603  
FXUS02 KWBC 020659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
258 AM EDT SUN SEP 02 2018  
 
VALID 12Z WED SEP 05 2018 - 12Z SUN SEP 09 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
GUIDANCE CONTINUES TO BE IN THE PROCESS OF REFINING THE DETAILS  
WITHIN AN AGREED-UPON NORTHEASTERN PACIFIC MEAN TROUGH, TO THE  
SOUTHEAST OF A STRONG RIDGE EXTENDING THROUGH THE BERING SEA.  
THERE IS STILL A GENERAL SIGNAL THAT DOWNSTREAM AMPLIFICATION OF  
THE PATTERN IN RESPONSE TO THE EXTRATROPICAL EVOLUTION OF CURRENT  
TYPHOON JEBI MAY SERVE TO REINFORCE OR FURTHER STRENGTHEN THE  
BERING SEA RIDGE LATE IN THE WEEK. TENDENCY FOR GUIDANCE TO TAKE  
SOME TIME TO ACCOUNT PROPERLY FOR SUCH STRONG EXTRATROPICAL  
TRANSITIONS PROVIDES ONE SOURCE OF UNCERTAINTY IN THE FORECAST.  
AS FAIRLY PROGRESSIVE MEAN FLOW BECOMES ESTABLISHED OVER SOUTHERN  
CANADA AND NORTHERN U.S. EXPECT THE INITIALLY STRONG EASTERN U.S.  
RIDGE ALOFT TO WEAKEN WITH TIME. TO THE SOUTH OF THE RIDGE A WAVE  
CURRENTLY OVER THE EASTERN BAHAMAS SHOULD REACH THE VICINITY OF  
THE CENTRAL GULF COAST BY THE START OF THE EXTENDED PERIOD ON WED.  
HOWEVER THERE IS CONSIDERABLE SPREAD REGARDING THE STRENGTH AND  
PATH OF THIS FEATURE AFTER MIDWEEK. A DEEP LAYER INVERTED TROUGH  
MAY REACH FLORIDA FROM THE BAHAMAS TOWARD FRI-SAT.  
 
OVER THE PAST 1-2 DAYS GUIDANCE CLUSTERING HAS IMPROVED SOMEWHAT  
FOR WHAT HAD APPEARED TO BE A VERY UNCERTAIN FORECAST WITHIN AND  
DOWNSTREAM FROM THE NORTHEASTERN PACIFIC MEAN TROUGH. SOLUTIONS  
HAVE CONVERGED/STABILIZED FAIRLY WELL FOR A LEADING SHORTWAVE  
ENERGY CROSSING CENTRAL/EASTERN CANADA, ULTIMATELY PUSHING A GREAT  
LAKES-PLAINS FRONT INTO THE MID-ATLANTIC BY LATE WEEK/WEEKEND.  
MEANWHILE SHORT RANGE ENERGY CLOSING OFF AN UPPER LOW OVER THE  
NORTHEAST PACIFIC SHOULD BE APPROACHING THE PACIFIC NORTHWEST  
COAST BY DAY 5 FRI WITH CONTINUED EAST-NORTHEAST EJECTING AS  
UPSTREAM ENERGY DROPPING SOUTH OVER ALASKA ULTIMATELY FORMS  
ANOTHER NORTHEASTERN PACIFIC UPPER LOW. DURING THE LATTER HALF OF  
THE PERIOD GFS/ECMWF AND ENSEMBLE MEAN CONSENSUS HAS BEEN FAIRLY  
STABLE SO FAR WITH THE SOUTHERN CANADA-NORTHERN U.S. LOW  
PRESSURE/FRONTAL SYSTEM ASSOCIATED WITH THE EJECTING SHORTWAVE.  
THE UPPER LOW WHICH GUIDANCE FORMS OVER THE NORTHEAST PACIFIC NEXT  
WEEKEND IS A NOTABLE TREND FROM 24 HOURS AGO WHEN THERE WAS LESS  
SEPARATION RELATIVE TO FLOW OVER NORTHERN-WESTERN CANADA. THE 18Z  
GFS PULLED THIS LATE-PERIOD UPPER LOW FARTHER WEST THAN MOST OTHER  
GUIDANCE BY DAY 7 SUN, DISCOURAGING ITS USE IN THE BLEND AT THAT  
TIME.  
 
