788  
FXUS02 KWBC 051528  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1127 AM EDT WED SEP 05 2018  
 
VALID 12Z SAT SEP 08 2018 - 12Z WED SEP 12 2018  
 
...HEAVY RAINS EXPECTED FOR PORTIONS OF THE MID-MISSISSIPPI AND  
OHIO VALLEYS LATE THIS WEEK INTO EARLY NEXT WEEK...  
 
   
..15 UTC UPDATE
 
 
MODEL/ENSEMBLE CONSENSUS THROUGHOUT THE MEDIUM RANGE WAS SLIGHTLY  
ABOVE AVERAGE, LEADING TO SOMEWHAT BETTER THAN AVERAGE FORECAST  
CONFIDENCE, PARTICULARLY WITH SYSTEMS TRAVERSING THE NORTHERN  
STREAM. AN AMPLIFIED FLOW REGIME IS EXPECTED TO SET UP ACROSS MUCH  
OF THE NORTHERN HEMISPHERE OVER THE NEXT SEVERAL DAYS IN RESPONSE  
TO THE EXTRATROPICAL TRANSITION OF FORMER TYPHOON JEBI ACROSS  
EASTERN ASIA. ENSEMBLE MEANS CONTINUE TO SUPPORT A HIGHLY  
ANOMALOUS (+4 STANDARD DEVIATIONS) UPPER RIDGE SETTING UP ACROSS  
THE BERING SEA BY LATE THIS WEEK, WITH ADDITIONAL FLOW  
AMPLIFICATION LIKELY DOWNSTREAM ACROSS NORTH AMERICA. A  
MULTI-MODEL CONSENSUS WAS THE FAVORED APPROACH TO THE FORECAST  
UPDATE THIS MORNING. THE DETERMINISTIC ECMWF/GFS/CMC SOLUTIONS  
FORMED A BASIS FOR THE FORECAST FOR DAYS 3-5 (SAT-MON). THIS  
CONSENSUS SOLUTION MATCHED THE OFFICIAL NHC FORECAST TRACK FOR  
TROPICAL DEPRESSION GORDON (FORECAST TO BE POST-TROPICAL BY THE  
MEDIUM RANGE TIME PERIOD), AND ALSO HANDLED RELATIVELY MINOR  
TIMING/AMPLITUDE DIFFERENCES WITH SHORTWAVE FEATURES TRAVERSING  
CANADA AND THE U.S. NORTHERN TIER. THE MOST SIGNIFICANT  
DIFFERENCES AMONG THE GUIDANCE REMAIN ALONG THE WEST COAST BY DAYS  
6-7 WITH EXPECTED UPPER TROUGH AMPLIFICATION DOWNSTREAM OF THE  
ANOMALOUS BERING RIDGE. IN GENERAL, A TREND HAS BEEN NOTED TOWARD  
THE ECMWF/ECENS SOLUTIONS SHOWING STRONGER HEIGHT FALLS ALONG THE  
WEST COAST, AS OPPOSED TO FARTHER WEST/OFFSHORE. WHILE GEFS/NAEFS  
MEMBERS REMAIN FARTHER WEST THAN THE ECENS, THEY HAVE TRENDED  
TOWARD IT. THUS, BY DAYS 6-7 (TUE-WED), WEIGHTING OF ECENS/NAEFS  
MEANS WAS INCREASED TO COMPRISE A MAJORITY OF THE WPC FORECAST  
BLEND.  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 0650 UTC)  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE EASTERN PACIFIC/WESTERN NORTH AMERICAN PATTERN WILL CONTINUE  
TO BE INFLUENCED BY VERY STRONG BERING SEA RIDGING WHICH AT TIMES  
MAY EXTEND INTO MAINLAND ALASKA. THIS RIDGE WILL CONTINUE TO  
PROMOTE A BROAD MEAN TROUGH OVER THE NORTHEASTERN PACIFIC. WHILE  
THE GENERAL SCENARIO HAS BEEN WELL ADVERTISED, GUIDANCE HAS HAD  
SOME DIFFICULTY IN RESOLVING EXACT DETAILS OF THE RIDGE. THIS IN  
TURN HAS BEEN LEADING TO OCCASIONALLY SIGNIFICANT SPREAD FOR THE  
CHARACTER OF THE DOWNSTREAM TROUGH AND SENSIBLE WEATHER EFFECTS  
OVER THE WESTERN U.S.. FARTHER EAST THE GREATEST FOCUS WILL BE ON  
POST-TROPICAL/EXTRATROPICAL GORDON WHOSE SURFACE REFLECTION IS  
MOST LIKELY TO TRACK NORTHEASTWARD THROUGH THE MIDDLE MS VALLEY  
AND LOWER GREAT LAKES. THE LOW TRACK WILL BE AROUND THE PERIPHERY  
OF A PERSISTENT WESTERN ATLANTIC/EAST COAST RIDGE ALOFT. EARLY IN  
THE PERIOD THE UPPER RIDGE WILL CONTINUE A WEAKENING TREND FROM  
THE SHORT RANGE TIME FRAME BUT THEN THERE IS A STRONG CONSENSUS  
THAT THE RIDGE WILL BEGIN TO REBUILD AFTER EARLY SUN.  
 
