571  
FXUS02 KWBC 150520  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
119 AM EDT WED AUG 15 2018  
 
VALID 12Z SAT AUG 18 2018 - 12Z WED AUG 22 2018  
   
..PATTERN OVERVIEW AND PREFERENCES
 
 
GUIDANCE CONTINUES TO AGREE ON AN AMPLIFICATION OF THE LARGE SCALE  
PATTERN TOWARD A CENTRAL/EASTERN NORTH AMERICA TROUGH AND WESTERN  
US RIDGE. THIS EVOLUTION WILL BE IN RESPONSE TO A DEEP UPPER  
TROUGH/LOW THAT DEVELOPS TO THE SOUTH OF THE ALASKA PENINSULA AND  
THE DOWNSTREAM RIDGE WHICH THEN BUILDS OVER WESTERN CANADA AND  
VICINITY.  
 
DURING FRI TO SAT OF THE FORECAST MODELS AND ENSEMBLES ARE IN GOOD  
AGREEMENT ON A WAVE OF LOW PRESSURE MOVING OFF THE NEW ENGLAND  
COAST AND TRAILING FRONT MOVING SOUTH ACROSS THE MID ATLANTIC.  
 
A SHORTWAVE ALOFT EMERGING INTO THE HIGH PLAINS THIS WEEKEND SPURS  
A DEFINED SURFACE WAVE TO DEVELOP,  
WITH THAT WAVE MERGING WITH A WAVE CAUSING A MERGER WITH LOW  
PRESSURE MOVING NORTHEAST FROM THE SOUTHERN PLAINS TO FORM A  
CONSOLIDATED CENTER OF LOW PRESSURE OVER THE MID MS VALLEY BY 12Z  
MON THAT MOVES  
NORTHEAST TOWARDS THE GREAT LAKES LATE MON AND DEPARTING INTO  
CANADA TUE.  
THE 12Z ECMWF WAS THE STRONGEST SOLUTION AND RESULTS IN HEAVIER  
RAIN ASSOCIATED WITH THE LOW PRESSURE.  
THE PREFERENCE CONTINUES TO BE HEDGING WITH THE 18Z GEFS/12Z ECMWF  
MEANS FOR BOTH THE INTENSITY OF THE SYSTEM AND RESULTANT RAINFALL  
UNTIL MORE SUPPORT FOR THE STRONGER SOLUTIONS DEVELOPS.  
THE ENSEMBLE MEANS OFFER GOOD AGREEMENT/CONTINUITY WITH LOW  
PRESSURE REACHING THE GREAT LAKES BY EARLY DAY 5 TUE 21 AUG.  
 
THE GFS/ECMWF/ENSEMBLE MEANS CENTER THE HIGH AMPLITUDE RIDGE OVER  
THE SOUTHWEST WITH ABOVE NORMAL HEIGHTS IN THE MIDDLE OF NEXT WEEK  
ACROSS THE NORTHWEST AS THE EAST PACIFIC RIDGE DRIFTS INLAND.  
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
NEW ENGLAND LOW PRESSURE PROVIDES WIDESPREAD SHOWERS/STORMS FRI  
THROUGH FRI NIGHT, ENDING AS THE WAVE MOVES OFFSHORE SAT.  
SHORTWAVE ENERGY THAT LIKELY REACHES THE PLAINS BY SUN SHOULD  
PROVIDE FOCUS FOR ORGANIZED CONVECTION AND LOCALLY HEAVY RAIN FROM  
THE HIGH PLAINS ON SAT TO ACROSS THE CENTRAL PLAINS TO MID MS  
VALLEY ON SUN. LOW PRESSURE MOVING NORTHEAST FROM THE MS VALLEY  
TOWARDS THE LAKES DRIVES SHOWERS/STORMS ON MON, AND AS THE LOW  
PRESSURE DEPARTS, SHOWERS/STORMS OCCUR ON THE TRAILING COLD FRONT  
IN THE LOWER LAKES TO THE APPALACHIANS AND SOUTHEAST TUE.  
 
TERRAIN/DIURNAL HEATING WILL PROMOTE SHOWERS/STORMS OVER PARTS OF  
THE SOUTHWEST WITH EXTREME SOUTHERN AREAS LIKELY TO SEE THE  
HIGHEST RAINFALL TOTALS WITH GREATER AVAILABLE MOISTURE. FLORIDA  
AND THE GULF COAST REGION SHOULD SEE DIURNAL SHOWERS/STORMS AS  
WELL THROUGH THE WEEKEND, WITH MORE ACTIVITY AS THE FRONT  
APPROACHES IN THE MIDDLE OF NEXT WEEK.  
 
THE LARGE SCALE TROUGH IN THE CENTRAL US WILL BRING AN EXPANDING  
AREA OF BELOW NORMAL TEMPERATURES EARLY NEXT WEEK. CURRENTLY  
EXPECT THE CORE OF COOLEST HIGH TEMPERATURES (10-15F BELOW NORMAL)  
TO BE CENTERED OVER THE NORTH-CENTRAL HIGH PLAINS SUN-MON. THE  
COLD FRONT MOVES ALL THE WAY TO THE GULF COAST SO THE SOUTHERN  
PLAINS EVEN GETS A DOSE OF COOLER CONDITIONS IN THE MIDDLE OF NEXT  
WEEK.  
 
DAYTIME HIGHS OVER THE INTERIOR WEST WILL TREND GRADUALLY WARMER  
WITH TIME AS RIDGING ALOFT REBUILDS. DAYTIME HIGHS ARE EXPECTED TO  
BE IN THE UPPER SINGLE DIGITS TO LOWER DOUBLE DIGITS ABOVE NORMAL  
ACROSS INTERIOR WA/OR/ID.  
 
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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