273  
FXUS02 KWBC 121556  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1156 AM EDT SUN AUG 12 2018  
 
VALID 12Z WED AUG 15 2018 - 12Z SUN AUG 19 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE MAIN WESTERLIES SETTLE OVER CANADA WHILE A SERIES OF SLOW  
MOVING FEATURES ARE PRIMED TO DRIFT EASTWARD ACROSS THE NATION  
UNDERNEATH NEXT WEEK. PINPOINTING WHICH SHORTWAVE WILL DOMINATE IS  
DIFFICULT TO SAY GIVEN A LACK OF RUN TO RUN MODEL CONTINUITY IN  
CONJUNCTION WITH THE WEAK STEERING FLOWS TO CONTEST WITH. WHILE  
THE INITIAL IMPULSE IN THE CHAIN SHOULD BE EXITING OFF THE  
MID-ATLANTIC COAST ON WEDNESDAY, THE FOCUS SHIFTS TOWARD THE NEXT  
ELONGATED TROUGH PUSHING ACROSS THE NORTH-CENTRAL U.S. THE  
GUIDANCE CONTINUES TO STRUGGLE WITH HOW AMPLIFIED THE FLOW WILL  
BECOME AS IT MOVES DOWNSTREAM INTO THE OHIO RIVER VALLEY LATE IN  
THE WORK WEEK. ENSEMBLE SPAGHETTI PLOTS SHOW A MIXED BAG OF  
SCENARIOS ALTHOUGH A VAST MAJORITY OF THESE SUPPORT SOME SURFACE  
WAVE DEVELOPMENT FROM THE MID-MS VALLEY TO THE GREAT LAKES AND  
NORTHEAST/CANADIAN MARITIMES WED INTO NEXT WEEKEND. RENEWED  
TROUGHING ALOFT COULD EMERGE INTO NEXT WEEKEND OVER THE CENTRAL  
U.S ALONG WITH SURFACE WAVE FORMATION/FOCUS. ENSEMBLE SPAGHETTI  
PLOTS SHOW GENERAL WEST-NORTHWESTERLY FLOW AND EMBEDDED  
SHORTWAVES. FINALLY ACROSS WESTERN NORTH AMERICA, MULTI-DAY  
ENSEMBLE COMPARISONS HAVE SHOWN THE ECMWF MEANS TO REALLY BACK OFF  
ON THE POLEWARD EXTENT OF THE BUILDING RIDGE. THE CORE OF THE  
UPPER RIDGE SHOULD STAY CENTERED OVER THE SOUTHWESTERN U.S. WHILE  
RIPPLES IN THE FLOW PUSH INLAND OVERTOP INTO THE PACIFIC NORTHWEST  
NEXT WEEKEND.  
 
THE WPC MEDIUM RANGE PRODUCT SUITE WAS PRIMARILY DERIVED FROM A  
COMPOSITE BLEND OF THE 06 UTC GEFS MEAN AND 00 UTC ECMWF/ECMWF  
ENSEMBLE MEAN. INCREASING GUIDANCE SPREAD LENDS FAVOR TO AN  
ENSEMBLE BASED FORECAST APPROACH. THAT SAID, THE 00 UTC ECMWF DOES  
OFFER A BIT MORE AMPLITUDE THAN OTHER MODELS AND CLUSTERS BETTER  
WITH ENSEMBLES. THIS SEEMS CONSISTENT WITH THE OVERALL FLOW/RECENT  
FLOW HISTORY AND WPC CONTINUITY.  
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
THREATS OF DAILY BOUTS OF MODERATE TO LOCALLY HEAVY RAINFALL  
SHOULD FOCUS FROM THE SOUTH-CENTRAL PLAINS AND MID-MS  
VALLEY/MID-SOUTH THROUGH THE THE OH VALLEY TO THE LOWER GREAT  
LAKES/NORTHEAST AS DEEPENED MOISTURE POOLS NEAR WAVY FRONTS.  
SURROUNDING LOCATIONS CAN EXPECT LESS FOCUSED DIURNALLY FORCED  
CONVECTION, BUT WITH SOME LOCALLY HEAVY DOWNPOURS. MEANWHILE,  
ACROSS AZ AND THE SOUTHERN HIGH PLAINS/ROCKIES IN FAVORABLE UPPER  
DIFFLUENT FLOW, MONSOONAL FLOW WILL DRIVE PERIODS OF DAILY  
CONVECTION AND SOME LOCAL RUNOFF ISSUES.  
 
SCHICHTEL  
 
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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