121  
FXUS02 KWBC 061519  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1118 AM EDT THU SEP 06 2018  
 
VALID 12Z SUN SEP 09 2018 - 12Z THU SEP 13 2018  
 
...HEAVY RAINFALL LIKELY TO EXTEND INTO THE OHIO VALLEY/EASTERN  
GREAT LAKES/NORTHEAST BY THIS WEEKEND/EARLY NEXT WEEK...  
 
   
..15 UTC UPDATE
 
 
AN AMPLIFIED FLOW PATTERN IS EXPECTED FROM THE NORTH PACIFIC  
ACROSS NORTH AMERICA DURING THE MEDIUM RANGE, WITH AN ANOMALOUS  
BLOCKING RIDGE IN PLACE ACROSS THE BERING SEA, DOWNSTREAM  
PERSISTENT NEGATIVE HEIGHT ANOMALOUS ACROSS THE NORTHWESTERN U.S.,  
AND A PERSISTENT UPPER LOW ACROSS NORTHERN CANADA. ONE MAJOR  
QUESTION IS WHETHER THIS AMPLIFICATION WILL CONTINUE FURTHER  
DOWNSTREAM WITH AN UPPER RIDGE EXPECTED TO DEVELOP OFF THE EASTERN  
SEABOARD ACROSS THE NORTH ATLANTIC, WHICH WILL BE MAJOR PLAYER IN  
THE EVENTUAL TRACK OF HURRICANE FLORENCE. ADDITIONALLY, TROPICAL  
DEPRESSION GORDON (EXPECTED TO BE A POST-TROPICAL CYCLONE) BY THE  
MEDIUM RANGE, WILL MOVE NORTHEASTWARD FROM THE MID-MISSISSIPPI  
VALLEY TO THE GREAT LAKES SUN-TUE, BRINGING HEAVY RAIN ALONG WITH  
IT. MODELS SHOWED REASONABLY GOOD CONSENSUS ON THE MEDIUM RANGE  
TRACK OF GORDON. IN GENERAL, MODEL SPREAD IN TERMS OF THE OVERALL  
SYNOPTIC PATTERN ACROSS NORTH AMERICA DURING THE MEDIUM RANGE  
REMAINED A BIT BELOW AVERAGE AND PREDICTABILITY SEEMS RELATIVELY  
GOOD. THE MAJOR EXCEPTION TO THIS IS THE EVENTUAL TRACK OF  
HURRICANE FLORENCE, FOR WHICH MODELS/ENSEMBLES CONTINUE TO SHOW A  
LARGE SPREAD DURING DAYS 6-7. PLEASE REFER TO NHC ADVISORIES FOR  
THE LATEST INFORMATION ON HURRICANE FLORENCE.  
 
A MULTI-MODEL DETERMINISTIC BLEND (INCLUDING THE ECMWF/GFS/CMC)  
WAS USED AS A BASIS FOR THE WPC FORECAST DURING DAYS 3-5, WITH  
ENSEMBLE WEIGHTING (ECENS/GEFS) BOOSTED DURING DAYS 6-7. THIS KEPT  
THE FORECAST CLOSE TO CONTINUITY, WITH NO MAJOR CHANGES FOR THIS  
UPDATE.  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 0643 UTC)  
 
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE LATEST RUNS OF GUIDANCE MAINTAIN CONTINUITY FAIRLY WELL IN  
SHOWING PRONOUNCED TROUGH AMPLIFICATION ALONG AND JUST OFFSHORE  
THE WEST COAST. CURRENT CONSENSUS SHOWS THIS EVOLUTION WILL BE IN  
RESPONSE TO THE OPENING UP OF A VERY STRONG RIDGE POSITIONED OVER  
THE BERING SEA AT THE START OF THE PERIOD AND LIKELY MERGING WITH  
A SEPARATE RIDGE BUILDING OVER THE PACIFIC. WHILE THERE IS GOOD  
AGREEMENT AND THUS FAIRLY HIGH CONFIDENCE FOR THE LARGE SCALE  
PATTERN THERE ARE ONGOING UNCERTAINTIES REGARDING EMBEDDED  
DETAILS. FARTHER EAST, THE POST-TROPICAL REFLECTION OF GORDON  
WILL TRACK NORTHEAST FROM THE MIDDLE MS VALLEY, BEHIND STRONG  
EASTERN CANADA/NORTHERN NEW ENGLAND HIGH PRESSURE. AS UPPER  
RIDGING REBUILDS ALONG/OFFSHORE THE EAST COAST AFTER EARLY SUN,  
ATTENTION WILL TURN TO HURRICANE FLORENCE WHOSE TRACK COULD REACH  
FAIRLY CLOSE TO THE EAST COAST TOWARD THE END OF THE PERIOD  
WED-THU.  
 
