896  
FXUS02 KWBC 120700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
200 AM EST WED DEC 12 2018  
 
VALID 12Z SAT DEC 15 2018 - 12Z WED DEC 19 2018  
   
..HEAVY PRECIPITATION THREAT FOR THE NORTHWEST
 
   
..HEAVY RAIN THREAT FOR THE EAST
 
 
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
MODELS AND ENSEMBLES AGREE TO SHOW A THREAT FOR PERIODS OF HEAVY  
PRECIPITATION ACROSS THE NORTHWEST/NORTHERN CA THIS PERIOD AS WELL  
AS A LINGERING HEAVY RAINFALL THREAT INTO THIS WEEKEND FROM THE  
SOUTHEAST/APPALACHIANS/MID-ATLANTIC TO SOUTHERN NEW ENGLAND.  
HOWEVER, SIGNIFICANT TIMING AND DEPTH DIFFERENCES AMONGST GUIDANCE  
PORTENDS BELOW AVERAGE FORECAST CONFIDENCE FOR DAILY FORECASTS  
SPECIFICS IN BOTH AREAS. ACCORDINGLY, THE WPC MEDIUM RANGE PRODUCT  
SUITE WAS PRIMARILY DERIVED FROM THE LATEST ECMWF ENSEMBLE MEAN  
THAT OFFERS A COMPROMISE GUIDANCE TIMING/STRENGTH COMPOSITE  
SOLUTION FOR BOTH AREAS.  
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
A LEAD DEEP CLOSED LOW WILL EJECT THIS WEEKEND FROM THE  
SOUTHEAST/SOUTHERN MID-ATLANTIC TO THE WESTERN ATLANTIC AND AMPLE  
GULF MOISTURE WILL FUEL LINGERING HEAVY PRECIPITATION TO THE EAST  
AND WRAPPING TO THE NORTH OF MAIN/RE-DEVELOPING SURFACE LOWS.  
HEAVY RAINFALL WILL ROBUSTLY SPREAD THROUGH THE APPALACHIANS AND  
EASTERN SEABOARD WITH ADDED INFUSION OF ATLANTIC MOISTURE WHERE  
RUNOFF PROBLEMS WILL BE ENHANCED IN AREAS WITH RECENT HEAVY  
SNOWFALL AND ONGOING RIVER FLOODING. SOME HEAVY SNOW MAY ALSO  
OCCUR IN A MARGINALLY COOLED AIRMASS FROM THE NORTH-CENTRAL  
APPALACHIANS TO THE INTERIOR NORTHEAST THIS WEEKEND ON THE  
NORTHERN PERIPHERY OF THE BROAD PRECIPITATION SHIELD.  
 
UPSTREAM, PRECIPITATION/WIND SUBSTANTIALLY INCREASES FROM COASTAL  
WASHINGTON TO NORTHERN CA INTO THE WEEKEND, WITH RESURGENCE  
EARLY-MID NEXT WEEK AS A SUCCESSION OF ENERGETIC PACIFIC  
SHORTWAVES/FRONTAL SYSTEMS PASS THROUGH THE REGION. HIGHER PACIFIC  
NORTHWEST TERRAIN WILL SEE THE HEAVIEST RAIN/MOUNTAIN SNOW (WITH  
VARYING SNOW LEVELS AS TEMPERATURES RISE AND FALL BETWEEN EACH  
SYSTEM, WITH SNOWS THEN WORKING TO THE NORTH-CENTRAL GREAT  
BASIN/ROCKIES. WHILE THIS ACTIVITY REMAINS CONCENTRATED WITHIN A  
PRONOUNCED NORTHERN STREAM FLOW, THE CONCURRENT DEVELOPMENT OF A  
SEPARATED SOUTHERN STREAM WILL MEANWHILE CARVE OUT A SOUTHWEST  
U.S. TO MEXICO CLOSED TROUGH ALOFT AND ASSOCIATED SURFACE SYSTEM  
THAT OFFERS HEAVY RAIN POTENTIAL FOR THE SOUTHERN PLAINS, BUT ONLY  
THE DETERMINISTIC CANADIAN MODEL IS PROGRESSIVE ENOUGH TO DEVELOP  
DEEP ACTIVITY BEFORE THE END OF DAY 7.  
 
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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