640  
FXUS02 KWBC 300650  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
250 AM EDT MON JUL 30 2018  
 
VALID 12Z THU AUG 02 2018 - 12Z MON AUG 06 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
DURING THE FIRST HALF OF THE PERIOD THERE WILL BE A  
WELL-ADVERTISED FLATTENING OF THE MEAN PATTERN, AS SOUTHWEST  
CANADA/PACIFIC NORTHWEST SHORTWAVE ENERGY PROGRESSES EASTWARD  
WHILE AN INITIAL MS RIVER UPPER TROUGH DISSIPATES THROUGH A  
COMBINATION OF WEAKENING IN PLACE AND EJECTION OF ENERGY AROUND A  
STRONG BERMUDA HIGH. A VERY STRONG RIDGE SOUTH OF THE ALEUTIANS  
WILL PROMOTE AMPLIFICATION OF AN EASTERN PACIFIC TROUGH AFTER  
EARLY FRI. CONSISTENT WITH TELECONNECTIONS RELATIVE TO THE  
PACIFIC PATTERN, GUIDANCE CONSENSUS EXPECTS FLOW OVER NORTH  
AMERICA TO EVOLVE TOWARD A WEST-CENTRAL RIDGE/EASTERN TROUGH BY  
LATE IN THE PERIOD AND JUST BEYOND.  
 
OVER THE EASTERN PACIFIC THE PRIMARY QUESTION OVER RECENT RUNS HAS  
BEEN THE LONGITUDE OF THE EVOLVING TROUGH. GFS/GEFS RUNS HAD BEEN  
ON THE WESTERN SIDE OF THE SPREAD AND NOT ESPECIALLY FAVORED BY  
TELECONNECTIONS RELATIVE TO THE UPSTREAM RIDGE'S CORRESPONDING  
HEIGHT ANOMALY CENTER. LATEST VERSIONS HAVE TRENDED FAVORABLY  
EASTWARD THUS ALLOWING FOR A MORE EVEN WEIGHT IN THE FORECAST  
BLEND. ON THE OTHER HAND THE 12Z ECMWF MAY ACTUALLY BE A LITTLE  
OVERDONE WITH THE EXTENT OF LEADING HEIGHT FALLS REACHING THE  
WEST--THUS FAVORING SOME INCLUSION OF THE 00Z/29 RUN TO BALANCE  
THE OPERATIONAL ECMWF COMPONENT OF THE FORECAST.  
 
FOR THE SHORTWAVE ENERGY PROGRESSING EASTWARD FROM THE PACIFIC  
NORTHWEST/SOUTHWESTERN CANADA THE UKMET CONTINUES TO LIE ON THE  
FAST SIDE OF THE ENVELOPE. BEST CONSENSUS OF THE GFS/ECMWF AND  
THEIR MEANS HAS TRENDED A LITTLE FASTER BY FRI OR SAT THOUGH.  
ASIDE FROM THIS TREND THERE IS REASONABLE CONTINUITY FOR THE FRONT  
REACHING THE NORTHERN PLAINS BY FRI-SAT. SPECIFICS BECOME  
INCREASINGLY MURKY AFTER SAT WITH VARYING IDEAS REGARDING HOW  
QUICKLY THE BEST DEFINED SHORTWAVE ALOFT WILL PROGRESS, WITH  
DETAIL UNCERTAINTIES FOR UPSTREAM IMPULSES AND HIGHER LATITUDE  
FLOW POSSIBLY PLAYING A ROLE. BY LATE IN THE PERIOD THE 12Z ECMWF  
IS A NOTABLE FASTER SOLUTION AT THE SURFACE AND ALOFT.  
 
FARTHER SOUTH/SOUTHEAST THERE ARE ONLY LOW-PREDICTABILITY DETAIL  
DIFFERENCES FOR THE MS VALLEY TROUGH ALOFT AND ASSOCIATED FRONTAL  
SYSTEM THAT WILL WEAKEN DURING THE FIRST HALF OF THE PERIOD.  
OTHERWISE THE BERMUDA HIGH WILL BE THE PREVAILING FEATURE  
AFFECTING THE EASTERN U.S. PATTERN. AROUND THE UPPER HIGH CENTER  
MOST SOLUTIONS SHOW A WEAKNESS TRACKING JUST OFF THE EAST COAST BY  
FRI ONWARD BUT WITH SIGNIFICANT TIMING DIFFERENCES.  
 