GUIDANCE AVAILABLE THROUGH THE 12Z/18Z CYCLES SHOWS MUCH LESS  
SPREAD OVER THE EASTERN PACIFIC/WESTERN U.S. THAN WHAT WAS SEEN 24  
HOURS AGO. MUCH OF THE CONVERGENCE IN GUIDANCE HAS BEEN ACHIEVED  
BY GFS/GEFS MEAN TRENDS TOWARD MORE AMPLIFIED TROUGHING ALONG WITH  
MORE SUPPRESSED RIDGING ALOFT OVER SOUTHERN PARTS OF THE WEST.  
ADJUSTMENTS THAT HAVE BEEN MADE WITH THE UPSTREAM RIDGE APPEAR TO  
SUPPORT THE CURRENT MAJORITY CLUSTER THAT DEVELOPS A DECENT CORE  
OF NEGATIVE HEIGHT ANOMALIES FAIRLY CLOSE TO VANCOUVER ISLAND BY  
THE END OF THE FORECAST PERIOD. THE 18Z GFS/GEFS MEAN AND 12Z  
ECMWF/ECMWF MEAN PROVIDED A GOOD REPRESENTATION OF THE CONSENSUS  
SCENARIO. THIS WOULD HAVE ONE SHORTWAVE CROSSING THE NORTHWEST ON  
SAT, FOLLOWED BY THE ARRIVAL OF ANOTHER TROUGH (WITH EMBEDDED  
UPPER LOW) AROUND MON-TUE.  
 
THE MOST PROMINENT TREND OVER THE PAST DAY FOR THE SURFACE SYSTEM  
ASSOCIATED WITH GORDON IS TOWARD A SLOWER AND FARTHER SOUTHWARD  
SOLUTION, IN RESPONSE TO THE STRENGTH/PERSISTENCE OF EASTERN  
CANADA/NEW ENGLAND HIGH PRESSURE. IN FACT THE 18Z GFS/12Z ECMWF  
AND THEIR MEANS SUGGEST THE POSSIBILITY THAT WITH UPPER SUPPORT  
EVENTUALLY OUTPACING THE SURFACE LOW, THERE MAY NOT BE A  
DISCERNIBLE FEATURE BY DAY 7 WED.  
 
BASED ON CURRENT CONSENSUS FOR SIGNIFICANT FEATURES, THE FORECAST  
STARTED WITH A BLEND OF THE 18Z GFS/GEFS MEAN AND 12Z ECMWF/ECMWF  
MEAN. THE EARLY-MIDDLE PART OF THE FORECAST TILTED MOSTLY TO THE  
OPERATIONAL RUNS WHILE THE REMAINDER OF THE FORECAST LEANED MORE  
TOWARD THE MEANS.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
EARLY IN THE WEEKEND GORDON SHOULD MERGE WITH A FRONT DRAPED  
ACROSS THE CENTRAL-EASTERN U.S., WITH THE EXTRATROPICAL WAVE  
PROGRESSING NORTHEASTWARD THROUGH THE MID MS VALLEY AND LOWER  
GREAT LAKES BEFORE POTENTIALLY WEAKENING JUST WEST OF OR OVER NEW  
ENGLAND. ABUNDANT TROPICAL MOISTURE INTERACTING WITH THE FRONTAL  
SYSTEM WILL LIKELY GENERATE HEAVY RAINFALL, WITH LATEST SOUTHWARD  
TRENDS IN THE GUIDANCE SUGGESTING AN AXIS OF HIGHEST RAINFALL  
TOTALS FROM THE MID MS VALLEY TO THE UPPER OHIO VALLEY/CENTRAL  
APPALACHIANS. SOME OF THIS MOISTURE SHOULD EXTEND INTO THE  
MID-ATLANTIC AND NORTHEAST. MEANWHILE THERE WILL ALSO BE AREAS OF  
RAINFALL FARTHER SOUTH, AHEAD OF A FRONT/TROUGH TRAILING FROM  
EXTRATROPICAL GORDON. FLORIDA WILL SEE DIURNALLY FAVORED  
CONVECTION WITH A SURFACE TROUGH PROVIDING SOME ADDED FOCUS DURING  
THE FIRST HALF OF THE PERIOD. A FRONTAL SYSTEM WILL BRING A  
FAIRLY NARROW BAND OF RAINFALL ACROSS PARTS OF THE NORTHERN PLAINS  
AND VICINITY DURING THE WEEKEND INTO MON. APPROACH OF ANOTHER  
FRONT MAY BRING A LITTLE MORE RAINFALL TO THIS REGION TOWARD  
MIDWEEK. THE EVOLVING PATTERN WILL LIKELY BRING MULTIPLE DAYS OF  
PRECIPITATION TO THE PACIFIC NORTHWEST, WITH MOST ACTIVITY  
EXPECTED TO BE ON THE LIGHT TO MODERATE SIDE.  
 
DURING THE FIRST HALF OF THE PERIOD EXPECT BELOW NORMAL HIGH  
TEMPERATURES FROM THE SOUTHERN/CENTRAL PLAINS INTO  
MID-ATLANTIC/NORTHEAST DUE TO A BROAD AREA OF CLOUDS/RAINFALL, OR  
COOL HIGH PRESSURE IN THE CASE OF NEW ENGLAND. BY THE FIRST HALF  
OF NEXT WEEK THE EVOLVING PATTERN WILL FAVOR COOL READINGS OVER  
THE NORTHWEST AND AN AXIS OF MODERATELY ABOVE NORMAL READINGS FROM  
THE SOUTHWEST INTO THE NORTHERN PLAINS.  
 
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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