875  
FXUS02 KWBC 020648  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
247 AM EDT THU AUG 02 2018  
 
VALID 12Z SUN AUG 05 2018 - 12Z THU AUG 09 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
LATEST GUIDANCE IS REINFORCING TRENDS THAT EMERGED 24-36 HOURS  
AGO, TOWARD THE IDEA OF AN UPPER LOW DROPPING INTO THE EASTERN  
PACIFIC BECOMING CUT OFF FROM THE WESTERLIES FOR MOST OF THE  
MEDIUM RANGE TIME FRAME--KEEPING THE FEATURE WELL OFFSHORE THE  
WEST COAST WHILE A STRONG RIDGE BUILDS OVER THE WESTERN U.S. AND  
INTO CANADA. IN RESPONSE TO THIS RIDGE EXPECT MEAN TROUGHING TO  
DEVELOP OVER EASTERN NORTH AMERICA, ULTIMATELY LEADING TO  
WEAKENING/SUPPRESSION OF THE UPPER RIDGE THAT SHOULD BE IN THE  
PROCESS OF BUILDING INTO THE SOUTHEAST DURING THE WEEKEND.  
 
BASED ON THE CURRENT CONSENSUS INCLUDING THE SLOWER-TRENDING 00Z  
CMC THE EASTERN PACIFIC UPPER LOW MAY NOT BEGIN TO EJECT  
NORTHEASTWARD UNTIL AROUND DAY 7 THU, SO LEADING HEIGHT FALLS MAY  
NOT REACH THE PACIFIC NORTHWEST UNTIL THEN. OPERATIONAL RUNS  
GENERALLY LED THE ENSEMBLES AS THE RAPID ADJUSTMENT IN GUIDANCE  
OCCURRED OVER THE PAST COUPLE DAYS, MERITING OPERATIONAL MODEL  
EMPHASIS EARLY-MID PERIOD AND AT LEAST HALF WEIGHT OF THE  
OPERATIONAL RUNS LATE.  
 
OVER THE EASTERN 2/3 OF THE CONTINENT, VARIOUS SHORTWAVE  
UNCERTAINTIES WILL BE THE PRIMARY FORECAST ISSUE. LOW AMPLITUDE  
FLOW INITIALLY OVER THE NORTHERN TIER WILL LIKELY CONTAIN AT LEAST  
A COUPLE TRACKABLE SHORTWAVES OR SHORTWAVE CLUSTERS--ONE OVER THE  
NORTHERN PLAINS/UPPER MS VALLEY AS OF EARLY SUN AND A SECOND  
PROGRESSING FROM THE NORTHWEST INTO THE NORTHERN PLAINS. THIS  
TRAILING FEATURE AND THEN ANOTHER SHORTWAVE POSSIBLY REACHING INTO  
CANADA DURING THE FIRST HALF OF NEXT WEEK SHOULD AMPLIFY INTO THE  
EXPECTED EASTERN MEAN TROUGH ALOFT. DEPTH AND TIMING OF SEPARATE  
SHORTWAVES VARY AMONG MODEL RUNS, ENSEMBLE MEMBERS, AND BETWEEN  
CONSECUTIVE CYCLES SO CONFIDENCE IS A LOT LOWER FOR THE COMPONENT  
FEATURES WITHIN THE MORE HIGHLY PREDICTABLE OVERALL TROUGH  
PATTERN. THIS FAVORS A MODEL/MEAN APPROACH ESPECIALLY BY THE  
LATTER HALF OF THE PERIOD.  
 
BASED ON GUIDANCE THROUGH THE 12Z/18Z CYCLES, THE UPDATED FORECAST  
USED INPUT FROM THE 12Z-18Z GFS/12Z ECMWF AND TO A LESSER EXTENT  
THE 12Z CMC/UKMET INTO DAY 5 TUE. THE FORECAST INTRODUCED THE 18Z  
GEFS/12Z ECMWF MEANS LATE TUE WITH ENSEMBLE WEIGHT REACHING 50  
PERCENT OF THE TOTAL BY DAY 7 THU.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
WITH THE LATEST CYCLE OF GUIDANCE EITHER CONSISTENT WITH OR  
FURTHERING TRENDS TOWARD KEEPING THE EASTERN PACIFIC UPPER LOW  
WELL OFFSHORE THROUGH THE PERIOD WHILE UPPER RIDGING BUILDS OVER  
THE WEST, CONFIDENCE IS INCREASING THAT MOST OF THE WESTERN U.S.  
SHOULD SEE A WARMING TREND TO ABOVE NORMAL TEMPERATURES BY  
TUE-THU. DRY CONDITIONS WILL LIKELY PREVAIL OVER MOST AREAS AS  
WELL, ASIDE FROM DIURNALLY FAVORED CONVECTION AT TIMES OVER PARTS  
OF THE FOUR CORNERS STATES. BEFORE THIS WARM-UP LOCATIONS FROM  
THE CENTRAL WEST COAST INTO THE NORTHERN ROCKIES MAY SEE BELOW  
NORMAL HIGHS ESPECIALLY ON SUN.  
 
FARTHER EASTWARD THE GRADUAL DEVELOPMENT OF UPPER TROUGHING WILL  
HELP TO PUSH AN INITIAL WAVY FRONT OVER THE NORTHERN PLAINS  
SOUTHEASTWARD WITH TIME. AS A RESULT THE FOCUS FOR AREAS OF  
POTENTIALLY HEAVY/STRONG CONVECTION WILL SHIFT FROM THE UPPER MS  
VALLEY/GREAT LAKES/NORTH-CENTRAL PLAINS TOWARD/INTO THE  
EASTERN-SOUTHERN STATES. SOME RAINFALL IS ALSO POSSIBLE WITH  
ANOTHER FRONT THAT MAY DROP INTO THE NORTHERN STATES LATE IN THE  
PERIOD BUT LESS AVAILABLE MOISTURE SHOULD LEAD TO LOWER TOTALS  
THAN WITH THE ACTIVITY EARLIER IN THE PERIOD. AHEAD OF THESE  
FRONTS EARLY IN THE WEEK, PORTIONS OF THE SOUTH AND EAST MAY SEE  
SCATTERED AND GENERALLY LIGHT-MODERATE SHOWERS/THUNDERSTORMS. FOR  
TEMPERATURES EAST OF THE ROCKIES EXPECT AREAS FROM THE GREAT LAKES  
INTO THE NORTHEAST TO SEE THE WARMEST READINGS VERSUS NORMAL  
(ESPECIALLY FOR MORNING LOWS) IN THE SUN-TUE TIME FRAME. THE  
EXPECTED PATTERN EVOLUTION SHOULD YIELD NEAR TO SLIGHTLY BELOW  
NORMAL HIGHS OVER A MAJORITY OF THE EAST BY NEXT WED-THU.  
 
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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