973  
FXUS02 KWBC 151607  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1207 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  
BUILDS THROUGH THE PERIOD OVER THE WESTERN U.S. WITH POSSIBLE WEAK  
SHORTWAVE 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 THIS REGION.  
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 GUIDANCE INITIALLY CENTERS THE UPPER 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 DETERMINISTIC 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 RACING IT EASTWARD INTO THE NORTHERN  
INTERMOUNTAIN WEST AND SORT OF BECOMING LOST/ABSORBED WITHIN THE  
AMPLIFYING RIDGE OVER THE EAST. THUS THE PREFERENCE FOR THIS  
SYSTEM WAS A NON-CMC BLEND, BUT GIVEN THE SPREAD IN INDIVIDUAL  
ENSEMBLE SOLUTIONS, THE EVOLUTION OF THIS REMAINS IN QUESTION.  
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
SHOWERS/STORMS ACROSS THE NORTHEAST AT THE END OF THIS WEEK MOVE  
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, WITH 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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