936  
FXUS02 KWBC 261540  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1039 AM EST WED DEC 26 2018  
 
VALID 12Z SAT DEC 29 2018 - 12Z WED JAN 02 2019  
   
..OVERVIEW
 
 
POSITIVELY-TILTED TROUGHING WILL RELOAD IN THE WEST AS SUBTROPICAL  
UPPER RIDGING LINGERS OVER THE BAHAMAS. THIS ALLOWS NORTHERN  
STREAM FLOW TO PROGRESS ALONG THE US/CANADIAN BORDER TO THE SOUTH  
OF AN UPPER LOW OVER HUDSON BAY. A FRONT WILL LINGER OVER THE GULF  
OF MEXICO WITH SURFACE WAVES MOVING NORTHEASTWARD THROUGH THE  
SOUTHEAST AS A PACIFIC SYSTEM PUSHES EAST SUN-TUE FROM  
NORTHWESTERN MONTANA TO THE GREAT LAKES.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
A CONSENSUS OF THE LATEST DETERMINISTIC GUIDANCE (00Z  
ECMWF/UKMET/CMC AND 06Z GFS) SERVED AS A GOOD STARTING POINT FOR  
THE FORECAST ON DAY 3 (SAT). DIFFERENCES SURROUNDING NORTHERN  
STREAM SHORTWAVE ENERGY AND THE ASSOCIATED LOW PRESSURE SYSTEM  
CROSSING THE NORTHERN PLAINS/MIDWEST SUN-MON RESULTED IN A TREND  
AWAY FROM THE GFS AND TOWARD HEAVIER ECMWF/CMC WEIGHTING DURING  
DAYS 4-5 (SUN-MON). THE GFS AND MOST GEFS MEMBERS WERE ON THE  
NORTHERN EDGE OF THE ENSEMBLE SPREAD WITH THE TRACK OF THIS  
SURFACE LOW, AND A SOLUTION CLOSER TO THE MORE SOUTHERN ECMWF/CMC  
TRACK WAS PREFERRED. FURTHER DIFFERENCES WITH ADDITIONAL SHORTWAVE  
ENERGY DIVING SOUTH ACROSS THE ROCKIES AND REINFORCING THE BROADER  
UPPER TROUGH MON-TUE RESULTED IN A CONTINUED TREND AWAY FROM THE  
GFS AND TOWARD ECMWF/CMC AND ECENS/NAEFS MEAN SOLUTIONS. THE GFS  
HAS CONTINUED TO BE MUCH MORE PROGRESSIVE THAN THE OVERALL  
CONSENSUS WITH THE LARGE SCALE UPPER TROUGH, WHILE THE  
AFOREMENTIONED PREFERRED SOLUTIONS KEEP HIGHER HEIGHTS ALONG THE  
EASTERN SEABOARD WEST OF THE SUBTROPICAL RIDGE, AND KEEP THE LOWER  
HEIGHTS THROUGH DAY 7 (WED) CONFINED TO FROM THE ROCKIES TO THE  
OHIO VALLEY. BY VERY LATE IN THE FORECAST PERIOD, RUN-TO-RUN  
CONTINUITY AMONG DETERMINISTIC SOLUTIONS DECLINES TO THE POINT  
THAT ECENS/NAEFS ENSEMBLE MEANS COMPRISED OVER 2/3 OF THE FORECAST  
BLEND.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
COLD FRONT IN THE EAST WILL LINGER IN THE GULF THROUGH THE PERIOD,  
SPREADING RAIN OVER THE SOUTHEAST IN BATCHES AS A SFC WAVE OR TWO  
RIDE ALONG THE BOUNDARY. THIS MAY EVEN FALL AS SNOW ON THE  
NORTHERN FRINGE GIVEN SUFFICIENTLY COLD AIR. NEXT PACIFIC SYSTEM  
ENTERS LATE SATURDAY INTO SUNDAY WITH WIDESPREAD BUT MOSTLY LIGHT  
TO LOCALLY MODEST RAIN/SNOW THROUGH MUCH OF THE INTERIOR WEST  
CONTINUING ON MONDAY BUT WINDING DOWN. THAT SURFACE LOW WILL MOVE  
ALONG THE US/CANADIAN BORDER AND DRAG ITS COLD FRONT TO THE SOUTH  
AND EAST. THIS WILL ALLOW GULF MOISTURE TO AGAIN FLOW NORTHWARD  
AND EXPAND/ENHANCE THE RAIN SHIELD THROUGH THE SOUTHEAST NEXT  
MONDAY/TUESDAY DEPENDING ON HOW THE FRONT IN THE GULF RESPONDS.  
 
TEMPERATURES WILL BE COOLER THAN AVERAGE IN THE WEST AND PLAINS  
WITH COLDEST ANOMALIES (10-20F BELOW AVERAGE) LIKELY FOCUSED OVER  
THE HIGH PLAINS AND SOUTHERN ROCKIES/WEST TEXAS. MUCH ABOVE  
AVERAGE WARMTH (PLUS 10-30F ANOMALIES) WILL BE DRAWN NORTHWARD  
AHEAD OF THE FRONT SATURDAY ALONG/EAST OF THE APPALACHIANS. COOLER  
AIR WILL FILTER INTO THE EAST IN THE WAKE OF THE COLD FRONTAL  
PASSAGE SATURDAY BUT WITH NEAR AVERAGE TEMPERATURES FOR LATE  
DECEMBER CONSIDERING THE ABOVE AVERAGE MID-LEVEL HEIGHTS. CENTRAL  
AND SOUTHERN FLORIDA WILL STAY UNDER THE INFLUENCE OF SUBTROPICAL  
RIDGING AND CONTINUE WITH ABOVE AVERAGE TEMPERATURES.  
 
RYAN/FRACASSO  
 
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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