EARLY IN THE PERIOD A BLEND OF THE 18Z GFS/12Z ECMWF AND THEIR  
MEANS--WITH MOST WEIGHT TO THE OPERATIONAL RUNS--BEST REFLECTED  
THE MOST COMMON IDEAS OF LATEST GUIDANCE. BY MID-LATE PERIOD  
AFOREMENTIONED QUESTION MARKS WITH THE 12Z ECMWF SUPPORTED ADDING  
SOME 00Z/29 ECMWF TO THE BLEND AS WELL AND ENSEMBLE MEAN INPUT  
INCREASED TO HALF WEIGHT BY DAY 7 MON GIVEN TYPICAL DETAIL  
UNCERTAINTY AT THAT TIME FRAME.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
WITHIN THE FAVORABLE AREA FOR LOCALLY HEAVY RAINFALL OVER THE  
SOUTHEAST/SOUTHERN HALF OF THE APPALACHIANS AND POSSIBLY EXTENDING  
INTO PARTS OF THE MID-ATLANTIC, THE BEST ORGANIZED ACTIVITY SHOULD  
BE LATE THIS WEEK WHEN FLOW BETWEEN THE MS VALLEY UPPER TROUGH AND  
BERMUDA HIGH WILL DIRECT THE MOST FOCUSED GULF/ATLANTIC MOISTURE  
INTO THE REGION. A WEAKENING COLD/STATIONARY FRONT WILL PROVIDE A  
LOW-LEVEL FOCUS FOR CONVECTION AS WELL. BY SAT-MON EXPECT  
SHOWERS/STORMS TO BECOME GRADUALLY LIGHTER AND MORE SCATTERED AS  
FLOW ALOFT BECOMES LESS DEFINED AND THE SURFACE RIDGE AXIS DRIFTS  
SOUTHWESTWARD. THE NORTHEASTERN PLAINS INTO THE GREAT LAKES WILL  
BE ANOTHER REGION WITH ENHANCED RAINFALL POTENTIAL DURING THE  
PERIOD. SOME OF THE RAIN WILL COME WITH A WAVY FRONT SETTLING  
INTO THE REGION LATE THIS WEEK. THEN EXPECT HEAVIER/STRONGER  
CONVECTION WITH THE SYSTEM EMERGING INTO THE NORTHERN PLAINS BY  
FRI WITH THE LEADING WARM FRONT PROVIDING AN ADDED FOCUS. THE  
COLD FRONT TRAILING FROM THIS SYSTEM MAY DECELERATE WITH TIME,  
ALLOWING FOR SOME PERSISTENCE/TRAINING LATER IN THE PERIOD.  
MEANWHILE DIURNALLY FAVORED CONVECTION OF VARYING INTENSITY WILL  
CONTINUE OVER PORTIONS OF THE FOUR CORNERS STATES. THE AMPLIFYING  
UPPER TROUGH NEARING THE WEST COAST LATER IN THE PERIOD MAY  
EVENTUALLY BRING SOME MOISTURE TO THE PACIFIC NORTHWEST.  
 
EXPECTED PATTERN EVOLUTION WILL LIKELY LEAD TO AN AREA OF ABOVE  
NORMAL MIN AND/OR MAX TEMPERATURES BECOMING ORIENTED FROM THE  
NORTH-CENTRAL/CENTRAL PLAINS THROUGH THE GREAT LAKES/NORTHEAST.  
ANOMALIES EXCEEDING 10F SHOULD BE SOMEWHAT MORE COMMON FOR MORNING  
LOWS. THE SOUTHEASTERN THIRD OF THE COUNTRY WILL SEE COOLEST  
DAYTIME HIGHS VERSUS NORMAL LATE THIS WEEK WHEN CLOUDS/RAINFALL  
SHOULD BE MOST PREVALENT, FOLLOWED BY A TREND TOWARD NORMAL  
VALUES. OVER THE WEST, MIN TEMPS WILL LIKELY REMAIN QUITE WARM  
OVER AND SOUTH OF THE GREAT BASIN THROUGH THE PERIOD WHILE  
SLIGHTLY BELOW NORMAL HIGHS SHOULD PREVAIL OVER THE NORTHWEST.  
 
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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