000 FOUS11 KWBC 080818 QPFHSD PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD 317 AM EST SUN NOV 08 2009 VALID 12Z SUN NOV 08 2009 - 12Z WED NOV 11 2009 DAYS 1/3... PACIFIC NORTHWEST/NORTHERN ROCKIES... DAY ONE IS EXPECTED TO BE A TRANSITIONAL PERIOD AS UPPER LEVEL FLOW STARTS AS ZONAL WITH UPPER LEVEL DIVERGENCE MAXIMA SUPPORTING CONTINUING SNOW SHOWERS IN THE WA CASCADES TO FAR NORTHERN ID/NORTHWEST MT. A RELATIVE LULL DEVELOPS THIS EVENING AS THE MID-UPPER LEVEL RIDGE MOVES ONSHORE AND SINKING MOTION EXPANDS ACROSS THE REGION. WARM ADVECTION AND MOISTENING CONDITIONS BRING A THREAT OF RENEWED PRECIP IN WESTERN WA THE LAST FEW HOURS OF THIS PERIOD AS THE NEXT UPPER LOW DRIFTS EAST FROM THE NORTHERN PACIFIC. A MDT RISK OF FOUR INCHES IS SHOWN IN THE NORTHERN WA CASCADES WHERE THE HIGHER QPF AMOUNTS ARE SHOWN IN THE MODELS/ENSEMBLE MEANS. ON DAY TWO...AS THE UPPER LOW OVER THE NORTHEAST PACIFIC DRIFTS CLOSER...THE UPPER LEVEL JET MOVES ONSHORE AND INTO THE PACIFIC NORTHWEST. UPPER LEVEL DIVERGENCE MAXIMA AND THE INCREASE IN PRECIPITABLE WATER VALUES SUPPORT ASCENT OCCURRING ACROSS THE WINDWARD SLOPES OF THE WA CASCADES AND MTNS OF NRN IDAHO/NERN WASHINGTON. WARM ADVECTION ON DEEP LAYER SOUTHWEST FLOW SHOULD KEEP SNOW LEVELS HIGH IN THE LATTER RANGES. MANUAL FORECAST USED A BLEND OF THE ECMWF/GFS/SREF MEAN TO GENERATE THE SNOW TOTALS/PROBABILITIES. ON DAY THREE...THE SLOW EASTWARD MOVEMENT OF THE UPPER LEVEL TROUGH LEADS TO CONTINUED DEEP LAYER SOUTHWEST FLOW. THE PRIMARY DIFFERENCE FROM DAY TWO IS GRADUAL COOLING OF TEMPERATURES ALOFT...WITH STEEPENING LAPSE RATES TO 7-8 DEG C/KM. THIS WILL PROMOTE CONVECTIVE ELEMENTS WITHIN THE SNOW SHOWERS. THE 00Z GFS WAS SLOWER TO MOVE THE TROUGH EAST AND THE 12-00Z ECMWF/21Z SREF MEAN/00Z NAM/0Z CANADIAN GLOBAL/00Z UKMET ALL EXTEND HIGHER PRECIP AMOUNTS FURTHER INLAND INTO WA THAN THE 00Z GFS. SNOW AMOUNTS/PROBABILITIES WERE COMPOSED OF WEIGHTING MORE TOWARDS THE ECMWF/UKMET/CANADIAN/NAM/SREF MEAN QPF AND THERMAL PROFILES. THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT. PETERSEN $$