681  
FXUS02 KWBC 160549  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
146 AM EDT THU AUG 16 2018  
 
VALID 12Z SUN AUG 19 2018 - 12Z THU AUG 23 2018  
   
..PATTERN OVERVIEW AND PREFERENCES
 
 
EARLY IN THE MEDIUM RANGE, 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. LATE RIN THE MEDIUM RANGE TOWARDS THE  
END OF NEXT WEEK, THE PATTERND EAMPLIFIES WITH REDUCTION IN THE  
AMPLITUDE OF THE EASTERN PACIFIC TROUGH, REDUCTION IN THE  
AMPLITUDE OF THE NORTHWEST RIDGE, AND REDUCTION IN AMPLITUDE OF  
THE EASTERN US TROUGH.  
 
A SHORTWAVE ALOFT MOVES ACROSS THE GREAT PLAINS THIS WEEKEND AND  
SPINS UP A DEFINED SURFACE LOW PRESSURE OVER THE CENTRAL U.S.  
WHICH LIFTS INTO THE UPPER GREAT LAKES BY DAY 5/TUESDAY 21 AUG.  
LOW RPESSURE THEN DEPARTS INTO CANADA WED WITH THE TRAILING FRONT  
CROSSING EASTERN STATES TO THE EAST COAST.  
THE MODELS ARE IN GOOD AGREEMENT ON THE DEVELOPMENT OF LOW  
PRESSURE BUT INTENSITY DIFFERENCES REMAIN. THE 12-18Z GFS RUNS  
WERE A BIT FURTHER SOUTH AND WEAKER THAN THE 12Z CANADIAN ECMWF.  
THE ECMWF/GEFS/CANADIAN ENSMEBLE MEANS PERSIST IN HAVING BETTER  
AGREEMENT ON THE LOW POSSITIONS AND INTENSITY THROUGH THE  
FORECAST. MORE WEIGHTING WAS GIVEN TO THE ENSMEBLE MEANS OF THE  
18Z GEFS AND 12Z ECMWF AS A RESULT, WITH LOW WEIGHTING OF THE  
OPERATIONAL 12Z ECMWF AND CANADIAN TO BOOST THE INTENSITY OF THE  
LOW PRESSURE INTENSITY A BIT.  
 
OUT WEST, THE GUIDANCE INITIALLY CENTERS THE UPPER RIDGE OVER THE  
SOUTHWEST. THERE IS SOME QUESTION IN THE DETAILS OF A 500 MB  
SHORTWAVE CROSSING THROUGH THE PACIFIC NORTHWEST, NORTHERN  
ROCKIES, AND NORTHERN GREAT BASIN EARLY IN THE WEEK. EACH OF THE  
ECMWF/CANADIAN/GFS HAVE A DIFFERENT AMPLITUDE AND THE MODELS ARE  
NOT QUITE IN SYNCH WITH THE MEANS, SO A MULTI-MODEL/ENSEMBLE MEAN  
BLEND WAS USED UNTIL BETTER AGREEMENT DEVELOPS ON THE EVOLUTION OF  
THE WAVE. THE CONSENSUS IS ONCE THE WAVE MOVES EAST IT DEAMPLIFIES  
AS IT CROSSES THE INTERMOUNTAIN WEST AND IT MOVES INTO CONFLUENT  
FLOW.  
 
THE MID LEVEL RIDGE BECOMES RESATABLISHED OVER THE NORTHWEST BY  
NEXT THU WITH THE APPROACH OF AN EAST PACIFIC TROUGH.  
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
SHORTWAVE ENERGY THAT 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 CENTERED ON  
TUESDAY. 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 CORE OF THE COOLEST HIGHS (10 TO 15 DEGREES BELOW NORMAL) IS  
FORECAST TO BE 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 LIKELY.  
 
PETERSEN  
 
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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