463  
FXUS02 KWBC 310655  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
254 AM EDT TUE JUL 31 2018  
 
VALID 12Z FRI AUG 03 2018 - 12Z TUE AUG 07 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
LATEST GUIDANCE CONTINUES TO SHOW THAT A STRONG UPPER RIDGE SOUTH  
OF THE ALEUTIANS WILL LEAD TO AN AMPLIFIED EASTERN PACIFIC TROUGH  
BY THIS WEEKEND. BEST CONSENSUS HAS THIS TROUGH PERSISTING OFF  
THE WEST COAST THROUGH EARLY NEXT WEEK BUT WITH DEVELOPING  
UNCERTAINTY OVER WHERE THE MAIN CORE OF TROUGH ENERGY WILL BE AT  
THAT TIME. MEANWHILE A MS VALLEY UPPER TROUGH ALREADY IN THE  
PROCESS OF WEAKENING AS OF EARLY FRI WILL CONTINUE TO WEAKEN  
RAPIDLY WHILE AN INITIAL BERMUDA HIGH WILL LIKELY BUILD INTO THE  
SOUTHEAST DURING THE WEEKEND AND THEN POSSIBLY CONTINUE TO  
RETROGRADE INTO THE PLAINS. BETWEEN THE PACIFIC TROUGH AND  
EASTERN RIDGE, EXPECT THE MAIN AXIS OF JET ENERGY TO FLOW FROM THE  
NORTHERN HALF OF THE WEST ACROSS THE NORTHERN TIER STATES AND  
SOUTHERN CANADA.  
 
FOR THE DEVELOPING EASTERN PACIFIC TROUGH, THE ECMWF MEAN HAS BEEN  
THE MOST STABLE SOLUTION OVER THE PAST 2-3 DAYS--AND HAS  
POSITIONED THE TROUGH FAIRLY CLOSE TO WHERE RECOMMENDED BY  
TELECONNECTIONS RELATIVE TO THE UPSTREAM RIDGE. EASTWARD GEFS  
TRENDS OVER THE PAST COUPLE DAYS HAVE BROUGHT ITS MEAN IN LINE  
WITH THE ECMWF MEAN. INDIVIDUAL MODELS AND ENSEMBLE MEMBERS STILL  
OFFER A FAIR AMOUNT OF SPREAD OVER WHERE THE TROUGH AND POSSIBLE  
EMBEDDED UPPER LOW WILL BE ON A PARTICULAR DAY. THE 12Z GFS/ECMWF  
ARE CLOSEST TO THE AFOREMENTIONED MEANS BUT THE 18Z GFS STRAYED  
BACK TO THE WEST (A TENDENCY OF EARLIER GFS RUNS). ON THE OTHER  
HAND SOME PRIOR ECMWF RUNS HAD BEEN A BIT EAST OF THE MEANS. IN  
VARYING WAYS THE NEW 00Z GFS AND RECENT CMC RUNS INDICATE  
POTENTIAL FOR FASTER EJECTION OF TROUGH ENERGY AT LEAST DUE IN  
PART TO APPROACH OF UPSTREAM ENERGY. SUCH DETAIL HAS LOW  
PREDICTABILITY 6-7 DAYS OUT IN TIME THUS RECOMMENDING THE ENSEMBLE  
MEANS AND 12Z GFS/ECMWF FOR THE DETERMINISTIC FORECAST. HOWEVER  
IT IS WITH THE UNDERSTANDING THAT A CYCLE OF EJECTION AND  
RELOADING IS ALSO A PLAUSIBLE SCENARIO THAT WOULD LEAD TO A  
SIMILAR MEAN TROUGH FROM THE MULTI-DAY MEAN PERSPECTIVE.  
 
THE FAST AND LOW AMPLITUDE NATURE OF MEAN FLOW ACROSS THE NORTHERN  
U.S./SOUTHERN CANADA LEADS TO ONGOING DIFFICULTY IN RESOLVING  
DETAILS OF SHORTWAVES ALOFT AND ASSOCIATED FRONTAL BOUNDARIES  
AFFECTING THE NORTHERN U.S.. THERE IS STILL FAIR CLUSTERING FOR A  
LEADING SYSTEM OVER/NORTH OF THE NORTHERN PLAINS FRI-SAT FOLLOWED  
BY DIVERGENCE ON EXACTLY HOW THE LEADING ENERGY ALOFT WILL EVOLVE  
AND ON DETAILS OF UPSTREAM ENERGY THAT FLOWS INTO THE WESTERN  
U.S.. GUIDANCE IS GENERALLY SIGNALING THAT BY LATE IN THE PERIOD  
AND BEYOND SOME OF THIS TRAILING ENERGY WILL ULTIMATELY SETTLE  
INTO A MODEST EASTERN NORTH AMERICA TROUGH THAT WOULD BE  
CONSISTENT WITH THE PATTERN UPSTREAM.  
 
