581  
FXUS02 KWBC 241536  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1135 AM EDT SUN JUN 24 2018  
 
VALID 12Z WED JUN 27 2018 - 12Z SUN JUL 01 2018  
   
..OVERVIEW
 
 
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.  
   
..MODEL PREFERENCES/DISCUSSION
 
 
THERE ARE TWO ISSUES IN THE FORECAST. ONE HAS TO DO WITH THE  
LONGWAVE TROUGH IN THE WEST AND HOW QUICKLY IT EJECTS. AFTER  
FRIDAY, THE 00Z CANADIAN AND 00Z ECMWF EJECT IT OUT QUICKER  
BRINGING A STRONG LOW INTO SOUTHERN CANADA AT A FASTER CLIP THAN  
THE OTHER GUIDANCE. NORMALLY, THE GUIDANCE IS TOO QUICK TO EJECT  
THE LONGWAVE TROUGH AND ANY SHORTWAVES MOVING AROUND ITS PERIPHERY  
TEND TO BE WEAK. SINCE THE ECMWF IS QUICKER THAN ITS OWN MEAN,  
DISCOUNTED IT FOR NEXT WEEKEND.  
 
THE OTHER ISSUE IS THE POSSIBILITY FOR A BAROCLINICALLY-INITIATED  
WARM CORE/CONVECTIVE LOW NEAR/OFFSHORE THE SOUTHEAST COAST. MOST  
OF THE GUIDANCE BRINGS CONVECTIVELY-INDUCED SHORTWAVES OUT OF THE  
SOUTHERN PLAINS THROUGH THE MID-MISSISSIPPI VALLEY AND CAROLINAS  
DURING THE SHORT RANGE PERIOD, CAUSING A LOW TO FORM IN THE  
CAROLINAS ALONG A WEAKENING FRONTAL BOUNDARY WHICH MOVES OFFSHORE  
BY WEDNESDAY MORNING. THE 00Z ECMWF WAS QUITE AGGRESSIVE WITH  
THIS SYSTEM AND STRENGTHENS IT TO HURRICANE STRENGTH BY NEXT  
SUNDAY -- LIKELY TOO STRONG. SINCE THERE IS A BUILDING RIDGE  
ACROSS THE NORTHERN MID-ATLANTIC STATES IN ALL THE GUIDANCE,  
WHATEVER FORMS OFFSHORE THE SOUTHEAST WOULD MOVE SOUTHWEST  
TOWARDS/OVER FL SOONER OR LATER, AND STUCK WITH CLUSTERING WITHIN  
THE SOUTHWEST SIDE OF THE 00Z GLOBAL ENSEMBLE GUIDANCE HERE WHICH  
RESULTED IN A SLOWER WESTWARD MOVEMENT THAN SHOWN BY  
YESTERDAY/OVERNIGHT WPC CONTINUITY ACROSS FL. AT ONE POINT, 35 OF  
THE 90 ECMWF/CANADIAN/GFS 00Z MEMBERS HAVE THE LOW, SO IT CAN'T BE  
DISCOUNTED. A COUPLE SIMILAR DEVELOPMENTS (CLARA 1977 & CLAUDETTE  
1985) FORMED AS A RESULT OF CONVECTIVE COMPLEXES MOVING OFF THE  
SOUTHEAST COAST; SUCH A DEVELOPMENT IS POSSIBLE.  
 
   
..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. ABOUT 20 DEGREES SOUTH OF  
THE AMPLIFYING MID-LEVEL POSITIVE HEIGHT ANOMALY, THE  
SOUTHEAST/FLORIDA WILL SEE AREAS OF LOCALLY HEAVY CONVECTION  
DURING THE PERIOD, WITH ENHANCEMENT EXPECTED FROM THE RETROGRADING  
SURFACE LOW/TROUGH FORECAST TO MOVE IN FROM OFFSHORE THE SOUTHEAST  
COAST.  
 
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. DAILY MAXIMUM  
RECORDS APPEAR TO BE MORE LIKELY FOR WARM LOWS THAN DAYTIME HIGHS,  
THOUGH A SMATTERING OF RECORD HIGHS ARE FORECAST ACROSS THE  
NORTHERN MID-ATLANTIC STATES AND PORTIONS OF NEW ENGLAND.  
INCREASED LOW-LEVEL MOISTURE WILL ALSO LEAD TO VERY HIGH HEAT  
INDEX VALUES OVER MANY AREAS. OTHERWISE, 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.  
 
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  

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page