585  
FXUS02 KWBC 200659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
259 AM EDT THU JUN 20 2024  
 
VALID 12Z SUN JUN 23 2024 - 12Z THU JUN 27 2024  
 
...HEAT WAVE PERSISTS FOR THE MID-ATLANTIC SUNDAY WITH ELEVATED  
HEAT OVER THE SOUTH/PLAINS THROUGH MUCH OF NEXT WEEK...  
   
..HEAVY RAINFALL THREAT OVER PARTS OF NEW ENGLAND INTO SUNDAY
 
 
   
..OVERVIEW
 
 
GUIDANCE SHOWS THE UPPER PATTERN SETTLING INTO A STABLE REGIME  
FEATURING A PERSISTENT UPPER HIGH ANCHORED OVER THE SOUTHERN  
ROCKIES AND VICINITY, WITH SOME RIDGING EXTENDING TO THE NORTH,  
WHILE THE NORTHEAST PACIFIC/WEST COAST AND EASTERN NORTH AMERICA  
SEE VARYING DEGREES OF MEAN TROUGHING. INDIVIDUAL SHORTWAVES WITHIN  
THE WESTERLIES CROSSING THE SOUTHERN CANADA/NORTHERN TIER U.S. WILL  
SUPPORT A SERIES OF PROGRESSIVE SOUTHERN CANADA SURFACE SYSTEMS AND  
TRAILING FRONTS EXTENDING INTO THE NORTHERN HALF TO TWO-THIRDS OF  
THE LOWER 48. A LEADING SYSTEM AFFECTING THE NORTHEAST ON SUNDAY MAY  
PRODUCE SOME AREAS OF HEAVY RAINFALL, WHILE THE TRAILING FRONT AND  
ANOTHER ONE BEHIND IT MAY PRODUCE EPISODES OF CONVECTION OVER THE  
CENTRAL/EASTERN U.S. EXPECT HAZARDOUS HEAT TO EXTEND INTO SUNDAY  
OVER THE MID-ATLANTIC, WITH HIGH HEAT INDICES LIKELY ACROSS THE  
SOUTH/PLAINS AND SOUTHWEST FOR MOST OF NEXT WEEK. THERE MAY BE A  
REBOUND OF SOME HEAT INTO THE MID-ATLANTIC AROUND MIDWEEK.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
LATEST DYNAMICAL AND MACHINE LEARNING (ML) GUIDANCE AGREES FAIRLY  
WELL FOR THE OVERALL PATTERN BUT DISPLAYS TYPICAL SPREAD AND RUN-  
TO-RUN VARIABILITY FOR THE DETAILS. FOR THE LEADING SYSTEM OVER  
EASTERN NORTH AMERICA, THE 18Z GFS STRAYED TO THE STRONG/NORTHWEST  
SIDE FOR THE SURFACE LOW WHILE THE NEW 00Z UKMET HAS COME IN ON THE  
SLOW SIDE FOR THE UPPER TROUGH BY TUESDAY. MODELS ARE STILL HAVING  
A TOUGH TIME DETERMINING WHEN THERE MAY BE A LOW EMBEDDED WITHIN  
THE UPPER TROUGH, AFFECTING THE SURFACE EVOLUTION. UPSTREAM, LATEST  
GUIDANCE IS HOLDING ONTO RECENT TRENDS TOWARD PROGRESSIVE PACIFIC  
ENERGY EVOLVING INTO A SOUTHERN CANADA/NORTHERN TIER U.S. SHORTWAVE  
THAT FEEDS INTO THE EASTERN NORTH AMERICA MEAN TROUGH, WHILE  
REMAINING UPPER LOW ENERGY NEAR/OFFSHORE BRITISH COLUMBIA MAY  
COMBINE WITH INCOMING NORTH PACIFIC ENERGY TO PRODUCE A WEST COAST  
UPPER TROUGH BY NEXT THURSDAY WITH AN INCREASINGLY BETTER DEFINED  
LEADING SURFACE SYSTEM. FARTHER EAST BY NEXT THURSDAY, IT WILL BE  
WORTH WATCHING DYNAMICAL VERSUS ML GUIDANCE AS THE 12Z ECMWF AND  
LATEST GFS RUNS ARE SOMEWHAT WEST OF MOST 12Z ECMWF-INITIALIZED ML  
RUNS FOR THE UPPER WEAKNESS THAT DEVELOPS OVER AND SOUTH OF THE  
TENNESSEE VALLEY.  
 
