548  
FXUS02 KWBC 240648  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
247 AM EDT SUN JUN 24 2018  
 
VALID 12Z WED JUN 27 2018 - 12Z SUN JUL 01 2018  
   
..OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
LATEST GUIDANCE CONTINUES TO SHOW THE MEAN PATTERN TRANSITIONING  
TOWARD AN AMPLIFIED WESTERN RIDGE/EASTERN TROUGH CONFIGURATION. A  
NOTABLE TREND OVER THE PAST DAY HAS BEEN TOWARD GREATER NORTHWARD  
AMPLITUDE OF THE EASTERN RIDGE BY NEXT WEEKEND, WITH THE CENTER OF  
THE EMBEDDED UPPER HIGH POSSIBLY DRIFTING AT LEAST AS FAR NORTH AS  
40N LATITUDE. THIS EVOLUTION WOULD DIRECT HIGHEST TEMPERATURE  
ANOMALIES ORIGINATING FROM THE CENTRAL PLAINS/MID-UPPER MS VALLEY  
INTO THE NORTHEAST. THE PRIMARY UNCERTAINTY WITH THE WESTERN  
TROUGH ALOFT IS FOR ITS DEPTH AND THE DETAILS OF INDIVIDUAL  
SHORTWAVES. THE LATTER WILL AFFECT THE FORECAST FOR WHAT MAY BE A  
CONVECTIVELY ACTIVE PERIOD ACROSS THE NORTHERN TIER STATES.  
 
FOR THE WESTERN TROUGH, GFS/GEFS RUNS HAVE BEEN A BIT ON THE  
STRONGER AND DEEPER SIDE OF THE SPECTRUM BUT WORTHY OF PARTIAL  
INCLUSION IN THE FORECAST GIVEN THE STRENGTH OF THE UPPER RIDGE  
BUILDING OVER THE EASTERN PACIFIC. MEANWHILE A GREATER PROPORTION  
OF ECMWF/CMC RUNS AND THEIR ENSEMBLES SUGGEST A SOMEWHAT MORE  
SHALLOW OVERALL TROUGH DUE TO FASTER EJECTION OF LEADING ENERGY IN  
RESPONSE TO UPSTREAM FLOW--WHICH IS ALSO PLAUSIBLE WITHIN THE  
CONTEXT OF A MULTI-DAY MEAN TROUGH. HOWEVER RECENT ECMWF RUNS  
HAVE BEEN INCONSISTENT WITH THE EVOLUTION OF TROUGH ENERGY BY NEXT  
WEEKEND. IN NEW 00Z GUIDANCE THUS FAR THE GFS HAS TRENDED TOWARD  
A 12Z ECMWF TYPE OF EJECTION LATE IN THE PERIOD ALBEIT WITH  
DIFFERING UPSTREAM DETAILS, WHILE THE CMC MAINTAINS A BROADER MEAN  
TROUGH ON MOST DAYS. POTENTIAL MERIT FOR VARYING CLUSTERS AND  
GFS/ECMWF RUN TO RUN VARIABILITY FAVOR A COMPROMISE APPROACH FOR A  
SINGLE DETERMINISTIC FORECAST.  
 
FARTHER EAST THE BEST-BEHAVED PART OF THE FORECAST IS FOR THE  
SYSTEM EXPECTED TO BE OVER THE GREAT LAKES AS OF DAY 3 WED.  
GUIDANCE OVER RECENT DAYS HAS TENDED TO SHOW MODERATE SPREAD WITH  
A CONSENSUS APPROACH MAINTAINING GOOD CONTINUITY. IN THE WAKE OF  
THIS FEATURE, AS PREVIOUSLY NOTED THE RECENT TREND IN MOST  
GUIDANCE IS FOR EASTERN RIDGING TO EXTEND FARTHER NORTHWARD. BY  
DAY 7 SUN THE PAST COUPLE ECMWF RUNS SHOW THE FARTHEST NORTH  
POSITION FOR THE UPPER HIGH, REACHING NEW ENGLAND.  
TELECONNECTIONS RELATIVE TO THE UPSTREAM PATTERN OFFER POTENTIAL  
FOR SOMEWHAT FLATTER FLOW THAN ADVERTISED BY THE  
STRONGEST/NORTHERN SOLUTIONS WITH THE EASTERN RIDGE, LEAVING A  
MODEL/MEAN BLEND AS A REASONABLE STARTING POINT LATE IN THE  
PERIOD. GUIDANCE IS SUGGESTING DEVELOPMENT OF A WEAKNESS ALOFT  
OVER THE SOUTHEAST/FLORIDA WITH POSSIBLE SURFACE REFLECTION IN THE  
FORM OF A TROUGH. PREDICTABILITY FOR SPECIFICS APPEARS TO BE  
BELOW AVERAGE GIVEN DIFFUSE NATURE OF THE FEATURE.  
 
