793  
FXUS02 KWBC 290650  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
249 AM EDT SUN JUL 29 2018  
 
VALID 12Z WED AUG 01 2018 - 12Z SUN AUG 05 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
A STRONG BERMUDA HIGH WILL EXTEND ITS INFLUENCE INTO THE EASTERN  
U.S. THROUGH THE PERIOD. THIS FEATURE WILL HOLD IN PLACE A  
STEADILY WEAKENING UPPER TROUGH OVER OR JUST EAST OF THE MS RIVER  
WHILE FLOW AROUND THE SURFACE HIGH WILL EVENTUALLY OVERWHELM A  
FRONT INITIALLY TRAILING FROM SURFACE LOW PRESSURE TRACKING  
NORTHEAST FROM THE GREAT LAKES. MEANWHILE FLOW ACROSS THE  
NORTHERN HALF OF THE WEST WILL FLATTEN WITH THE EASTWARD  
PROGRESSION OF ENERGY ASSOCIATED WITH A MODEST UPPER LOW NEAR  
NORTHERN VANCOUVER ISLAND AS OF EARLY WED. THE SHORTWAVE WILL  
BRING AN AREA OF SURFACE LOW PRESSURE ACROSS SOUTHERN CANADA AND  
THE NORTHERN U.S.. GUIDANCE CONTINUES TO ADVERTISE A DRAMATIC  
PATTERN CHANGE OVER THE PACIFIC AS A STRENGTHENING NORTH  
PACIFIC/ALEUTIANS RIDGE ENCOURAGES TROUGHING TO DEVELOP OVER THE  
NORTHEASTERN PACIFIC. THE LEADING EDGE OF THE TROUGH MAY BEGIN TO  
INFLUENCE THE WEATHER OVER THE PACIFIC NORTHWEST TOWARD THE END OF  
THE PERIOD.  
 
FOR DEPICTING PRIMARY FEATURES OF INTEREST A BLEND CONSISTING OF  
MOSTLY THE 18Z GFS/12Z ECMWF WITH THE REST COMPOSED OF THEIR  
ENSEMBLE MEANS WORKED WELL FOR DAYS 3-5 WED-FRI. CMC RUNS THROUGH  
12Z/28 HAD BEEN TOO WEAK WITH THE WESTERN ATLANTIC RIDGE ALOFT,  
ALLOWING THE EAST-CENTRAL U.S. TROUGH TO EXTEND TOO FAR EASTWARD.  
THE 00Z CMC HAS FINALLY TRENDED CLOSE TO CONSENSUS THAT HAS ALSO  
INCLUDED THE UKMET. ON THE OTHER HAND UKMET RUNS HAVE TENDED TO  
BE ON THE FAST SIDE OF THE SPREAD WITH THE SYSTEM TRACKING ALONG  
THE WESTERN/CENTRAL U.S.-CANADIAN BORDER. CMC TENDENCY OVER THE  
PAST COUPLE RUNS HAS BEEN TOWARD SLOWER THAN CONSENSUS TIMING FOR  
THIS SYSTEM.  
 
