO SNOW SHOULD SUBSIDE AS DAY 3 PROGRESSES. ...MAINE... MODELS SHOW A PAIR OF LOWS IN THE COASTAL TO OFFSHORE WATERS WITH THE INITIAL SURGE OF 700 MB ASCENT OCCURRING AS THE FIRST LOW MOVES NORTH AND THE 700 MB FRONT SETTING UP ACROSS MAINE. SNOW DEVELOPS AS COLD AIR IS INGESTED INTO THE SYSTEM AS IT DEEPENS WITH WET BULB COOLING OCCURRING AS WELL. UNCERTAINTY PERSISTS IN DOWNEAST MAIN AS INITIAL CONDITIONS ARE TOO WARM FOR SNOW...WITH THE COLD AIR MORE LIKELY TO MAINTAIN SUSTAINED SNOW FURTHER INLAND. ON DAY 2 (SUN)...A LARGE UPPER DIVERGENCE/MID LEVEL CONVERGENCE COUPLET THE FIRST SEVERAL HOURS OF THE PERIOD SHOULD PRODUCE A NARROW STRIPE OF HEAVY SNOW...WITH AMOUNTS UP TO A FOOT ACROSS PORTIONS OF EASTERN MAINE. THIS SECOND LOW ON DAY 2 BECOMES STRONG AND THUS HEAVIER AMOUNTS ARE INDICATED ON DAY 2 AS MID LEVEL FRONTOGENESIS AIDS ASCENT. DIFFERENCES BEGIN TO DEVELOP AS THE 00Z NAM IS ON THE LEFT SIDE OF THE SUITE OF TRACK OPTIONS...AND CANADIAN GLOBAL ON THE RIGHT. THE 00Z GFS/UKMET/ECMWF FORM THE MIDDLE CLUSTER AND THUS FORM THE PREFERRED SUITE OF SOLUTIONS. AS THE LOW GOES BY..ENHANCED COLD ADVECTION ALLOWS THE RAIN TO SNOW TRANSITION ZONE TO MOVE EAST TOWARDS THE COAST. THE TYPICAL TRACK UNCERTAINTY IS AUGMENTED BY PRECIP TYPE UNCERTAINTY AND SHARP QPF GRADIENTS SO THE FORECAST IS NOT A CONFIDENT ONE TONIGHT. ...CENTRAL/SOUTHERN APPALACHIANS... SNOW HAS BEGUN IN THE HIGHER ELEVATIONS IN EASTERN TN AND WESTERN NC. A ZONE OF MID LEVEL DEFORMATION CONTINUES INTO THIS MORNING BEFORE THE 700 MB CIRCULATION DEPARTS FOR THE COAST AND ASCENT SUBSIDES THIS AFTERNOON. THE SUBSIDENCE RESULTS IN DRYING AND THEN A SHARP DECREASE IN SNOWFALL COVERAGE AND INTENSITY. COLD AIR ALOFT WILL SURGE ACROSS THE APPALACHIAN SPINE WITH DEFORMATION LIFT AIDED BY WITH UPSLOPE FLOW FOR SNOWFALL IN THE MOUNTAINS AND ONLY MINOR ACCUMULATIONS AWAY FROM THE TERRAIN. WITH THE FORECASTS REMAINING IN GOOD AGREEMENT....WPC RELIED HEAVILY ON A BLEND OF CONTINUITY WITH THE 21Z SREF MEAN/00Z WRF ARW QPF AND TEMPERATURE PROFILES TO GENERATE SNOW AMTS/PROBABILITIES. THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT. PETERSEN $$

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