445  
FXUS02 KWBC 041516  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1115 AM EDT TUE SEP 04 2018  
 
VALID 12Z FRI SEP 07 2018 - 12Z TUE SEP 11 2018  
   
..15 UTC UPDATE
 
 
MODELS/ENSEMBLE HAVE NOT CHANGED SIGNIFICANTLY COMPARED TO THE  
OVERNIGHT FORECAST ISSUANCE. MODEL SPREAD IS RELATIVELY LOW EARLY  
IN THE MEDIUM RANGE AND A BLEND OF MOSTLY DETERMINISTIC GUIDANCE  
INCLUDING THE ECMWF AND GFS WAS USED AS A FORECAST STARTING POINT.  
FROM DAY 5 (SUN) ONWARD, THE GFS WAS FASTER THAN THE CONSENSUS  
WITH THE REMNANTS OF TROPICAL STORM GORDON ACROSS THE OHIO  
VALLEY/GREAT LAKES. ADDITIONALLY, A BIMODAL DISTRIBUTION OF  
SOLUTIONS TAKES SHAPE ACROSS THE PACIFIC NORTHWEST BY DAYS 6-7.  
UPPER TROUGH AMPLIFICATION APPEARS LIKELY EITHER IN THE VICINITY  
OF THE PACIFIC NORTHWEST OR PERHAPS OFFSHORE, DOWNSTREAM OF  
ANOMALOUS (+4 STANDARD DEVIATIONS IN 500 HPA HEIGHTS) UPPER RIDGE  
AMPLIFICATION IN THE BERING SEA. THE ECMWF/ECENS CONTINUE TO BE  
THE STRONGEST WITH THE UPPER RIDGE ACROSS THE BERING, AND ALSO  
STRONGEST WITH PUSHING HEIGHT FALLS INTO THE PACIFIC NORTHWEST BY  
EARLY NEXT WEEK. THE GFS/CMC AND NAEFS KEEP THE STRONGEST HEIGHT  
FALLS A BIT FARTHER WEST OVER THE NORTH PACIFIC. A FAIRLY HIGH  
DEGREE OF CONSISTENCY WAS NOTED IN THE ECMWF/ECENS SOLUTIONS,  
ALTHOUGH UNDERDISPERSION IN THE ECENS COULD BE A PROBLEM GIVEN THE  
RATHER TIGHTLY CLUSTERED ENSEMBLE MEMBERS AROUND THE MEAN. GIVEN  
THESE CONSIDERATIONS, A TREND TOWARD HEAVIER ENSEMBLE MEAN  
WEIGHTING WAS SHOWN FROM DAY 5 ONWARD, WITH APPROXIMATELY 60/40  
WEIGHTING TOWARD THE ECENS/ECMWF CAMP AND GFS/NAEFS CAMP,  
RESPECTIVELY.  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 0658 UTC)  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
FOR ABOUT THE FIRST HALF OF THE PERIOD A BLEND OF THE 12Z ECMWF  
AND VARYING WEIGHTS OF THE 18Z GFS/12Z CMC REPRESENTED THE BEST  
IDEAS OF GUIDANCE THROUGH THE 18Z CYCLE OF GUIDANCE. THE MAJORITY  
TOTAL WEIGHT OF THE ECMWF/CMC ACCOUNTED FOR RECENT TENDENCIES OF  
THE GFS TO LOSE DEFINITION OF T.S. GORDON LEADING INTO THE START  
OF THE MEDIUM RANGE PERIOD. GFS RUNS DO SHOW A BETTER DEFINED LOW  
BY THE WEEKEND IN RESPONSE TO THE SHORTWAVE ENERGY PROGRESSING  
ACROSS THE PLAINS/MS VALLEY. THE BLEND (WITH MINOR ADJUSTMENTS  
FOR THE OFFICIAL NHC FORECAST AND YESTERDAY'S WPC-NHC COORDINATED  
TRACK INTO MON) YIELDED GOOD CONTINUITY AS FASTER ECMWF  
TRENDS/SLOWER GFS TRENDS WITH THE PLAINS INTO GREAT LAKES SURFACE  
LOW HAVE NARROWED THE GUIDANCE SPREAD. THE NEW 00Z GFS HAS  
TRENDED SLOWER THAN THE 18Z RUN SO THERE IS NOW EVEN LESS SPREAD  
THAN BEFORE. FARTHER UPSTREAM THERE IS DECENT AGREEMENT INTO THE  
WEEKEND WITH SOME LINGERING QUESTION MARKS ON THE FINER DETAILS.  
GUIDANCE STILL SHOWS A VERY STRONG BERING SEA RIDGE ALOFT (LIKELY  
AIDED IN PART BY THE EXTRATROPICAL EVOLUTION OF TYPHOON JEBI WELL  
TO THE WEST) SUPPORTING A NORTHEAST PACIFIC MEAN TROUGH. A  
SHORTWAVE OFF THE PACIFIC NORTHWEST COAST AS OF EARLY FRI WILL  
PROGRESS INLAND AND SUPPORT SOUTHERN CANADA LOW PRESSURE WITH A  
TRAILING FRONT. AT THE SAME TIME ENERGY DROPPING SOUTH FROM  
ALASKA WILL CLOSE OFF ANOTHER PACIFIC LOW.  
 