DAYS 3-5 SUN-TUE EMPHASIZED THE 18Z GFS AND 12Z ECMWF/UKMET WITH A  
SMALL MINORITY OF 18Z GEFS/12Z ECMWF MEAN INPUT. AN AVERAGE OF  
THOSE THREE OPERATIONAL MODELS PROVIDED A GOOD INTERMEDIATE TRACK  
FOR POST-TROPICAL GORDON WITH ONLY MODEST ADJUSTMENTS IN  
CONTINUITY. OVER THE PAST DAY ECMWF RUNS HAVE HELD FAIRLY STEADY  
WITH TRACK/TIMING WHILE THE GFS HAS BEEN CONTINUING A LONGER-TERM  
SLOWER/SOUTHWARD TREND. BY DAY 4 MON ADJUSTMENTS IN THE NEW 00Z  
GFS HAVE BROUGHT ITS SOLUTION CLOSE TO THE FAVORED INTERMEDIATE  
TRACK BASED ON 12Z/18Z GUIDANCE. MEANWHILE THE BLEND YIELDED  
LITTLE CHANGE IN THE FORECAST UPSTREAM. A LEADING SHORTWAVE  
EJECTING FROM THE INITIALLY ESTABLISHED NORTHEAST PACIFIC TROUGH  
WILL BRING A FRONTAL SYSTEM INTO THE NORTHERN PLAINS BY THE START  
OF THE PERIOD SUN, FOLLOWED BY EVENTUAL DISSIPATION OVER THE  
NORTHERN TIER AS BEST DYNAMICS LIFT INTO CANADA. AT THE SAME TIME  
A LOW AT THE SURFACE AND ALOFT WILL DRIFT TOWARD VANCOUVER ISLAND,  
PUSHING A LEADING COLD FRONT INTO THE NORTHWEST.  
 