BASED ON 12Z/18Z GUIDANCE COMPARISONS, THE UPDATED FORECAST STARTED  
WITH A COMPOSITE OF 12Z MODELS (18Z GFS LOOKING TOO SHALLOW WITH  
THE EASTERN TROUGH BY MONDAY) FOR ABOUT THE FIRST HALF OF THE  
PERIOD. THE THE BLEND INCORPORATED SOME 18Z GEFS/12Z ECENS MEANS  
WHILE ULTIMATELY TRANSITIONING THE GFS COMPONENT TOWARD THE 18Z RUN  
BY NEXT THURSDAY (12Z GFS TRENDING FLAT WITH THE LATE-PERIOD  
EASTERN TROUGH).  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE SOUTHEASTERN CANADA SURFACE LOW AND TRAILING FRONT AFFECTING  
THE NORTHEAST ON SUNDAY AND PERHAPS TO A LESSER DEGREE INTO MONDAY  
WILL HAVE THE POTENTIAL TO PRODUCE SOME AREAS OF LOCALLY HEAVY  
RAINFALL ACROSS THE REGION. THERE IS CONTINUED SPREAD IN THE  
GUIDANCE FOR EXACTLY HOW THIS SYSTEM WILL EVOLVE AND THUS FOR  
RAINFALL DETAILS. THE DAY 4 EXCESSIVE RAINFALL OUTLOOK COVERING  
SUNDAY-SUNDAY NIGHT MAINTAINS A SLIGHT RISK AREA OVER PARTS OF  
NORTHERN-CENTRAL NEW ENGLAND WITH MODEST ADJUSTMENTS BASED ON  
LATEST MODEL/ENSEMBLE GUIDANCE AND FIRST-GUESS FIELDS. THE  
SURROUNDING MARGINAL RISK THAT EXTENDS AS FAR SOUTH AS THE CENTRAL  
APPALACHIANS IS ALSO CLOSE TO CONTINUITY AND REFLECTS THE  
COMBINATION OF POTENTIAL FOR LOCALLY HEAVY ACTIVITY AND TERRAIN  
SENSITIVITY. THE DAY 4 ERO MAINTAINS A MARGINAL RISK AREA OVER FAR  
SOUTHERN TEXAS WHERE MOIST FLOW TO THE NORTH OF A SECOND POSSIBLE  
WESTERN GULF SURFACE LOW MAY PRODUCE SOME ENHANCED RAINFALL. IN  
THE DAY 5 ERO PERIOD (MONDAY- MONDAY NIGHT), GUIDANCE IS STARTING  
TO SUGGEST AN AREA OF POSSIBLE CONVECTION OVER THE UPPER  
MIDWEST/WESTERN GREAT LAKES AHEAD OF AN ADVANCING WARM FRONT. WET  
GROUND CONDITIONS DUE TO PRIOR RAINFALL FAVOR STARTING WITH A  
MARGINAL RISK AREA, THOUGH CONFIDENCE IN DETAILS IS SOMEWHAT BELOW  
AVERAGE FOR NOW.  
 
ANOMALOUS MOISTURE SHOULD PERSIST OVER THE FOUR CORNERS STATES,  
AND ARIZONA IN PARTICULAR, DURING THE DAYS 4-5 PERIOD AND  
POTENTIALLY LEAD TO SOME LOCALLY HEAVY RAINFALL. HOWEVER WILL AWAIT  
A MORE COHERENT SIGNAL IN THE GUIDANCE BEFORE DEPICTING ANY RISK  
AREAS. PARTS OF THE SOUTHEAST/FLORIDA MAY ALSO SEE EPISODES OF  
DIURNALLY ENHANCED CONVECTION BUT GROUND CONDITIONS ARE ON THE  
DRIER SIDE WITH FFG VALUES QUITE HIGH, SO AGAIN FAVORED NO RISK  
AREAS FOR THE TIME BEING. BEYOND EARLY TUESDAY, A CENTRAL INTO  
EASTERN U.S. FRONT WILL HELP TO FOCUS AREAS OF SHOWERS AND  
THUNDERSTORMS WHILE PORTIONS OF THE SOUTH AND THE ROCKIES SHOULD  
CONTINUE TO SEE DIURNALLY FAVORED CONVECTION. A LATE-PERIOD FRONT  
REACHING THE NORTHWEST MAY PRODUCE SOME LIGHT RAINFALL (AS MAY A  
FRONT ON SUNDAY).  
 
THE MID-ATLANTIC SHOULD CONTINUE TO HAVE TEMPERATURES 10-15F ABOVE  
NORMAL INTO SUNDAY WITH SOME DAILY RECORDS POSSIBLE FOR MAX/WARM  
MIN TEMPERATURES. THE FRONT PROGRESSING INTO THE EAST WILL BRIEFLY  
SUPPRESS THE HEAT SOUTHWARD NEXT MONDAY BUT THE HOT TEMPERATURES  
SHOULD RETURN NORTHWARD TOWARD MIDWEEK AHEAD OF ANOTHER FRONT.  
MEANWHILE A BROAD AREA OF TEMPERATURES GENERALLY 5-15F ABOVE NORMAL  
WILL EXTEND FROM THE SOUTH INTO THE PLAINS AS WELL AS CALIFORNIA  
INTO THE INTERIOR WEST. NEAR TO SLIGHTLY BELOW NORMAL HIGHS OVER  
THE PACIFIC NORTHWEST SHOULD BEGIN TO EXPAND SOUTHEASTWARD BY NEXT  
THURSDAY. BEST POTENTIAL FOR MAXIMUM HEAT INDEX VALUES OF 105-110F  
OR SO WILL BE FROM THE SOUTH INTO THE SOUTHERN HALF OR SO OF THE  
PLAINS AS WELL AS OVER THE DESERT SOUTHWEST.  
 
RAUSCH  
 
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC MEDIUM  
RANGE HAZARDS OUTLOOK CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION FORECAST (QPF), EXCESSIVE RAINFALL  
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT  
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:  
 
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/#PAGE=ERO  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW  
 

 
 
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