392  
FXUS02 KWBC 010310  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1010 PM EST MON DEC 31 2018  
 
VALID 12Z FRI JAN 04 2019 - 12Z TUE JAN 08 2019  
   
..OVERVIEW
 
 
A PHASING/STRENGTHENING NEGATIVELY TILTED TROUGH STRETCHING ACROSS  
THE NORTHEAST PACIFIC FROM OFFSHORE ALASKA TO OFFSHORE THE WEST  
COAST OF THE LOWER 48 IS EXPECTED TO HELP EXPAND/STRENGTHEN  
MID-LEVEL POSITIVE HEIGHT ANOMALIES ACROSS THE GREAT LAKES/MIDWEST  
WITH TIME, WHICH ALLOWS TROUGHING ACROSS ATLANTIC CANADA TO  
STRENGTHEN DOWNSTREAM.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
THE GUIDANCE SHOWS SMALL DIFFERENCES WITH THE FIRST SYSTEM  
TRACKING FROM THE MID-MISSISSIPPI VALLEY ACROSS THE MID-ATLANTIC  
STATES OFFSHORE NOVA SCOTIA FRIDAY INTO SUNDAY; A COMPROMISE OF  
THE 00Z GFS/00Z ECMWF/00Z UKMET SHOULD WORK OUT FINE HERE.  
HOWEVER, DIFFERENCES CROP UP WITHIN THE DETAILS OF THE NEGATIVELY  
TILTED TROUGH OFFSHORE WESTERN NORTH AMERICA, INCLUDING A SYSTEM  
MOVING ACROSS THE SOUTHERN TIER OF THE LOWER 48. THE 18Z GFS WAS  
THE PREFERRED PIECE OF DETERMINISTIC GUIDANCE WITH THIS SYSTEM AS  
IT BEST MATCHES THE 12Z ECMWF/12Z BIAS-CORRECTED NAEFS MEAN.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
A DEEP LAYER CYCLONE FRIDAY AND SATURDAY SHOULD PROMOTE A QUICK  
SHOT OF MODERATE TO HEAVY RAINFALL FROM THE MID-MISSISSIPPI VALLEY  
TO THE MID-ATLANTIC AND SOUTHERN NEW ENGLAND COASTS. MUCH OF THE  
PRECIP ASSOCIATED WITH THIS SYSTEM SHOULD BE IN THE FORM OF RAIN,  
BUT SOME CHANGEOVER TO FREEZING RAIN/SNOW IS POSSIBLE ON THE FAR  
NORTHERN EDGE OF THE PRECIPITATION SHIELD, THOUGH ANY  
ACCUMULATIONS APPEAR LIGHT AT THIS TIME.  
 
OUT WEST, THE FIRST SHORTWAVE ON FRIDAY SHOULD BRING RAIN/MOUNTAIN  
SNOWS INTO THE PACIFIC NORTHWEST WHILE THE APPROACHING TROUGH THIS  
WEEKEND INTO EARLY NEXT WEEK SPREADS PRECIPITATION INTO CALIFORNIA  
(WITH HEAVIER PRECIP LIKELY FOCUSED ALONG FAVORED TERRAIN IN  
SOUTHERN CALIFORNIA) AND EVENTUALLY MOVING INTO THE SOUTHWEST AND  
COLORADO ROCKIES BEFORE THE HEAVY PRECIPITATION FOCUS SHIFTS INTO  
THE NORTHWEST EARLY NEXT WEEK. HEAVY MOUNTAIN SNOWS ARE POSSIBLE  
ESPECIALLY FOR THE SIERRA NEVADAS AND HIGHER ELEVATIONS OF ARIZONA  
BEFORE SHIFTING INTO THE PACIFIC NORTHWEST LATE IN THE PERIOD.  
 
THE BEST CHANCE FOR BELOW TO MUCH BELOW NORMAL TEMPERATURES SHOULD  
BE EARLY ON ACROSS THE FOUR CORNERS INTO THE SOUTHERN PLAINS WHERE  
AFTERNOON HIGHS COULD BE 10-20F BELOW NORMAL. THE WESTERN U.S.  
SHOULD STAY NEAR NORMAL THROUGHOUT THE MEDIUM RANGE PERIOD WHILE  
TEMPERATURES EAST OF THE ROCKIES SHOULD BE NEAR TO ABOVE NORMAL.  
 
ROTH/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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