GIVEN THE RELATIVE AGREEMENT OF THE 12Z GFS/ECMWF WITH THE 18-12Z  
GEFS/12Z ECMWF MEANS FOR THE EASTERN PACIFIC TROUGH AS WELL AS  
DECENT ALIGNMENT ELSEWHERE (WITH THE 12Z UKMET/CMC DIFFERING OVER  
ONE OR MORE AREAS) THOSE RUNS SERVED AS THE OPERATIONAL INPUT FOR  
THE UPDATED FORECAST. ENSEMBLE MEANS CONSTITUTED A SMALL MINORITY  
EARLY IN THE PERIOD AND REACHED A TOTAL HALF WEIGHT BY DAY 7 TUE.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
BY THE LATTER HALF OF THE PERIOD THE AMPLIFYING UPPER TROUGH OFF  
THE WEST COAST HAS THE POTENTIAL TO BRING MORE MOISTURE INTO PARTS  
OF THE PACIFIC NORTHWEST/VANCOUVER ISLAND THAN MIGHT TYPICALLY BE  
EXPECTED FOR THE FIRST HALF OF AUGUST. RAINFALL AMOUNTS WILL BE  
SENSITIVE TO EXACT DETAILS ALOFT AND INDIVIDUAL MODEL SOLUTIONS  
ARE CURRENTLY QUITE VARIED FOR MULTI-DAY TOTALS. ALSO OVER THE  
WEST, DIURNALLY FAVORED CONVECTION WITH SOME LOCALLY  
MODERATE-HEAVY ACTIVITY WILL CONTINUE OVER THE FOUR CORNERS STATES.  
 
FARTHER EASTWARD EXPECT AREAS OF ENHANCED RAINFALL FROM THE GULF  
COAST NORTHEASTWARD AT THE START OF THE PERIOD TO BECOME LIGHTER  
AND MORE SCATTERED WITH TIME AS RIDGING ALOFT BUILDS INLAND FROM  
THE ATLANTIC. AT THE SAME TIME THE AREA EXTENDING FROM THE  
NORTHERN PLAINS INTO THE GREAT LAKES WILL BECOME A PROMINENT FOCUS  
FOR PERIODS OF POTENTIALLY HEAVY/STRONG CONVECTION. THE SYSTEM  
REACHING THE NORTHERN PLAINS LATE THIS WEEK WILL PROVIDE AN  
INITIAL FOCUS FOR ACTIVITY. THEN THE ORIENTATION OF FLOW ALOFT  
WILL ENCOURAGE PERSISTENCE OF A MEAN FRONTAL BOUNDARY OVER OR NEAR  
THIS REGION, LEADING TO THE POSSIBILITY OF REPEAT  
ACTIVITY/TRAINING AT SOME LOCATIONS.  
 
EXPECT HIGH TEMPERATURES TO BE BELOW NORMAL (MOSTLY SINGLE-DIGIT  
ANOMALIES) DURING THE PERIOD OVER THE NORTHWEST WHILE FARTHER  
SOUTH READINGS SHOULD SLOWLY DECREASE TO OR A FRACTION BELOW  
NORMAL. MIN TEMPS WILL GENERALLY BE WARMER RELATIVE TO  
CLIMATOLOGY. TO THE EAST OF THE ROCKIES THERE WILL BE GRADUALLY  
INCREASING COVERAGE OF ABOVE NORMAL TEMPERATURES. HIGHEST  
ANOMALIES WITH A FEW POCKETS EXCEEDING 10F ABOVE NORMAL (SOMEWHAT  
MORE LIKELY FOR MIN TEMPS) SHOULD BE WITHIN AN AXIS FROM THE  
NORTHERN-CENTRAL PLAINS INTO THE NORTHEAST. BELOW AVERAGE HIGHS  
OVER THE SOUTHEAST TO START THE PERIOD SHOULD TREND TOWARD NORMAL  
LEVELS AS CLOUDS/RAINFALL BECOME LESS ORGANIZED.  
 
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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