641  
FXUS02 KWBC 281601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 AM EST FRI DEC 28 2018  
 
VALID 12Z MON DEC 31 2018 - 12Z FRI JAN 04 2019  
   
..OVERVIEW
 
 
THE MEDIUM RANGE PERIOD (MON-FRI) FEATURES A POSITIVELY TILTED  
TROUGH IN THE WEST SLOWLY TRANSLATING EASTWARD AS SUBTROPICAL  
UPPER RIDGING OVER THE BAHAMAS RETREATS INTO THE ATLANTIC. THIS  
ALSO ALLOWS PROGRESSION OF NORTHERN STREAM UPPER FLOW ALONG THE  
U.S.-CANADIAN BORDER SOUTH OF AN UPPER LOW OVER HUDSON BAY.  
INITIAL EASTERN PACIFIC/NORTHWESTERN NORTH AMERICAN RIDGING IS  
FORECAST TO PUSH INTO THE LOWER 48 WITH CORRESPONDING PROGRESSION  
OF THE DOWNSTREAM UPPER TROUGH. THIS OVERALL PATTERN EVOLUTION  
LIKELY RESULTS IN THE HEAVIEST PRECIPITATION (MOSTLY IN THE FORM  
OF RAIN) OVER THE EASTERN THIRD TO HALF OF THE COUNTRY.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
THERE REMAINS GOOD ENOUGH AGREEMENT TO WARRANT A DETERMINISTIC  
MODEL BLEND (BETWEEN THE 06Z GFS/00Z FV3-GFS/00Z ECMWF) FOR DAYS  
3-4 (MON-TUES) WHICH CLUSTERS WELL WITH THE ENSEMBLE CONSENSUS AND  
PREVIOUS SHIFT CONTINUITY REGARDING UPPER TROUGHING MOVING INTO  
THE SOUTHERN PLAINS. BEYOND THIS, DIFFERENCES BEGIN TO ARISE IN  
BOTH THE EVOLUTION OF THE TROUGH (RECENT RUNS OF SOME MODELS  
DEPICT A CLOSED OR NEARLY CLOSED LOW FORMING OVER THE SOUTHEAST)  
AND ALSO HOW QUICKLY TO ADVANCE THE TROUGH INTO THE EAST.  
MEANWHILE, THERE IS SOME QUESTION ON HOW QUICKLY, IF AT ALL, UPPER  
RIDGING INTO THE WESTERN U.S. MID-LATE WEEK GETS BROKEN DOWN (THE  
00Z CMC IS BY FAR THE STRONGEST IN MAINTAINING THAT RIDGE) AND  
ALSO WITH AMPLITUDE AND TIMING OF A SHORTWAVE MOVING INTO THE  
PACIFIC NORTHWEST LATE WEEK AND INTO NEXT WEEKEND. IN LIGHT OF  
THIS, OPTED TO RELY MUCH MORE ON THE ENSEMBLE MEANS LATE IN THE  
MEDIUM RANGE PERIOD WITH ONLY SLIGHT CONTRIBUTIONS FROM THE 00Z  
ECMWF TO ADD SOME DETAIL AS IT SEEMED TO PROVIDE A SOLUTION MOST  
REASONABLY IN LINE WITH THE ENSEMBLE MEANS. NOTE THAT YESTERDAYS  
12Z ECENS MEAN WAS USED DUE TO DELAYED AVAILABILITY OF THE 00Z  
RUN.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
A COUPLE OF WAVES OVER THE EASTERN HALF OF THE COUNTRY WILL  
PROMOTE HEAVY RAINFALL POTENTIAL FROM THE MISSISSIPPI VALLEY TO  
THE TENNESSEE OR OHIO VALLEY AND INTO THE MID-ATLANTIC AND THE  
SOUTHEAST. A TRAILING WAVY FRONTAL BOUNDARY MAKING ONLY GRADUAL  
PROGRESS OVER THE SOUTHEAST WILL LIKELY PROVIDE FOCUS FOR  
ADDITIONAL BANDS OF ENHANCED RAINFALL FROM TUES ONWARD, WITH THE  
BEST SIGNAL FOR HIGHEST 5-DAY QPF TOTALS CURRENTLY EXTENDING FROM  
THE CENTRAL GULF COAST THROUGH GEORGIA. 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  
MID-WEEK. IN THE SOUTHWEST, AN INITIAL UPPER LOW EARLY IN THE WEEK  
WILL LIKELY RESULT IN ANOTHER ROUND OF PRECIPITATION FOR PARTS OF  
EASTERN ARIZONA AND NEW MEXICO, WITH THE HIGHEST POTENTIAL FOR  
ACCUMULATING SNOWS CONFINED TO THE HIGHER MOUNTAINS. THE PACIFIC  
JET SHOULD REACH FAR ENOUGH SOUTH FOR THE PACIFIC NORTHWEST TO SEE  
AN INCREASE OF RAIN/HIGH ELEVATION SNOWS BY NEXT THU/FRI WITH SOME  
MEANINGFUL TOTALS LIKELY TO EXTEND AT LEAST INTO THE  
OLYMPICS/WASHINGTON CASCADES.  
 
THE NORTHERN PLAINS STATES WILL SEE A COUPLE OF 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 NEXT WEEK.  
 
SANTORELLI/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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