760  
FXUS02 KWBC 280625  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
124 AM EST FRI DEC 28 2018  
 
VALID 12Z MON DEC 31 2018 - 12Z FRI JAN 04 2019  
   
..OVERVIEW
 
 
POSITIVELY-TILTED TROUGHING IN THE WEST WILL SLOWLY TRANSLATE  
EASTWARD AS SUBTROPICAL UPPER RIDGING SLOWLY RETREATS FROM THE  
BAHAMAS INTO THE ATLANTIC. THIS WILL ALLOW NORTHERN STREAM FLOW TO  
PROGRESS ALONG THE U.S.-CANADIAN BORDER TO THE SOUTH OF AN UPPER  
LOW OVER HUDSON BAY. WITH TIME A STRONG NORTHERN PACIFIC JET WILL  
SAG SOUTHWARD, PUSHING AN INITIAL EASTERN PACIFIC/NORTHWESTERN  
NORTH AMERICA RIDGE INTO WESTERN PARTS OF THE LOWER 48 WITH  
CORRESPONDING PROGRESSION OF THE DOWNSTREAM UPPER TROUGH. THIS  
PATTERN EVOLUTION WILL LIKELY PRODUCE HEAVIEST PRECIPITATION (THE  
MAJORITY IN THE FORM OF RAIN) OVER THE EASTERN HALF/SOUTHERN  
TWO-THIRDS OF THE COUNTRY.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
A GENERAL MODEL BLEND REPRESENTS CURRENT CONSENSUS WELL FOR DAYS  
3-4 MON-TUE EXCEPT FOR THE 12Z UKMET WHICH CONTINUED ITS TREND OF  
FORECASTING A FARTHER WESTWARD EXTEND OF LOWER HEIGHTS IN THE WEST  
THAN NEARLY ALL THE ENSEMBLES. THE 18Z GFS/12Z FV3-GFS AND 12Z  
ECMWF CLUSTERED WELL WITH THE ENSEMBLE CONSENSUS IN LIGHT OF  
CONTINUITY AS THE TROUGH MOVES INTO THE SOUTHERN PLAINS LATE  
WEDNESDAY. AFTER THAT, UPSTREAM UNCERTAINTY IN THE NORTHEAST  
PACIFIC MOVES INTO THE PAC NW AND THE MODELS/ENSEMBLES DIVERGE IN  
HOW QUICKLY TO ADVANCE THE EASTERN TROUGH LATE THURSDAY (EVEN  
CLOSED OFF PER THE 12Z ECMWF) SO OPTED TO RELY MUCH MORE ON THE  
ENSEMBLE MEANS WITH ONLY THE 12Z FV3-GFS TO ADD SOME DETAIL AS IT  
MAINTAINED BETTER SYSTEM TIMING/AMPLITUDE.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
MULTIPLE WAVES OVER THE EASTERN HALF OF THE COUNTRY WILL PROMOTE  
HEAVY RAINFALL POTENTIAL FROM THE MISSISSIPPI VALLEY EASTWARD  
THROUGH THE OHIO VALLEY INTO PARTS OF THE MID-ATLANTIC/SOUTHEAST.  
SOME MOISTURE WILL LIKELY EXTEND FARTHER NORTHWARD AT TIMES  
DEPENDING ON THE TRACK/STRENGTH OF THE SURFACE FEATURE. A TRAILING  
WAVY FRONTAL BOUNDARY MAKING ONLY GRADUAL PROGRESS OVER THE  
SOUTHEAST WILL LIKELY PROVIDE A FOCUS FOR ADDITIONAL BANDS OF  
ENHANCED RAINFALL FROM NEW YEAR'S DAY (TUE) ONWARD. THE BEST  
SIGNAL FOR HIGHEST 5-DAY TOTALS CURRENTLY EXTENDS FROM THE CENTRAL  
GULF COAST THROUGH GEORGIA. SOME MOISTURE REACHING THE GREAT  
LAKES/NEW ENGLAND MAY BE IN THE FORM OF SNOW, ESPECIALLY WITH LAKE  
EFFECT ACTIVITY BEHIND LOW PRESSURE EXITING NEW ENGLAND ON TUE.  
FARTHER WEST, AN AREA OF PRECIPITATION--MOSTLY SNOW--WILL SPREAD  
SOUTHWARD THROUGH AND JUST WEST OF THE ROCKIES WITH LIGHT AMOUNTS  
INTO PARTS OF THE NORTHERN PLAINS/UPPER GREAT LAKES. THE PACIFIC  
JET SHOULD REACH FAR ENOUGH SOUTH FOR THE PACIFIC NORTHWEST TO SEE  
AN INCREASE OF RAIN/HIGH ELEVATION SNOW BY NEXT THU/FRI. HIGHEST  
AMOUNTS SHOULD REMAIN NORTH OVER VANCOUVER ISLAND BUT MEANINGFUL  
TOTALS MAY EXTEND AT LEAST INTO THE OLYMPICS/WASHINGTON CASCADES.  
 
THE NORTHERN PLAINS WILL SEE A COUPLE VERY COLD DAYS WITH  
TEMPERATURES 15-30F BELOW NORMAL FOLLOWED BY A RAPID WARM-UP BY  
THU. MANY AREAS FROM THE INTERIOR WEST THROUGH THE REST OF THE  
PLAINS WILL ALSO SEE A PERIOD OF CHILLY/COLD TEMPERATURES,  
GENERALLY 10-25F BELOW NORMAL. COLD ANOMALIES SHOULD BE MOST  
WIDESPREAD IN THE MON-WED TIME FRAME UNTIL THE TROUGH WEAKENS AND  
MOVES EASTWARD. IN CONTRAST THE EAST WILL SEE ABOVE NORMAL  
TEMPERATURES UNTIL COOLER AIR MOVES IN WED-THU. GREATEST ANOMALIES  
SHOULD BE IN THE PLUS 20-30F RANGE FOR LOW TEMPERATURES ON TUE,  
WHILE HIGH TEMPERATURES SHOULD BE 10-20F ABOVE NORMAL MON-TUE.  
EXPECT FLORIDA TO BE THE LAST TO SEE THE LATE-PERIOD COOLING TREND  
OVER THE EAST AS THE COLD FRONT FINALLY MOVES EASTWARD, WHILE THE  
WEST COAST STATES SHOULD EXPERIENCE TEMPERATURES WITHIN A FEW  
DEGREES OF NORMAL ON MOST DAYS.  
 
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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