932  
FXUS02 KWBC 271600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 AM EST THU DEC 27 2018  
 
VALID 12Z SUN DEC 30 2018 - 12Z THU JAN 03 2019  
   
..OVERVIEW
 
 
POSITIVELY-TILTED TROUGHING WILL RELOAD OVER THE NORTHERN PLAINS  
AND THE WEST WHILE SUBTROPICAL UPPER RIDGING SLOWLY RETREATS  
EASTWARD PAST THE BAHAMAS. 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 SUN-MON. PREVIOUS SPREAD HAS NARROWED CONSIDERABLY WITH  
GFS/GEFS GUIDANCE IN PARTICULAR TRENDING OVER THE PAST DAY TO A  
MORE AMPLIFIED/WESTWARD FORECAST WITH THE TROUGH ENERGY DIGGING  
INTO THE WESTERN HALF OF THE COUNTRY. EARLIER TRENDS WITH THIS  
EVOLUTION HAD BEEN SUGGESTING AN ATTEMPT TO CONVERGE ON AN  
INTERMEDIATE SOLUTION BUT STILL CLOSER TO ECMWF-CMC MODEL/ENSEMBLE  
SOLUTIONS. BASED ON THE CURRENT CONSENSUS, EXPECT EJECTING  
SOUTHERN ROCKIES/NORTHWESTERN MEXICO TROUGH ENERGY TO SUPPORT A  
WAVE THAT LIFTS NORTHEASTWARD AHEAD OF U.S.-CANADIAN BORDER LOW  
PRESSURE TRACKING TOWARD THE GREAT LAKES.  
 
DAYS 5-7 REQUIRES SOME REFINEMENT OF OPERATIONAL MODEL INPUT AND  
INCREASING ENSEMBLE MEAN WEIGHT AS GUIDANCE DIVERGES. THE 00Z  
UKMET BECOMES ONE OF THE MORE EXTREME SOLUTIONS FOR WESTWARD  
EXTENT OF UPPER TROUGHING OVER THE WEST, LEADING TO ITS REMOVAL  
FROM THE BLEND AFTER MON. IN ADDITION THE 06Z GFS BEGINS TO STRAY  
SOMEWHAT FASTER THAN CONSENSUS WITH THE UPPER TROUGH AXIS, LEADING  
TO A TRANSITION FROM 06Z TO 00Z GFS RUNS FOR THE GFS COMPONENT OF  
THE BLEND INTO WED--AFTER WHICH TIME THE 00Z GFS ALSO BECOMES  
SOMEWHAT FASTER THAN THE 06Z GEFS/00Z ECMWF MEANS (WHICH ARE  
FAIRLY SIMILAR). MULTI-DAY TRENDS HAVE BEEN IN FAVOR OF SLOWING  
DOWN GFS PROGRESSION SO CONTINUE TO LEAN TOWARD A SOMEWHAT SLOWER  
SOLUTION. ON THE OTHER HAND, WHILE GUIDANCE IS SOMETIMES BIASED  
TOWARD BREAKING DOWN RIDGES TOO QUICKLY THERE IS SO FAR NOT MUCH  
EVIDENCE THAT THE EASTERN PACIFIC/WESTERN NORTH AMERICAN RIDGE  
WILL BE ABLE TO RESIST THE STRONG PACIFIC FLOW TO THE EXTENT  
DEPICTED IN THE 00Z CMC MEAN. DAYS 6-7 WED-THU INCREASE TOTAL  
ENSEMBLE MEAN WEIGHT TO 50-70 PERCENT RESPECTIVELY, WITH  
AFOREMENTIONED GFS CONSIDERATIONS LEADING TO EXCLUDING IT AFTER  
WED. THIS TIME FRAME USED COMPONENTS OF THE PAST TWO ECMWF RUNS  
DUE TO DIFFERING DETAILS OVER THE EAST AND OFF THE PACIFIC  
NORTHWEST COAST.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
MULTIPLE WAVES OVER THE EASTERN HALF OF THE COUNTRY WILL PROMOTE  
HEAVY RAINFALL POTENTIAL FROM AREAS JUST WEST OF THE LOWER HALF OF  
THE MISSISSIPPI VALLEY TO PARTS OF THE MID-ATLANTIC/SOUTHEAST.  
SOME MOISTURE WILL LIKELY EXTEND FARTHER NORTHWARD AT TIMES. ONE  
PRONOUNCED EPISODE SHOULD OCCUR AROUND SUN-MON AS A WAVE TRACKS  
NORTHEASTWARD FROM THE WESTERN GULF OF MEXICO THROUGH THE LOWER  
MISSISSIPPI VALLEY AND INTO THE NORTHEAST. 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  
INTO THE TENNESSEE VALLEY/SOUTHERN APPALACHIANS. 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, DURING SUN-MON AMPLIFYING UPPER  
TROUGHING AND LEADING NORTHERN TIER LOW PRESSURE WILL SPREAD AN  
AREA OF PRECIPITATION--MOSTLY SNOW--SOUTHWARD THROUGH AND JUST  
WEST OF THE ROCKIES AS WELL AS 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. HIGHEST AMOUNTS SHOULD  
REMAIN NORTH OVER VANCOUVER ISLAND BUT MEANINGFUL TOTALS MAY  
EXTEND AT LEAST INTO THE OLYMPICS/WASHINGTON CASCADES.  
 
AFTER A BRIEF SURGE OF WARMTH ON SUN (PLUS 10-20F ANOMALIES), 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 TEMPERATURES, GENERALLY 10-25F BELOW  
NORMAL. COLD ANOMALIES SHOULD BE MOST WIDESPREAD IN THE MON-WED  
TIME FRAME. 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, WHILE THE WEST COAST STATES SHOULD  
EXPERIENCE TEMPERATURES WITHIN A FEW DEGREES OF NORMAL ON MOST  
DAYS.  
 
RAUSCH  
 
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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