A BLEND AMONG THE 18Z GFS AND 12Z ECMWF/UKMET/CMC REPRESENTED  
CONSENSUS WELL DURING THE FIRST HALF OF THE PERIOD WHILE GUIDANCE  
SPREAD/VARIABILITY FAVORED A TREND TOWARD A MORE EVEN  
MODEL/ENSEMBLE WEIGHT BY THE WEEKEND. THE 18Z GEFS/12Z ECMWF MEAN  
PROVIDED THE ENSEMBLE INPUT FOR DAYS 6-7 SAT-SUN WHILE OPERATIONAL  
ECMWF VARIABILITY LED TO INCLUDING A SMALL COMPONENT OF ITS PAST  
TWO RUNS BY THAT TIME FRAME. THE BLEND HELD ONTO MODEST WEIGHT OF  
18Z GFS/12Z CMC RUNS AS WELL.  
 
   
..WEATHER THREATS/HIGHLIGHTS
 
 
THE SYSTEM INITIALLY OVER THE GREAT LAKES WILL LIKELY SUPPORT SOME  
AREAS OF MODERATE TO HEAVY RAINFALL EARLY IN THE PERIOD, MOST  
LIKELY FROM EASTERN GREAT LAKES/UPPER OHIO VALLEY INTO  
WESTERN-NORTHERN NEW ENGLAND. AS THE UPPER PATTERN BECOMES MORE  
AMPLIFIED AND A MEAN FRONTAL BOUNDARY/LOW PRESSURE PERSIST OVER  
THE PLAINS AND VICINITY, THERE WILL BE THE POTENTIAL FOR MULTIPLE  
CONVECTIVE EPISODES AND ACCOMPANYING HEAVY RAINFALL FROM OVER OR  
JUST EAST OF THE NORTHERN ROCKIES INTO THE UPPER MS VALLEY/UPPER  
GREAT LAKES. SPECIFICS WILL DEPEND ON LESS PREDICTABLE DETAILS OF  
SHORTWAVES EJECTING FROM THE WESTERN TROUGH ALOFT. CURRENTLY THE  
BEST SIGNAL FOR HIGHEST 5-DAY RAINFALL TOTALS EXISTS OVER THE  
UPPER MS VALLEY/UPPER GREAT LAKES. WITH LIGHTER AMOUNTS, THE  
WESTERN UPPER TROUGH WILL BRING SCATTERED RAINFALL TO OTHER AREAS  
WITHIN THE NORTHERN HALF OF THE REGION. THE SOUTHEAST/FLORIDA  
WILL SEE AREAS OF LOCALLY HEAVY CONVECTION DURING THE PERIOD.  
ENHANCEMENT TO RAINFALL MAY COME FROM A WEAKENING FRONT EARLY IN  
THE PERIOD AND THEN THE DEVELOPING UPPER WEAKNESS/SURFACE TROUGH.  
 
AN EXPANDING AREA OF HOT/HUMID CONDITIONS WILL LIKELY COVER A  
LARGE PORTION OF THE CENTRAL-EASTERN U.S. BY LATE THIS WEEK AND  
WEEKEND. CURRENTLY EXPECT THE AXIS OF HIGHEST MIN/MAX TEMPERATURE  
ANOMALIES OF PLUS 10-20F TO EXTEND FROM THE CENTRAL PLAINS THROUGH  
THE GREAT LAKES/OHIO VALLEY AND INTO THE NORTHEAST. ANY DAILY  
RECORDS APPEAR TO BE MORE LIKELY FOR WARM LOWS THAN DAYTIME HIGHS.  
INCREASED LOW-LEVEL MOISTURE WILL ALSO LEAD TO VERY HIGH HEAT  
INDEX VALUES OVER MANY AREAS. ON THE OTHER HAND THE UPPER TROUGH  
AMPLIFYING INTO THE WEST WILL BRING A PRONOUNCED COOLING TREND.  
LOCATIONS IN THE NORTHERN ROCKIES/HIGH PLAINS SHOULD SEE HIGHS AS  
MUCH AS 10-15F BELOW NORMAL BY FRI-SUN WITH LESS EXTREME ANOMALIES  
ELSEWHERE.  
 
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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