781  
FXUS02 KWBC 231433  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1032 AM EDT SAT JUN 23 2018  
 
VALID 12Z TUE JUN 26 2018 - 12Z SAT JUN 30 2018  
   
..OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE MODEL GUIDANCE SHOWS GOOD AGREEMENT ON A SHIFT TOWARDS  
INCREASING WESTERN TROUGHING/INCREASED RIDGING NEAR THE  
MID-ATLANTIC STATES BY THE END OF THE PERIOD. THERE ARE SOME  
DETAIL ISSUES IN REGARDS TO HOW STRONG THE WESTERN TROUGH GETS AND  
HOW CLOSED THE SYSTEM MOVING THROUGH THE GREAT LAKES ON WEDNESDAY  
MIGHT BE AS IT MOVES OVER THE TOP OF THE SOUTHEASTERN RIDGE, BUT  
OVERALL AGREEMENT IS SO GOOD THAT CONFIDENCE IS ABOVE AVERAGE IN  
THE FORECAST.  
 
THE DAYS 3-7 PRESSURES, WINDS, AND 500 HPA HEIGHTS ARE BASED ON A  
DETERMINISTIC BLEND OF THE 00Z CANADIAN, 00Z ECMWF, 00Z UKMET, AND  
06Z GFS THROUGH THURSDAY. FOR FRIDAY INTO NEXT SATURDAY, SLOWLY  
INCREASED THE CONTRIBUTION OF THE 00Z NAEFS MEAN AND 00Z ECMWF  
ENSEMBLE MEAN TO COMPROMISE 40-45% OF THE DISTRIBUTION BY THE END  
OF THE PERIOD. THE REMAINDER OF THE WEATHER GRIDS ARE BASED ON A  
MORE EVEN BLEND OF THE NATIONAL BLEND OF GUIDANCE, WPC CONTINUITY,  
AND THE NAEFS ENSEMBLE MEAN GUIDANCE. THE PLAN FOR THE DAYS 4-7  
QPF IS TO USE AN EVEN BLEND OF THE 12Z NATIONAL BLEND OF MODELS,  
06Z GFS, 00Z ECMWF, AND 00Z CANADIAN GUIDANCE. SHOULD THE 12Z GFS  
PROVE USEFUL, IT COULD BE INCLUDED.  
 
   
..WEATHER THREATS/HIGHLIGHTS
 
 
THE SYSTEM TRACKING EAST-NORTHEAST FROM THE NORTHERN PLAINS/UPPER  
MS VALLEY TUE ONWARD WILL LIKELY PRODUCE ONE OR MORE EPISODES OF  
LOCALLY HEAVY RAINFALL. BEST POTENTIAL FOR AREAS ACTIVE  
CONVECTION SHOULD EXIST FROM THE UPPER-MID MS VALLEY THROUGH THE  
GREAT LAKES AND OHIO VALLEY. SOME POCKETS OF MODERATE TO HEAVY  
RAINFALL MAY EXTEND INTO THE NORTHEAST BUT SHOULD GENERALLY BE  
MORE SCATTERED AS THE SYSTEM WEAKENS. THE TRAILING FRONT  
PROGRESSING ACROSS THE NORTHERN TIER WILL STALL OVER THE PLAINS  
WITH AT LEAST A PORTION LIKELY TO RETURN NORTH AS A WARM FRONT BY  
FRIDAY INTO NEXT SATURDAY. THE COMBINATION OF THIS LOW-LEVEL  
FOCUS AND INCREASING TENDENCY FOR SHORTWAVE IMPULSES TO EJECT FROM  
THE EVOLVING WESTERN TROUGH ALOFT MAY PROMOTE PERIODS OF  
CONVECTION/RAINFALL--SOME POTENTIALLY HEAVY--FROM THE NORTHERN  
ROCKIES EASTWARD BY LATE WEEK/WEEKEND. THE WESTERN TROUGH SHOULD  
ALSO BRING UNSETTLED WEATHER TO OTHER PARTS OF THE NORTHERN HALF  
OF THE WEST THOUGH WITH LIGHTER AMOUNTS. MEANWHILE EXPECT PERIODS  
OF CONVECTION OVER THE SOUTHEAST/FLORIDA DURING THE PERIOD WITH  
SOME HEAVY ACTIVITY POSSIBLE. A GRADUALLY WEAKENING FRONT OVER  
THE REGION WILL PROVIDE SOME ADDED LOW-LEVEL FOCUS DURING THE  
FIRST HALF OF THE PERIOD.  
 
THE FORECAST IS STILL ON TRACK FOR HOT/HUMID CONDITIONS TO BECOME  
ESTABLISHED OVER AN INCREASINGLY LARGE PORTION OF THE  
CENTRAL/EASTERN STATES. LOCATIONS FROM THE CENTRAL PLAINS TO MID  
ATLANTIC/NORTHEAST WILL HAVE THE BEST POTENTIAL FOR SEEING ONE OR  
MORE DAYS WITH MIN AND/OR MAX READINGS REACHING AS HIGH AS 10-15F  
ABOVE NORMAL, WITH POSSIBLE RECORDS APPEARING TO BE CONFINED TO  
WARM OVERNIGHT LOWS FROM WEDNESDAY ONWARD. DUE TO INCREASED  
LOW-LEVEL MOISTURE WITHIN A MORE SUMMER-LIKE REGIME, HIGHEST HEAT  
INDEX VALUES SHOULD EXPAND FROM THE SOUTH EARLY IN THE PERIOD TO A  
LARGE AREA CENTERED OVER/NEAR THE MID MS VALLEY AS WELL AS  
SOUTHERN MID ATLANTIC/SOUTHEAST. THE GREAT BASIN/FOUR CORNERS  
REGION WILL SEE WARM TEMPERATURES BUT WITH SOME GRADUAL COOLING  
LATER IN THE PERIOD WITH THE ARRIVAL OF THE WESTERN TROUGH. THIS  
FEATURE SHOULD BRING COOLEST ANOMALIES (5-15F BELOW NORMAL FOR  
HIGHS) INTO THE NORTHERN PARTS OF THE WEST BY NEXT SATURDAY.  
 
ROTH/RAUSCH  
 
WPC MEDIUM RANGE FORECASTS OF 500 MB HEIGHTS, SURFACE SYSTEMS,  
WEATHER GRIDS, QUANTITATIVE PRECIPITATION, AND WINTER WEATHER  
OUTLOOK PROBABILITIES ARE FOUND AT:  
 
HTTP://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTP://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTP://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTP://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTP://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  

 
 
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