FARTHER EAST THE GREATEST UNCERTAINTY EXISTS FROM DAY 4 THU ONWARD  
WITH THE ULTIMATE EVOLUTION/PATH OF THE FEATURE WHICH IS MOST  
LIKELY TO BE OVER/NEAR SOUTHEASTERN LOUISIANA AS OF EARLY WED.  
RECENT GFS RUNS HAVE BEEN WEAKEST AT THE SURFACE AND TAKE THE MID  
LEVEL ENERGY/SURFACE REFLECTION QUICKLY NORTHWARD AROUND THE  
WESTERN PERIPHERY OF THE EASTERN RIDGE ALOFT. IN CONTRAST ECMWF  
RUNS HAVE HELD ONTO ENOUGH OF A WESTWARD EXTENSION OF THE RIDGE  
ALOFT TO YIELD A TRACK INTO SOUTHEASTERN TEXAS. CMC RUNS ARE  
BETWEEN THE TWO EXTREMES WHILE THE UKMET IS CLOSE TO THE ECMWF  
SCENARIO. NOT SURPRISINGLY THE ENSEMBLES SHOW AN EQUALLY  
DIVERGENT ARRAY OF POSSIBILITIES. IMPORTANT DETAILS ARE SMALL  
ENOUGH IN SCALE AS TO HAVE LOWER THAN DESIRED PREDICTABILITY,  
KEEPING CONFIDENCE LOW IN ANY SPECIFIC SOLUTION. THERE IS SOME  
AGREEMENT ON A GENERAL WEAKNESS ALOFT OVER THE SOUTHERN PLAINS BY  
MID-LATE PERIOD SO AT LEAST HAVING SOME SURFACE TROUGHING AS A  
WEATHER-FOCUSING FEATURE SEEMS LIKE A REASONABLE STARTING POINT.  
ASSUMING LESS RECURVATURE OF ENERGY/LOW PRESSURE THAN DEPICTED IN  
THE GFS, REMAINING CONSENSUS HOLDS ONTO MORE HIGH PRESSURE NEAR  
THE EAST COAST TOWARD THE END OF THE PERIOD.  
 
THE UPDATED FORECAST STARTED WITH A BLEND CONSISTING OF MOSTLY THE  
18Z GFS/12Z ECMWF EARLY IN THE PERIOD, WITH GRADUALLY INCREASING  
18Z GEFS/12Z ECMWF MEAN INPUT WITH TIME SO THAT DAY 7 SUN REACHED  
75 PERCENT TOTAL ENSEMBLE MEAN WEIGHT. THE FORECAST GRADUALLY  
PHASED OUT GFS INPUT TOWARD THE LATTER PART OF THE PERIOD.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
LOCATIONS ALONG AND INLAND FROM THE CENTRAL-WESTERN GULF COAST  
WILL NEED TO MONITOR THE POTENTIAL FOR HEAVY RAINFALL IN  
ASSOCIATION WITH THE FEATURE CURRENTLY FORECAST TO BE NEAR  
SOUTHEAST LOUISIANA AS OF EARLY WED. THE SPREAD IN POSSIBLE  
SOLUTIONS AFTER WED LEADS TO LOW CONFIDENCE IN SPECIFICS FOR  
LOCATION/INTENSITY OF RAINFALL DURING THE REST OF THE PERIOD BUT  
CONSIDERABLE MOISTURE WILL PROMOTE AREAS OF HEAVY RAIN. MEANWHILE  
THE CENTRAL PLAINS THROUGH THE MIDWEST/GREAT LAKES AND PERHAPS  
EVEN LOCATIONS FARTHER TO THE EAST WILL BE ANOTHER AREA AT RISK  
FOR HEAVY RAINFALL, AS A WAVY FRONT STALLS OVER CENTRAL LATITUDES  
OF THE CENTRAL-EASTERN U.S.. EVEN IF GFS SPECIFICS ARE NOT  
ESPECIALLY FAVORED, THERE REMAINS THE POTENTIAL THAT SOME GULF  
MOISTURE MAY EVENTUALLY FEED INTO THE ACTIVITY NEAR THE FRONT.  
FARTHER TO THE SOUTHWEST EXPECT PERIODS OF CONVECTION OVER THE  
SOUTHERN ROCKIES AND HIGH PLAINS. THE PACIFIC NORTHWEST MAY SEE  
SOME PRECIPITATION WITH THE SHORTWAVE APPROACHING LATE IN THE WEEK  
BUT WITH RELATIVELY LIGHT AMOUNTS AT MOST LOCATIONS.  
 
DURING MID-LATE WEEK THE FRONT INITIALLY FROM THE GREAT LAKES INTO  
CENTRAL PLAINS WILL SERVE AS A DIVIDE BETWEEN VERY WARM/HOT AND  
HUMID CONDITIONS OVER THE EAST AND COOLER READINGS OVER PARTS OF  
THE PLAINS. SOME LOCATIONS FROM THE GREAT LAKES/OHIO VALLEY INTO  
MID-ATLANTIC/NORTHEAST SHOULD SEE PLUS 10F OR GREATER TEMPERATURE  
ANOMALIES. GENERALLY EXPECT MIN TEMPS TO BE FARTHER ABOVE NORMAL  
WITH SOME RECORD WARM LOWS MOST LIKELY ON WED-THU. THE  
NORTHERN-CENTRAL PLAINS MAY SEE ONE OR MORE DAYS WITH HIGHS 10F OR  
MORE BELOW NORMAL. HIGH PRESSURE BEHIND THE FRONT WILL BRING A  
PRONOUNCED COOLING TREND INTO AT LEAST THE NORTHERN HALF OF THE  
EAST BY NEXT WEEKEND. PASSAGE OF SHORTWAVE ENERGY ACROSS THE  
NORTHWEST WILL LEAD TO A COOLING TREND OVER THAT REGION BY SAT.  
 
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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