534  
FXUS02 KWBC 301601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 AM EST SUN DEC 30 2018  
 
VALID 12Z WED JAN 02 2019 - 12Z SUN JAN 06 2019  
   
..OVERVIEW
 
 
MEDIUM RANGE PERIOD FEATURES A CLOSED LOW IN THE SOUTHERN HIGH  
PLAINS DAY 3 (WED) PROGRESSING EASTWARD TOWARDS THE MID-ATLANTIC  
BY DAY 5 (FRI) WHILE ADDITIONAL WAVES OF ENERGY SLIDE THROUGH NEAR  
THE U.S./CANADIAN BORDER IN THE NORTHERN STREAM. OUT WEST, THE  
FIRST SHORTWAVE CROSSES INTO THE PACIFIC NORTHWEST BY LATE WEEK,  
WITH ANOTHER DEEPER (POSSIBLE CLOSED LOW?) TROUGH ARRIVING BY THE  
WEEKEND. THIS PATTERN WOULD FAVOR HEAVY RAINFALL FROM THE GULF  
COAST TO THE NORTHEAST, AND INCREASING PRECIPITATION IN THE  
PACIFIC NORTHWEST/WEST COAST.  
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
THE BIGGEST QUESTION IN THE MEDIUM RANGE REMAINS THE CLOSED LOW  
PROGRESSING EASTWARD ACROSS THE SOUTH AND EVENTUALLY LIFTING INTO  
THE MID-ATLANTIC AND OFF THE NORTHEAST COAST BY THE WEEKEND. EVEN  
AS EARLY AS DAY 3, TIMING DIFFERENCES START TO ARISE WITH THE UK  
MET REMAINING THE SLOWEST/STRONGEST (OVER THE SOUTHWEST) AND THE  
CMC/FV3-GFS ON THE FASTER SIDE (OVER THE CENTRAL/SOUTHERN PLAINS)  
WITH THE LATEST ECMWF/GFS RUNS SITTING SOMEWHERE IN THE MIDDLE.  
AFTER THIS TIME, THINGS REALLY TAKE A TURN FOR THE WORST AS THE  
THE 06Z GFS LIFTS THE ENERGY QUICKLY NORTHEASTWARD OPENING UP INTO  
MORE OF AN OPEN WAVE TYPE OF STRUCTURE, WHILE THE ECMWF/UK MET  
HOLD STRONG TO A SLOWER CLOSED LOW. LOOKING AT ENSEMBLE SPAGHETTI  
PLOTS, THERE REMAINS A SLIGHT TREND TOWARDS A SLOWER/STRONGER  
SOLUTION LIKE HAS CONSISTENTLY BEEN DEPICTED RUN AFTER RUN BY BOTH  
THE ECMWF AND THE UK MET. AFTER DAY 3, THE GFS IS WILDLY  
INCONSISTENT RUN TO RUN WITH EVOLUTION OF THIS SYSTEM. ALSO  
CONCERNING, IS THAT THE DETERMINISTIC GFS IS EVEN FASTER THAN ITS  
ENSEMBLE MEAN WHICH ALSO WOULD GIVE A BIT MORE CREDIBILITY TO THE  
SLOWER SOLUTIONS. THEREFORE, WPC PREFERS AN EARLY PERIOD BLEND  
HEAVILY WEIGHTED TOWARDS THE ECMWF, WITH INCREASING PERCENTAGES OF  
THE ENSEMBLE MEANS LATER IN THE PERIOD. THIS ALSO PROVIDES A  
FORECAST CLOSE TO THAT OF PREVIOUS SHIFT CONTINUITY.  
 
OUT IN THE WEST, DIFFERENCES IN THE DETAILS AND SLIGHT TIMING  
DIFFERENCES ARISE WITH THE SECOND TROUGH APPROACHING THE WEST  
COAST AT THE END OF THE PERIOD. GIVEN A DAY 6-7 FORECAST, A BLEND  
OF THE ECENS/GEFS MEAN SUFFICED WITH SOME SMALL PERCENTAGE OF THE  
ECMWF JUST GIVEN ITS BETTER RUN-TO-RUN CONTINUITY ACROSS THE  
BOARD.  
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
CLOSED LOW PROGRESSING IN THE SOUTH WILL PROMOTE HEAVY RAINFALL  
POTENTIAL FROM THE LOWER MISSISSIPPI VALLEY TO THE MID-ATLANTIC  
AND NORTHEAST DURING THE MID TO LATER HALF OF THIS WEEK. A SURFACE  
LOW LIFTING THROUGH THE SOUTHEAST INTO THE MID-ATLANTIC IS LIKELY  
TO PROVIDE A FOCUS FOR ENHANCED RAINFALL WITH MODELS CONTINUING TO  
SHOW THE HEAVIEST AXIS OF RAINFALL SETTING UP FROM MISSISSIPPI TO  
THE CENTRAL ATLANTIC COAST. MUCH OF THE PRECIPITATION ASSOCIATED  
WITH THIS STORM SHOULD BE IN THE FORM OF RAIN, BUT SOME CHANGEOVER  
TO SNOW IS POSSIBLE ON THE FAR NORTHERN EDGE OF THE PRECIPITATION  
SHIELD THOUGH ACCUMULATION POTENTIAL AT THIS POINT (FROM ROUGHLY  
THE MID-MISSISSIPPI VALLEY TO THE NORTHERN MID-ATLANTIC) REMAINS  
LOW. THE BEST CHANCE FOR ACCUMULATING SNOWS WILL BE ACROSS  
NORTHERN NEW ENGLAND AS THE SURFACE LOW DEEPENS ALONG THE  
NORTHEAST U.S. COAST. OUT WEST, RAIN/MOUNTAIN SNOWS MOVE INTO THE  
PACIFIC NORTHWEST BY THURSDAY, WITH MORE PRECIPITATION ARRIVING  
INTO THE WEST COAST/CALIFORNIA/EVENTUALLY THE SOUTHWEST NEXT  
WEEKEND.  
 
THE FOUR CORNERS REGION TO THE SOUTHERN PLAINS WILL BE THE COLD  
SPOT IN THE BEGINNING OF THE MEDIUM RANGE PERIOD AS AFTERNOON  
HIGHS ARE FORECAST TO BE 15-25F BELOW NORMAL. NORTHERN TIER STATES  
INTO THE EAST TRANSITION TO ABOVE NORMAL WITH THE HIGHEST  
ANOMALIES (15-20F ABOVE NORMAL) EXPECTED IN THE NORTHERN PLAINS  
THUR-FRI. THE WEST SHOULD STAY AT OR WITHIN A FEW DEGREES OF  
NORMAL THROUGH THE MEDIUM RANGE PERIOD.  
 
SANTORELLI  
 
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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