578  
FXUS02 KWBC 151600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1200 PM EDT WED AUG 15 2018  
 
VALID 12Z SAT AUG 18 2018 - 12Z WED AUG 22 2018  
   
..PATTERN OVERVIEW AND PREFERENCES
 
 
THE GUIDANCE CONTINUES TO AGREE ON AN AMPLIFICATION OF THE LARGE  
SCALE PATTERN TOWARD A CENTRAL/EASTERN NORTH AMERICA TROUGH, AND A  
WESTERN U.S./CANADA RIDGE. THIS EVOLUTION IS IN RESPONSE TO A DEEP  
UPSTREAM UPPER LOW/TROUGH WHICH DEVELOPS SOUTH OF ALASKA.  
INITIALLY, THE MEDIUM RANGE PERIOD STARTS OUT WITH AN ELONGATED  
TROUGH ACROSS THE EASTERN U.S., KICKING A SURFACE LOW OUT OF NEW  
ENGLAND, AND A TRAILING FRONT DROPPING THROUGH THE MID-ATLANTIC.  
MEANWHILE, A SHORTWAVE ALOFT EMERGES INTO THE HIGH PLAINS THIS  
WEEKEND AND SPINS UP A DEFINED SURFACE WAVE OVER THE CENTRAL U.S.  
WHICH LIFTS INTO THE UPPER GREAT LAKES BY DAY 6/TUESDAY. RIDGING  
BUILDING THROUGH THE PERIOD OVER THE WESTERN U.S. WHICH POSSIBLE  
LOBES OF UPPER LEVEL ENERGY STREAKING THROUGH.  
 
WHILE THERE IS RELATIVELY GOOD AGREEMENT THROUGH THE ENTIRE PERIOD  
ON THE OVERALL SYNOPTIC SETUP, THERE REMAINS QUESTIONS WITH  
INDIVIDUAL DETAILS, PARTICULARLY REGARDING THE WELL DEFINED  
SURFACE LOW LIFTING THROUGH THE NORTH-CENTRAL STATES. THE 00Z  
ECMWF CONTINUES TO ADVERTISE THE STRONGEST/DEEPEST SOLUTION (A  
995MB LOW OVER THE UPPER GREAT LAKES ON DAY 6), ONE OF WHICH, IF  
IT WERE TO VERIFY, COULD BE RECORD LOW PRESSURE FOR PARTS OF THE  
UPPER GREAT LAKES. THE REMAINDER OF THE GUIDANCE IS VARYING  
DEGREES WEAKER, WITH THE 06Z GFS COMING IN THE FLATEST/WEAKEST.  
UNTIL A MORE CLEAR PATH IS PAVED HERE, THE PREFERENCE CONTINUES TO  
BE A GENERAL MODEL BLEND BETWEEN THE LATEST GFS/ECMWF/CMC, WHICH  
RESULTS IN SOMETHING IN BETWEEN THE TWO EXTREMES AND A SOLUTION  
VERY CLOSE TO THAT OF THE LATEST ENSEMBLE MEAN GUIDANCE AND WPC  
CONTINUITY.  
 
OUT WEST, THE GFS/ECMWF/ENSEMBLE MEANS INITIALLY CENTER THE HIGH  
AMPLITUDE RIDGE OVER THE SOUTHWEST, WITH LIKELY BUILDING INTO THE  
REST OF THE WESTERN STATES AS HIGH AMPLITUDE RIDGING DRIFTS INLAND  
FROM THE EAST PACIFIC. THERE IS SOME QUESTION IN THE DETAILS OF A  
SHORTWAVE CROSSING THROUGH THE PACIFIC NORTHWEST IN THE MIDDLE OF  
THE PERIOD, WITH THE LATEST RUN OF THE CMC ADVERTISING A DEEP  
CLOSED LOW CENTERED OVER THE REGION BY DAY 7. THE DETERMINISTIC  
GFS/ECMWF AND THEIR ENSEMBLE MEANS ARE MUCH MORE MODEST WITH THIS  
SHORTWAVE AS IT RACES EASTWARD INTO THE NORTHERN INTERMOUNTAIN  
WEST AND SORT OF BECOMES LOST/ABSORBED WITHIN THE AMPLIFYING RIDGE  
OVER THE EAST. THUS THE PREFERENCE FOR THIS SYSTEM WAS A NON-CMC  
BLEND.  
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
SHOWERS/STORMS ACROSS THE NORTHEAST AT THE END OF THIS WEEK MOVES  
OFFSHORE BY THE WEEKEND, BUT SHOULD CONTINUE ACROSS THE  
CAROLINAS/TENNESSEE VALLEY AS THE TRAILING FRONT EVENTUALLY LIFTS  
NORTHWARD AS A WARM FRONT. SHORTWAVE ENERGY THAT LIKELY REACHES  
THE PLAINS ON SUNDAY WILL PROVIDE FOCUS FOR ORGANIZED CONVECTION  
AND LOCALLY HEAVY RAINFALL FROM THE HIGH PLAINS TO THE  
MID-MISSISSIPPI VALLEY THIS WEEKEND. LOW PRESSURE MOVING NORTHEAST  
FROM THE CENTRAL PLAINS/MS VALLEY TOWARDS THE GREAT LAKES BRINGS  
WITH IT LIKELY SHOWERS AND STORMS, MODEST PRECIP ALSO LIKELY ALONG  
THE TRAILING COLD FRONT MOVING THROUGH THE LOWER  
LAKES/APPALACHIANS/SOUTHEAST BY THE END OF THE PERIOD. MONSOONAL  
MOISTURE INTO THE SOUTHWEST DRIVES DIURNALLY DRIVEN SHOWERS AND  
STORMS ACROSS THE REGION AND INTO THE ROCKIES, WITH THE BEST  
CHANCE FOR HIGHEST RAINFALL TOTALS OVER PARTS OF ARIZONA/NEW  
MEXICO DUE TO GREATER AVAILABLE MOISTURE.  
 
THE LARGE SCALE TROUGHING IN THE CENTRAL U.S. WILL BRING AN  
EXPANDING AREA OF BELOW NORMAL TEMPERATURES, WITH THE CORE OF THE  
COOLEST HIGHS (10 TO 15 DEGREES BELOW NORMAL) CENTERED OVER THE  
NORTH-CENTRAL PLAINS SUNDAY-MONDAY. MEANWHILE, TEMPERATURES OVER  
THE INTERIOR WEST SHOULD TREND GRADUALLY WARMER WITH TIME AS THE  
RIDGE ALOFT BUILDS, WITH DAYTIME HIGHS OF 5 TO 10+ DEGREES ABOVE  
NORMAL POSSIBLE.  
 
SANTORELLI  
 
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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