DAYS 6-7 WED-THU STEADILY TRENDED THE FORECAST MORE TOWARD THE  
ENSEMBLE MEANS, BOTH TO DOWNPLAY LOWER-CONFIDENCE DETAILS WITHIN  
THE EASTERN PACIFIC/WEST COAST UPPER TROUGH AS WELL AS TO YIELD A  
SOLUTION CLOSER TO THE MIDDLE OF THE ENVELOPE FOR HURRICANE  
FLORENCE. WITHIN THE WESTERN TROUGH THERE IS REASONABLE AGREEMENT  
THAT THE UPPER LOW NEARING VANCOUVER ISLAND ON TUE SHOULD  
SUBSEQUENTLY OPEN UP WITH SURROUNDING TROUGH ENERGY EJECTING  
INLAND. THEN A NUMBER OF OPERATIONAL RUNS SUGGEST ANOTHER BUNDLE  
OF ENERGY DROPPING THROUGH NORTHWESTERN CANADA WILL FORM ANOTHER  
CLOSED LOW REACHING CLOSE TO VANCOUVER ISLAND. INCORPORATING BOTH  
OPERATIONAL AND THE MORE OPEN ENSEMBLE MEAN SOLUTIONS SEEMS  
REASONABLE GIVEN TYPICALLY LOWER DETAIL PREDICTABILITY THIS FAR  
OUT IN TIME. SUCH A BLEND ALSO TEMPERS EMERGING DIFFERENCES IN  
TROUGH ORIENTATION NEAR THE COAST TOWARD DAY 7 THU. THE 18Z  
GFS/12Z CMC SHOWED A SHARPER/NEUTRALLY TILTED TROUGH WHILE OTHER  
GUIDANCE SHOWED SOMEWHAT MORE SOUTHWESTWARD ELONGATION. EITHER  
SCENARIO COULD BE PLAUSIBLE FROM A MULTI-DAY MEAN PERSPECTIVE.  
AHEAD OF THE TROUGH EXPECT A MEAN FRONTAL BOUNDARY TO SETTLE FROM  
THE NORTHERN PLAINS SOUTHWEST THROUGH CALIFORNIA, PARALLEL TO FLOW  
ALOFT. AS FOR FLORENCE'S TRACK, CONFIDENCE IN ANY SOLUTION IS  
RATHER LOW AS MODEL/ENSEMBLE MEMBERS HAVE BEEN DIVERGING  
CONSIDERABLY AFTER ABOUT SUN. BY DAY 7 THU 18Z GFS/12Z ECMWF RUNS  
THAT BRING FLORENCE TO OR NEAR THE EAST COAST ARE IN THE WESTERN  
PART OF THE OVERALL ENSEMBLE SPREAD. THE NEW 00Z GFS HAS RETURNED  
A BIT TO THE EAST.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
EXPECT AN AREA OF HEAVY RAINFALL TO ACCOMPANY THE NORTHEASTWARD  
PROGRESSION OF POST-TROPICAL GORDON FROM THE MIDDLE MS VALLEY SUN  
ONWARD. GUIDANCE IS STILL IN THE PROCESS OF FINE-TUNING THE EXACT  
TRACK AND TIMING BUT THE OVERALL CONSENSUS HAS HELD FAIRLY STEADY  
OVER THE PAST DAY, HIGHLIGHTING BEST POTENTIAL FOR HIGHEST TOTALS  
TO EXTEND ACROSS THE OHIO VALLEY AND INTO THE EASTERN GREAT LAKES.  
RAINFALL AMOUNTS SHOULD DECREASE SOMEWHAT AS THE MOISTURE  
CONTINUES INTO THE NORTHEAST. A FRONT TO THE EAST MAY PROVIDE  
SOME FOCUS FOR ACTIVITY OVER THE MID-ATLANTIC. LOCATIONS TO THE  
SOUTHWEST OF THE OHIO VALLEY, THROUGH PARTS OF TEXAS, MAY ALSO SEE  
LOCALLY ENHANCED RAINFALL AHEAD OF A SURFACE FRONT/TROUGH. BY  
NEXT WED-THU THE TRACK OF HURRICANE FLORENCE WILL LIKELY PROVIDE  
THE GREATEST UNCERTAINTY FOR SENSIBLE WEATHER EFFECTS NEAR THE  
EAST COAST. EVEN IF THE TRACK REMAINS FAR ENOUGH OFFSHORE TO KEEP  
THE REGION DRY, SOME COASTAL AREAS MAY SEE BRISK WINDS. ACROSS  
FLORIDA EXPECT A CONTINUATION OF DIURNALLY FAVORED CONVECTION.  
 
THE EVOLVING PATTERN OVER THE EASTERN PACIFIC/WESTERN NORTH  
AMERICA WILL BRING MOISTURE INTO THE PACIFIC NORTHWEST AFTER SUN,  
WITH PRECIPITATION LIKELY PERSISTING THROUGH THE END OF THE  
PERIOD. EXPECT HIGHEST TOTALS TO BE CONFINED TO VANCOUVER ISLAND  
WITH A FAIRLY RAPID DECREASE AS ONE TRAVELS SOUTHWARD, WITH MOST  
MEASURABLE RAINFALL STAYING NORTH OF CALIFORNIA. LOCATIONS FROM  
THE EXTREME NORTHERN ROCKIES ACROSS THE NORTHERN PLAINS MAY SEE AT  
LEAST A COUPLE EPISODES OF LIGHT TO LOCALLY MODERATE PRECIPITATION  
AS FEATURES EJECT FROM THE UPSTREAM TROUGH ALOFT.  
 
THE COOLING TREND OVER THE NORTHWEST SHOULD BRING MULTIPLE DAYS OF  
HIGHS 5-10F BELOW NORMAL NEXT WEEK. WITHIN THE LEADING WARM  
SECTOR FROM THE SOUTHWEST INTO THE NORTHERN PLAINS, SOME NORTHERN  
PLAINS LOCATIONS MAY SEE A COUPLE DAYS UP TO 10-15F ABOVE NORMAL.  
FARTHER EAST, BELOW NORMAL DAYTIME HIGHS (LOCALLY 10-15F BELOW  
NORMAL) FROM THE CENTRAL-SOUTHERN PLAINS INTO  
MID-ATLANTIC/NORTHEAST ON SUN WILL STEADILY MODERATE THEREAFTER.  
MIN TEMPERATURES IN PARTICULAR SHOULD TREND ABOVE NORMAL OVER THE  
EAST BY TUE-THU.  
 
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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