THE REMAINDER OF THE FORECAST TRANSITIONED TOWARD SOMEWHAT MORE  
18Z GEFS/12Z NAEFS INFLUENCE RELATIVE TO THE ECMWF/ECMWF MEAN BY  
DAY 7 TUE AS THE MODELS/ENSEMBLES SHOW INCREASINGLY DIVERGENT  
IDEAS FOR LARGER SCALE FLOW ASPECTS, WITH CONFLICTING  
CONSIDERATIONS RECOMMENDING AN INTERMEDIATE SOLUTION. A GREATER  
PROPORTION OF GUIDANCE INDICATES THAT THE BERING SEA RIDGE AND A  
NORTHERN PACIFIC UPPER LOW SHOULD BE AT A FARTHER SOUTH LATITUDE  
THAN DEPICTED IN THE ECMWF/ECMWF MEAN. IN ADDITION  
TELECONNECTIONS RELATIVE TO THE RIDGE'S CORRESPONDING POSITIVE  
HEIGHT ANOMALY CENTER IN THE ECMWF SCENARIO WOULD SUGGEST FLATTER  
MEAN FLOW OVER THE WESTERN U.S. VERSUS THE TROUGH DEPICTED IN THE  
ECMWF. TELECONNECTIONS RELATIVE TO EITHER THE GFS/GEFS OR ECMWF  
RIDGE FAVOR FLAT OR MODESTLY CYCLONIC FLOW OVER EASTERN NORTH  
AMERICA--ANOTHER NOD TO THE NON-EC SOLUTIONS. AT THE SAME TIME  
THE FAVORED MEAN FLOW OVER THE NORTHEAST PACIFIC COULD ULTIMATELY  
ALLOW FOR SLIGHTLY GREATER PROGRESSION THAN THE GFS/GEFS BY THE  
END OF THE PERIOD. THUS THE AFOREMENTIONED BLEND PROVIDED THE  
BEST COMPOSITE OF THESE CONSIDERATIONS BY NEXT MON-TUE.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
EXPECT WEAKENING TROPICAL CYCLONE GORDON TO REACH NEAR THE  
SOUTH-CENTRAL PLAINS LATE THIS WEEK AND THEN PROGRESS  
NORTHEASTWARD AS AN EXTRATROPICAL LOW. MOISTURE FROM GORDON AND  
INTERACTION WITH A WAVY FRONT INITIALLY STALLED JUST TO ITS NORTH  
AS OF FRI WILL ULTIMATELY PRODUCE A THREAT FOR HEAVY RAINFALL OVER  
SOME LOCATIONS FROM THE SOUTH-CENTRAL PLAINS THROUGH THE MIDWEST  
AND GREAT LAKES/NORTHEAST. WHILE THERE IS STILL UNCERTAINTY WITH  
SPECIFICS, THERE IS SOMEWHAT BETTER GUIDANCE AGREEMENT ON  
TIMING/RAINFALL COVERAGE THAN 24 HOURS AGO SO CONFIDENCE IS  
GRADUALLY IMPROVING. THERE MAY ALSO BE AN ENHANCED RAINFALL FOCUS  
OVER THE MID-ATLANTIC NEAR THE STALLED FRONT. MEANWHILE  
PERSISTENT MOISTURE OVER THE SOUTHERN TIER STATES SHOULD ALLOW FOR  
PERIODS OF RAINFALL, WHILE ENERGY ALOFT AND A SURFACE TROUGH MAY  
ENHANCE RAIN AT TIMES OVER FLORIDA. THE SHORTWAVE ALOFT  
APPROACHING/CROSSING THE PACIFIC NORTHWEST LATE THIS WEEK/WEEKEND  
SHOULD PRODUCE A PERIOD OF LIGHT-MODERATE PRECIPITATION OVER THAT  
REGION AND THEN EVENTUALLY THE NORTHERN PLAINS/UPPER MS VALLEY.  
THE NEXT PACIFIC SYSTEM SHOULD BRING SOME MOISTURE INTO THE  
PACIFIC NORTHWEST EARLY NEXT WEEK BUT ONLY WITH MEDIUM TO LOWER  
CONFIDENCE REGARDING PRECISE TIMING AND INTENSITY/SOUTHWARD EXTENT.  
 
HIGH PRESSURE BUILDING INTO THE NORTHEAST WILL BRING A PRONOUNCED  
COOLING TREND TO THAT REGION. THE SOUTHERN MID-ATLANTIC/SOUTHEAST  
WILL REMAIN SOUTH OF THE STALLED SURFACE FRONT AND SHOULD SEE  
ABOVE NORMAL HIGH TEMPERATURES MOST DAYS, WHILE A BROADER PORTION  
OF THE EAST WILL SEE WARM MIN TEMPERATURES IN THE MOIST FLOW AHEAD  
OF GORDON AND ITS EXTRATROPICAL EVOLUTION. THE CENTRAL PLAINS  
INTO MIDWEST WILL SEE WELL BELOW NORMAL HIGHS FRI-SAT FOLLOWED BY  
A TREND TO NEAR NORMAL TEMPERATURES. THE OVERALL PATTERN SHOULD  
FAVOR SLIGHTLY BELOW NORMAL HIGHS OVER THE NORTHWEST AFTER FRI  
WHILE THE SOUTHERN HALF OF THE WEST SHOULD SEE ABOVE NORMAL  
READINGS FOR MOST OF THE PERIOD.  
 
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  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page