DAYS 6-7 SAT-SUN TRANSITIONED THE BLEND TO A MORE EVEN  
MODEL/ENSEMBLE WEIGHT WITH A SLIGHT TILT IN THE ECMWF/ECMWF MEAN  
DIRECTION, PLUS USING THE LAST TWO ECMWF RUNS FOR THE OPERATIONAL  
ECMWF COMPONENT DUE TO INCREASING DETAIL DIFFERENCES. AN OVERALL  
CONSENSUS AMONG GFS/ECMWF RUNS AND THEIR MEANS HAS BEEN FAIRLY  
STABLE FOR THE SYSTEM NEAR THE CANADIAN BORDER THUS FAR (ALBEIT  
WITH SOME VARIABILITY/SPREAD AMONG INDIVIDUAL SOLUTIONS/RUNS), IN  
SPITE OF WHAT HAVE BEEN SOME SIGNIFICANT DIFFERENCES FOR PACIFIC  
EVOLUTION. RECENT GFS/GEFS RUNS HAVE BEEN ON THE WESTERN SIDE OF  
THE SPREAD FOR THE OVERALL PACIFIC RIDGE-TROUGH PATTERN. THROUGH  
THE 00Z CYCLE THEY HAVE BEEN NUDGING TOWARD CONSENSUS FOR THE  
RIDGE BUT CONTINUE TO DEPICT A DEEP/WESTWARD TROUGH AND EMBEDDED  
UPPER LOW. TELECONNECTIONS RELATIVE TO THE POSITIVE HEIGHT  
ANOMALY CENTER NEAR THE ALEUTIANS IN D+8 CHARTS FAVOR A  
NORTHEASTERN PACIFIC TROUGH CLOSER TO THE ECMWF/ECMWF MEAN, THOUGH  
THE AMPLIFIED NATURE OF THE PATTERN ALLOWS FOR PARTIAL INCLUSION  
OF THE GFS/GEFS SCENARIO.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE SOUTHEAST CONTINUES TO SHOW THE BEST POTENTIAL FOR HIGHEST  
RAINFALL TOTALS DURING THE PERIOD. FLOW AROUND THE BERMUDA HIGH  
WILL SUPPORT A PERSISTENT FEED OF LOW LEVEL ATLANTIC/EASTERN GULF  
MOISTURE INTO THE REGION. THIS MOISTURE SHOULD INTERACT WITH THE  
SLOW MOVING FRONT OVER THE EAST DURING THE FIRST HALF OF THE  
PERIOD. AFTER THE FRONT DISSIPATES EXPECT CONVECTION TO BE  
SOMEWHAT LESS ORGANIZED WITH INCREASING FOCUS ALONG TERRAIN AND  
LOCALIZED BOUNDARIES. CONVECTION/RAINFALL THROUGH THE PERIOD MAY  
BE PROBLEMATIC OVER AREAS THAT HAVE RECEIVED SIGNIFICANT RAINFALL  
OVER THE PAST TWO WEEKS. EXPECT SOME MOISTURE/RAINFALL TO EXTEND  
INTO THE MID-ATLANTIC/NORTHEAST, WITH POCKETS OF MODERATE-HEAVY  
ACTIVITY BUT IN GENERAL LESS EXTREME TOTALS THAN OVER THE  
SOUTHEAST.  
 
PERIODS OF CONVECTION WITH VARYING INTENSITY WILL CONTINUE OVER  
THE FOUR CORNERS STATES WITH SHOWERS/STORMS POSSIBLY EXTENDING  
INTO THE SOUTHERN HIGH PLAINS AT TIMES. ACROSS THE NORTHERN  
STATES, ADVANCING SHORTWAVE ENERGY AND SURFACE LOW  
PRESSURE/LEADING WARM FRONT SHOULD ENCOURAGE CONVECTIVE  
DEVELOPMENT WITHIN AN AREA FROM THE NORTHERN PLAINS INTO WESTERN  
GREAT LAKES FROM LATE WEEK INTO THE WEEKEND. SOME THUNDERSTORMS  
MAY PRODUCE HEAVY RAINFALL.  
 
ACROSS THE WEST, FLATTER FLOW ALOFT OVER NORTHERN AREAS WILL BRING  
TEMPERATURES DOWN TO NEAR OR SLIGHTLY BELOW NORMAL. EXPECT THE  
GREAT BASIN/SOUTHWEST TO SEE ABOVE NORMAL MIN TEMPS THROUGH THE  
PERIOD BUT HIGHS WILL LIKELY BE MORE MODERATE. A WAVY FRONT  
PUSHING INTO THE NORTHERN U.S. WILL BRING AN AREA OF WELL BELOW  
NORMAL READINGS INTO THE NORTHERN PLAINS/UPPER MS VALLEY WED-THU.  
THEN FLATTENING OF THE UPPER PATTERN WILL ENCOURAGE A WARMING  
TREND TO ABOVE NORMAL LEVELS, PROGRESSING FROM THE NORTHERN PLAINS  
INTO GREAT LAKES DURING FRI-SUN. CLOUDS/RAINFALL SHOULD KEEP THE  
SOUTHEAST BELOW NORMAL FOR HIGHS ON MOST DAYS. FLOW AROUND THE  
BERMUDA HIGH WILL PROMOTE ABOVE NORMAL LOWS OVER THE  
MID-ATLANTIC/NORTHEAST.  
 
RAUSCH  
 
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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