142  
FXUS02 KWBC 220527  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
126 AM EDT SAT SEP 22 2018  
 
VALID 12Z TUE SEP 25 2018 - 12Z SAT SEP 29 2018  
   
..PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
CONFIDENCE REMAINS HIGH THAT AN INCREASINGLY AMPLIFIED UPPER-LEVEL  
FLOW PATTERN WILL TAKE FROM THE NORTH PACIFIC ACROSS NORTH AMERICA  
DURING THE MEDIUM RANGE. AN ANOMALOUS UPPER RIDGE AXIS IS EXPECTED  
TO GRADUALLY STRENGTHEN THROUGH THE FORECAST PERIOD OFF THE NORTH  
AMERICA WEST COAST EXTENDING NORTH ACROSS ALASKA TO THE ARCTIC.  
THIS WILL FAVOR DOWNSTREAM TROUGHING ACROSS THE NORTHERN PLAINS  
AND MIDWEST, WHICH ALONG WITH A PERSISTENT UPPER LOW OVER THE HIGH  
LATITUDES OF CANADA, WILL FAVOR TRANSPORT OF CHILLY POLAR AIR  
SOUTHWARD INTO THE U.S., FROM THE NORTHERN ROCKIES TO THE UPPER  
MIDWEST. A SERIES OF UPPER SHORTWAVES (EACH ACCOMPANIED BY SURFACE  
FRONTAL SYSTEMS) WILL GRADUALLY CARVE OUT THE UPPER TROUGH THROUGH  
THE PERIOD. THE AMPLITUDE OF A SHORTWAVE DIGGING INTO THE  
NORTHWESTERN STATES BY FRI-SAT WILL LIKELY PLAY A SIGNIFICANT ROLE  
IN THE EVOLUTION OF THE UPPER TROUGH FARTHER EAST, AND INTRODUCES  
A FAIRLY SIGNIFICANT DEGREE OF UNCERTAINTY INTO THE FORECAST BY  
LATE IN THE MEDIUM RANGE, PARTICULARLY WITH RESPECT TO TO THE  
DEGREE THAT POLAR AIR IS ABLE TO PENETRATE SOUTHWARD INTO THE U.S.  
NORTHERN TIER.  
 
GIVEN ABOVE AVERAGE MODEL CONSENSUS EARLY IN THE FORECAST PERIOD,  
A MULTI-MODEL DETERMINISTIC BLEND (INCLUDING THE  
ECMWF/GFS/UKMET/CMC) WAS USED AS A BASIS FOR THE WPC FORECAST FOR  
DAYS 3-5 (TUE-THU). FOR DAYS 6-7 (FRI-SAT), WEIGHTING OF ENSEMBLE  
MEANS (ECENS/NAEFS) WAS INCREASED TO COMPRISE A MAJORITY OF THE  
FORECAST BLEND. AS MENTIONED ABOVE, DESPITE GENERAL LARGE SCALE  
AGREEMENT, UNCERTAINTY WITH RESPECT TO THE DETAILS BEGINS TO  
QUICKLY ESCALATE BY DAY 7, PARTICULARLY WITH RESPECT TO THE DEGREE  
OF SHORTWAVE AMPLIFICATION ACROSS THE NORTHWESTERN U.S., AND THE  
IMPACT ON HEIGHTS AND UPPER FLOW DOWNSTREAM ACROSS THE NORTHERN  
PLAINS AND MIDWEST. THE 12Z ECMWF REMAINED THE MOST AGGRESSIVE  
SOLUTION WITH TRANSPORTING COLD CANADIAN AIR SOUTHWARD INTO THE  
U.S. NORTHERN TIER BY LATE NEXT WEEK (BY MEANS OF AN IMPRESSIVE  
1040 HPA SURFACE HIGH SLIDING SOUTHWARD ALONG THE EASTERN SLOPE OF  
THE CANADIAN ROCKIES). THE GFS, HOWEVER, DIGS THE SHORTWAVE A BIT  
MORE ACROSS THE NORTHWEST AND KEEPS HIGHER HEIGHTS IN PLACE ACROSS  
THE CENTRAL U.S. WITH LESS TRANSPORT OF COLD AIR SOUTHWARD INTO  
THE CENTRAL U.S. (BUT PERHAPS MORE INTO THE NORTHERN GREAT BASIN).  
 
   
..SENSIBLE WEATHER HIGHLIGHTS
 
 
A COLD FRONT CROSSING THE CENTRAL/EASTERN STATES TUE-WED (IN  
ASSOCIATION WITH THE FIRST IN THE SERIES OF UPPER SHORTWAVES  
CROSSING THE NORTHERN TIER) WILL HAVE ACCESS TO RELATIVELY DEEP  
TROPICAL MOISTURE, AND AREAS OF HEAVY RAINFALL WILL BE POSSIBLE  
FROM THE SOUTHERN PLAINS/LOWER MISSISSIPPI VALLEY TO THE  
NORTHEAST. THERE IS SOME SUGGESTION AMONG THE GUIDANCE THAT  
ADDITIONAL MOISTURE ASSOCIATED WITH A TROPICAL DISTURBANCE OFF THE  
EASTERN SEABOARD COULD FEED NORTHWARD INTO THE FRONTAL SYSTEM,  
POTENTIALLY ENHANCING RAINFALL ACROSS PORTIONS OF THE NORTHERN  
MID-ATLANTIC OR NORTHEAST. THE FRONTAL BOUNDARY IS EXPECTED TO  
BECOME STATIONARY FROM THE SOUTHERN PLAINS TO THE SOUTHEAST  
WED-THU AS THE SECOND COLD FRONT EVENTUALLY APPROACHES, CONTINUING  
TO FOCUS LOCALLY HEAVY RAINS ACROSS THOSE AREAS THROUGH LATE WEEK.  
 
THE PASSAGE OF EACH COLD FRONT IS EXPECTED TO SPREAD SUCCESSIVELY  
COOLER TEMPERATURES INTO AREAS FROM THE NORTHERN ROCKIES TO THE  
MIDWEST THROUGH THE NEXT WEEK, WITH POTENTIALLY THE COOLEST  
TEMPERATURES ARRIVING BY FRI-SAT. HIGH TEMPERATURES AS LOW AS  
15-20 DEG F BELOW AVERAGE ARE POSSIBLE BY LATE NEXT WEEK  
(POTENTIALLY EVEN LOWER IF THE COOLER SOLUTIONS WERE TO VERIFY).  
MEANWHILE, AREAS ACROSS THE SOUTHEAST AND THE SOUTHWEST, AWAY FROM  
THE INFLUENCE OF THE AMPLIFYING UPPER TROUGH, WILL REMAIN 5-10 DEG  
ABOVE AVERAGE THROUGH MOST OF THE FORECAST PERIOD.  
 
RYAN  
 
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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