784  
FXUS02 KWBC 091530  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1029 AM EST SUN DEC 09 2018  
 
VALID 12Z WED DEC 12 2018 - 12Z SUN DEC 16 2018  
   
..HEAVY RAIN THREAT FOR PARTS OF THE EAST FRI-SUN
 
 
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
THE WEATHER PATTERN OVER THE NEXT WEEK IS EXPECTED TO REMAIN QUITE  
ACTIVE ACROSS THE NORTHWEST AND BECOME INCREASINGLY WET FOR MUCH  
OF THE EASTERN THIRD OF THE NATION LATER IN THE WEEK. THE MODELS  
CONTINUE TO WAVER (SOMETIMES SIGNIFICANTLY) WITH THE DETAILS IN AN  
OTHERWISE RATHER PREDICTABLE LONGWAVE UPPER PATTERN THAT IS  
WELL-REPRESENTED IN THE LATEST ENSEMBLE MEANS AND ONGOING WPC  
CONTINUITY. ACCORDINGLY, THE WPC MEDIUM RANGE PRODUCT SUITE WAS  
PRIMARILY DERIVED FROM A BLEND OF THE LATEST GEFS/ECMWF ENSEMBLE  
MEANS AND CONTINUITY WITH MINORITY WEIGHTING APPLIED TO THE 00Z  
ECMWF/UKMET EARLY IN THE PERIOD (FAVORING THE SLOWER SOLUTIONS)  
BUT THE 06Z GFS LATER IN THE PERIOD (FAVORING THE BEST ENSEMBLE  
CLUSTERING WELL AWAY FROM THE MUCH FARTHER SOUTH ECMWF IN THE  
SOUTHEAST). CONFIDENCE IS NO BETTER THAN AVERAGE WITH THE TRACK OF  
THE SYSTEM NEXT SATURDAY IN THE EAST, BUT THE SIGNAL FOR MODEST TO  
LOCALLY HEAVY RAIN REMAINS FOCUSED ON THE SOUTHERN APPALACHIANS.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
PRECIPITATION/WIND PERIODICALLY INCREASES INTO THE NORTHWEST  
MIDWEEK AND AGAIN INTO THE WEEKEND AS HEIGHTS FALL WITH A  
SUCCESSION OF APPROACHING SHORTWAVES/FRONTAL SYSTEMS. HIGHER  
PACIFIC NORTHWEST TERRAIN WILL OBSERVE THE HEAVIEST RAIN/MOUNTAIN  
SNOW (WITH VARYING SNOW LEVELS AS UPPER TEMPERATURES RISE AND FALL  
BETWEEN EACH SYSTEM), WITH ACTIVITY THEN WORKING TO THE  
NORTH-CENTRAL GREAT BASIN/ROCKIES. PRECIPITATION MAY BE ENHANCED  
WITH STRONGER-THAN-FORECAST SURFACE LOWS THAT MAY TRACK NEAR/OVER  
NORTHWESTERN WASHINGTON FRIDAY BUT THESE WAVES HAVE LITTLE  
PREDICTABILITY AT THIS TIME RANGE.  
 
BY THURSDAY, A PACIFIC SHORTWAVE WILL DIVE FROM THE ROCKIES INTO  
THE SOUTHERN PLAINS TO REINFORCE UPPER TROUGHING WHILE SPAWNING  
SURFACE CYCLOGENESIS OVER OKLAHOMA. GULF MOISTURE/WARM ADVECTION  
INCREASING AHEAD OF THE DEEPENING SYSTEM WILL HELP TO EXPAND THE  
PRECIPITATION SHIELD TO THE EAST AND THEN WRAP AROUND TO THE NORTH  
OF THE SURFACE LOW. RELATIVELY SLOW MOVEMENT OF THE SYSTEM WILL  
SUPPORT A HEAVY RAIN THREAT AND SOME COLD SECTOR (MARGINALLY COLD  
AT BEST) SNOWS THROUGH THE MIDWEST/GREAT LAKES AND  
CENTRAL/NORTHERN APPALACHIANS TO THE NORTHWEST OF THE UNCERTAIN  
LOW TRACK. RAINFALL MAY BE HEAVY FROM THE EAST SIDE OF THE  
APPALACHIANS TO THE COASTAL PLAIN WITH THE INFUSION OF ATLANTIC  
MOISTURE AS WELL, WHICH MAY FALL OVER AREAS CURRENTLY RECEIVING  
APPRECIABLE SNOWFALL (ESPECIALLY OVER THE SOUTHERN APPALACHIANS).  
TEMPERATURES WILL TREND MILDER IN THE EAST (SUPPORTING A MOSTLY  
RAIN EVENT NEXT WEEKEND) BUT THE TRACK OF THE SURFACE LOW WILL  
DETERMINE THE AREAL EXTENT OF WINTRY WEATHER (MOST UNCERTAINTY IN  
THE NORTHEAST).  
 
FRACASSO/SCHICHTEL  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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