928  
FXUS02 KWBC 011601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1200 PM EDT SAT SEP 01 2018  
 
VALID 12Z TUE SEP 04 2018 - 12Z SAT SEP 08 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
AN IMPRESSIVE MID/UPPER-LEVEL RIDGE WILL DOMINATE THE PATTERN  
ACROSS THE EASTERN SEABOARD THROUGHOUT MUCH OF THE FORECAST.  
THROUGH MID-WEEK, 500-MB HEIGHT ANOMALIES SHOULD SIT AROUND 2 TO  
2.5 SIGMA ABOVE CLIMATOLOGY. GRADUAL AMPLIFICATION WITHIN THE  
NORTHERN STREAM SHOULD EFFECTIVELY LOWER HEIGHTS IN THIS RIDGE  
BEYOND DAY 5/THURSDAY AS A BACKDOOR FRONT SWINGS DOWN THROUGH THE  
NORTHEASTERN U.S. WITHIN THE SUBTROPICS, THE 00Z ECMWF AND ITS  
ENSEMBLE MEMBERS HAVE TRENDED TOWARD A MORE DEFINED SURFACE  
CYCLONE TRACKING ALONG THE NORTHERN GULF OF MEXICO. COMPARED TO  
PREVIOUS MODEL CYCLES, THE 00Z ECMWF IS 5 TO 10 MB DEEPER WITH  
SUPPORT FROM THE 00Z CMC. BASED ON THIS LATEST ENSEMBLE PLOT,  
THERE IS INCREASING CONFIDENCE IN SOME SORT OF CIRCULATION TO  
AFFECT THE GULF COAST DURING THE PERIOD. THERE DOES REMAIN A  
DEARTH OF SUPPORT FROM THE GEFS MEMBERS WHICH HAS BEEN A THEME THE  
PAST FEW DAYS. AS THIS SYSTEM REACHES THE WESTERN GULF COAST BY  
THURSDAY, A BROAD INVERTED TROUGH IS PRIMED TO REACH THE BAHAMAS  
BY DAY 6/FRIDAY WITH THE BEST SUPPORT FROM THE 06Z/00Z GFS AND 00Z  
ECMWF.  
 
ACROSS THE WESTERN/CENTRAL SECTIONS OF NORTH AMERICA, MULTI-DAY  
ENSEMBLE TRENDS HAVE SHOWN A TENDENCY TO PUSH A LONGWAVE TROUGH  
FARTHER BACK TOWARD THE EASTERN PACIFIC EARLY ON WHILE BEING MORE  
AMPLIFIED WITH A SYSTEM CROSSING CENTRAL CANADA. THE LATTER WAS  
DEPICTED IN SUCH A MANNER MULTIPLE RUNS AGO FROM THE ECMWF WHERE  
FUTURE GUIDANCE EVENTUALLY MOVED IN THIS DIRECTION. THUS, VALUE  
THE CONTINUITY OF THE ECMWF RATHER HIGHLY HERE. THIS AMPLIFIED  
FLOW WILL ULTIMATELY FLATTEN THE DOWNSTREAM RIDGE ACROSS THE  
EASTERN U.S. FARTHER WEST, THE SLOWING TREND IN THE GUIDANCE SEEMS  
TO BE SUPPORTED ACROSS THE BOARD ALTHOUGH OPERATIONAL RUN-TO-RUN  
DIFFERENCES ARE EVIDENT. MOST NOTABLY, THE 00Z GFS STOOD OUT ON  
THE QUICKER SIDE WHICH SUGGESTED MORE RELIANCE ON THE LATER 06Z  
RUN. ENSEMBLE MEANS ARE IN AGREEMENT WITH THIS SLOWER SCENARIO  
ALTHOUGH PLENTY OF SPREAD DOES EXIST AHEAD OF THE MEAN POSITION OF  
THE TROUGH.  
 
WITH SUFFICIENT MODEL AGREEMENT THROUGH DAY 4/WEDNESDAY, WENT WITH  
A COMBINATION OF THE 06Z GFS AND 00Z ECMWF/CMC BEFORE GRADUALLY  
INCORPORATING ENSEMBLE MEAN INFLUENCES. GENERALLY TOOK A 50/50  
APPROACH BETWEEN THE 06Z GFS/GEFS MEAN AND 00Z ECMWF/ECMWF  
ENSEMBLE MEAN THROUGH THE REMAINDER OF THE PERIOD. WAS A BIT LEERY  
IN PLACING TOO MUCH ON THE 00Z ECMWF SOLUTION BY DAY 6/7,  
SEPTEMBER 7/8, GIVEN RATHER AMPLIFIED FLOW ACROSS EASTERN CANADA.  
THUS, KEPT ITS CONTRIBUTIONS MORE CONSERVATIVE IN NATURE AT THAT  
POINT.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
STRONG RIDGING FROM THE MISSISSIPPI RIVER OUT TO THE EASTERN  
SEABOARD WILL SUPPORT A LENGTHY PERIOD OF ABOVE AVERAGE  
TEMPERATURES. THE MOST INTENSE ANOMALIES SHOULD SIT AROUND 10 TO  
15 DEGREES ABOVE CLIMATOLOGY CENTERED OVER THE GREAT LAKES INTO  
THE NORTHEASTERN U.S. SEVERAL DAYS OF DAILY RECORDS ARE LIKELY  
GIVEN THE WARM OVERNIGHT LOWS. SOME COOLING TREND IS LIKELY BY  
FRIDAY AND INTO NEXT WEEKEND GIVEN THE APPROACH OF THE BACKDOOR  
FRONT. FARTHER WEST, WELL BELOW AVERAGE TEMPERATURES ARE LIKELY  
ACROSS THE MIDDLE OF THE COUNTRY GIVEN AN ACTIVE PRECIPITATION  
PATTERN ACCOMPANIED BY EXTENSIVE CLOUD COVER. HIGHS MAY STRUGGLE  
TO GET OUT OF THE 60S ACROSS THE NORTH-CENTRAL U.S. ON TUESDAY AND  
WEDNESDAY.  
 
REGARDING PRECIPITATION, MULTIPLE ROUNDS OF SEMI-ORGANIZED  
CONVECTION IS LIKELY FROM THE CENTRAL U.S. NORTHEASTWARD INTO THE  
GREAT LAKES WERE A FRONTAL ZONE WILL STALL. WHILE OVERALL 24-HOUR  
AMOUNTS ARE NOT TERRIBLY HIGH, THE MULTI-DAY NATURE OF THESE  
EVENTS COULD OFFER A THREAT FOR HYDROLOGIC ISSUES AT SOME POINT  
DURING THE PERIOD. ALONG THE GULF COAST, THE MENTIONED TROPICAL  
DISTURBANCE COULD POSE A THREAT FOR HEAVY RAINFALL FROM EAST TO  
WEST. EVEN IF THE SYSTEM DOES NOT INTENSIFY INTO A NAMED STORM, IT  
WILL STILL BRING HEAVY RAINFALL POTENTIAL GIVEN THE ANOMALOUS GULF  
MOISTURE AND SUFFICIENT MID/UPPER-LEVEL DYNAMICS.  
 
RUBIN-OSTER  
 
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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