648  
FXUS02 KWBC 270635  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
134 AM EST THU DEC 27 2018  
 
VALID 12Z SUN DEC 30 2018 - 12Z THU JAN 03 2019  
   
..OVERVIEW
 
 
POSITIVELY-TILTED TROUGHING WILL RELOAD IN THE WEST AS SUBTROPICAL  
UPPER RIDGING SLOWLY RETREATS EASTWARD PAST THE BAHAMAS. THIS WILL  
ALLOW 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 PERHAPS SEVERAL SURFACE WAVES  
MOVING NORTHEASTWARD THROUGH THE SOUTHEAST OR SOUTHERN  
MID-ATLANTIC AS A PACIFIC SYSTEM PUSHES EASTWARD SUN-TUE FROM  
MONTANA TO THE UPPER MIDWEST AND GREAT LAKES INTO THE NORTHEAST.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
A CONSENSUS OF THE LATEST DETERMINISTIC GUIDANCE (18Z GFS AND 12Z  
ECMWF/UKMET/CMC/FV3-GFS) SERVED AS A GOOD STARTING POINT FOR THE  
FORECAST SUN-MON. DIFFERENCES SURROUNDING NORTHERN STREAM  
SHORTWAVE ENERGY AND THE ASSOCIATED LOW PRESSURE SYSTEM CROSSING  
THE NORTHERN PLAINS/MIDWEST SUN-MON FAVORED AN ENSEMBLE CONSENSUS  
NEAR THIS CLUSTER (AWAY FROM THE 12Z GFS/GEFS). WAVY FRONT IN THE  
GULF WILL RELEASE SEVERAL WAVES OF LOW PRESSURE ALONG ITS BOUNDARY  
AS UPPER RIDGING OVER THE BAHAMAS PREVENTS ANY EASTWARD MOVEMENT  
OF THE FRONT UNTIL LATE IN THE FORECAST. MODELS/ENSEMBLES HAVE HAD  
POOR CONTINUITY WITH THESE SMALLER FEATURES (UNEXPECTEDLY) SO A  
CONSENSUS APPROACH SEEMED PRUDENT.  
 
BY NEXT WED/THU, MODELS/ENSEMBLES DIFFER ON HOW QUICKLY TO  
PROGRESS THE REINFORCING HEIGHT FALLS IN THE WEST EASTWARD --  
GFS/CANADIAN GENERALLY QUICKER THAN THE ECMWF. THE SLOWER ECMWF  
ENSEMBLES HAVE BEEN TRENDING JUST A BIT QUICKER WHILE THE QUICKER  
GEFS ENSEMBLES HAVE BEEN TRENDING JUST A BIT SLOWER BUT THEIR  
MEANS WERE STILL WERE ~10 DEG LONGITUDE APART BY NEXT THURSDAY.  
PREFERRED THE SLOWER SIDE OF THE ENSEMBLES (NEAR THE 12Z ECMWF  
ENSEMBLE MEAN) GIVEN THE BEST OVERLAP OF ENSEMBLE CLUSTERS NEAR  
THE 12Z ECMWF/FV3-GFS AND ITS STABILITY.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
COLD FRONT IN THE EAST WILL LINGER IN THE GULF THROUGH THE PERIOD,  
SPREADING RAIN OVER THE SOUTHEAST AND SOUTHERN MID-ATLANTIC IN  
BATCHES AS A FEW SURFACE WAVES RIDE ALONG THE BOUNDARY. THIS MAY  
EVEN FALL AS SNOW ON THE NORTHERN FRINGE GIVEN SUFFICIENTLY COLD  
AIR AND/OR AT HIGHER ELEVATIONS. A PACIFIC SYSTEM WILL PRODUCE  
WIDESPREAD BUT MOSTLY LIGHT TO LOCALLY MODEST RAIN/SNOW THROUGH  
MUCH OF THE INTERIOR WEST CONTINUING ON MONDAY BUT DIMINISHING  
WITH TIME. BULK OF GULF MOISTURE WILL BE FOCUSED FARTHER EAST  
ALONG THE WAVY FRONT THAT WILL FLOW NORTHWARD AND EXPAND/ENHANCE  
THE RAIN SHIELD THROUGH THE SOUTHEAST ESPECIALLY MONDAY AS LOW  
PRESSURE LIFTS FROM LOUISIANA TO OFF THE NORTHEAST COAST. ONCE THE  
FRONTS CLEAR THE COAST LATE TUESDAY HIGH PRESSURE SHOULD DRY  
THINGS OUT FOR MUCH OF THE CENTRAL/EASTERN STATES EXCEPT IN THE  
SOUTHEAST. TAIL-END OF THE FRONT WILL FINALLY MAKE SOME EASTWARD  
ADVANCE LATER NEXT WEEK AS UPPER RIDGING TO ITS EAST RELENTS.  
 
TEMPERATURES WILL BE QUITE COLD IN PARTS OF THE SOUTHERN  
ROCKIES/WEST TEXAS AND UPPER MIDWEST EARLY IN THE WEEK WITH  
ANOMALIES ABOUT 10-25F BELOW AVERAGE. THIS WILL MEAN LOW  
TEMPERATURES IN THE NEGATIVE TEENS OVER PARTS OF ND/MN TUE/WED AND  
HIGHS NEAR ZERO. ABOVE AVERAGE TEMPERATURES (PLUS 10-25F  
ANOMALIES) WILL BE DRAWN NORTHWARD AHEAD OF THE PACIFIC FRONT  
MON/TUE EAST OF THE MISSISSIPPI. CENTRAL AND SOUTHERN FLORIDA WILL  
STAY UNDER THE INFLUENCE OF SUBTROPICAL RIDGING UNTIL ABOUT NEXT  
THURSDAY AND CONTINUE TO SEE ABOVE AVERAGE TEMPERATURES.  
 
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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