888  
FXUS02 KWBC 180659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
259 AM EDT TUE JUN 18 2024  
 
VALID 12Z FRI JUN 21 2024 - 12Z TUE JUN 25 2024  
 
...RECORD-BREAKING HEAT FROM THE MIDWEST TO THE EAST COAST EXPECTED  
TO MODERATE BY EARLY NEXT WEEK...  
 
...HEAVY RAINFALL THREAT FROM THE NORTHERN PLAINS INTO UPPER  
GREAT LAKES INTO FRIDAY-SATURDAY...  
 
   
..OVERVIEW
 
 
LATEST MODELS AND ENSEMBLES CONTINUE TO SHOW THE LARGE SCALE  
PATTERN TRANSITIONING TOWARD A MORE TYPICAL EARLY SUMMER REGIME  
LATE WEEKEND AND EARLY NEXT WEEK, AS INITIALLY STRONG RIDGING OVER  
THE EASTERN HALF OF THE COUNTRY GIVES WAY TO A LEADING TROUGH  
CROSSING THE NORTHERN TIER U.S. AND SOUTHERN CANADA (REACHING THE  
EAST COAST BY AROUND NEXT TUESDAY) WHILE A SYSTEM REACHING THE  
NORTHEAST PACIFIC AND WEST COAST THIS WEEKEND CONTINUES EASTWARD  
THEREAFTER. ALTHOUGH WEAKENING, THE EASTERN UPPER RIDGE WILL STILL  
SUPPORT A BROAD AREA OF HAZARDOUS HEAT WITH POTENTIAL FOR DAILY RECORDS  
EXTENDING FROM THE OHIO VALLEY/LOWER GREAT LAKES INTO THE MID-  
ATLANTIC FRIDAY INTO THE WEEKEND. THE NORTHERN PLAINS INTO UPPER  
GREAT LAKES SHOULD SEE A CONTINUED THREAT FOR HEAVY RAINFALL  
THROUGH FRIDAY-SATURDAY AS DEVELOPING NORTHERN PLAINS INTO GREAT  
LAKES/ONTARIO LOW PRESSURE AND ASSOCIATED FRONTS INTERACT WITH  
ANOMALOUS MOISTURE ALREADY IN PLACE. SOME OF THIS RAINFALL WILL  
EXTEND INTO THE NORTHEAST. A TRAILING NORTHEAST PACIFIC SYSTEM WILL  
LIKELY BRING A WELL-DEFINED FRONT INTO THE WEST BY SUNDAY,  
CONTINUING EAST THEREAFTER. TO THE SOUTH OF THE UPPER RIDGE  
ULTIMATELY SETTLING OVER THE FAR SOUTHERN TIER, POTENTIAL TROPICAL  
CYCLONE ONE (SEE NHC FOR UPDATES) SHOULD DISSIPATE OVER MEXICO BY  
FRIDAY WHILE NHC IS MONITORING POTENTIAL FOR ANOTHER FEATURE OVER  
THE WESTERN GULF MID-LATE PERIOD.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE LATEST FORECAST BASED ON 12Z/18Z GUIDANCE STARTED WITH AN  
OPERATIONAL MODEL COMPOSITE EARLY IN THE PERIOD AND THEN GRADUALLY  
INCREASED ENSEMBLE INPUT (18Z GEFS AND 12Z ECENS/CMCENS) TO A TOTAL  
OF 60 PERCENT BY THE END OF THE PERIOD NEXT TUESDAY. ECMWF INPUT  
WAS SPLIT BETWEEN THE 12Z AND 00Z/17 RUN MID-LATE PERIOD DUE TO  
SOME QUESTIONABLE DETAILS IN THE 12Z RUN.  
 
AT LEAST IN PRINCIPLE, AN AVERAGE OF DYNAMICAL AND ECMWF MACHINE  
LEARNING (ML) GUIDANCE AGREED FAIRLY WELL WITH THE UPPER RIDGE  
SETTLING OVER THE FAR SOUTHERN TIER WHILE A FAIRLY VIGOROUS SYSTEM  
DEVELOPS/TRACKS ALONG THE NORTHERN TIER/SOUTHERN CANADA THROUGH THE  
PERIOD, LEADING TO A MODERATE UPPER TROUGH/LEADING COLD FRONT NEAR  
THE EAST COAST BY NEXT TUESDAY. THE MEAN OF ML MODELS WAS CLOSE TO  
THE DYNAMICAL GUIDANCE AVERAGE FOR THE SURFACE SYSTEM'S DEPTH,  
WHILE DISPLAYING SOME DETAIL DIFFERENCES AFTER MOST GUIDANCE AGREED  
TO AN EMBEDDED UPPER LOW ALOFT AS OF SUNDAY. WITH THE TRAILING  
PACIFIC SYSTEM, SPORADIC GFS RUNS (LIKE THE 12Z AND NEW 00Z  
VERSIONS) HAVE BEEN SHEARING SOME UPPER ENERGY WHILE LEAVING THE  
MAIN UPPER LOW BEHIND NEAR THE BRITISH COLUMBIA COAST. THE 18Z GFS  
LOOKED MORE LIKE CONSENSUS. MEANWHILE THE ML MODELS ARE GENERALLY  
SHOWING SOMEWHAT HIGHER HEIGHTS AND LESS POTENTIAL FOR A WEAKNESS  
AT THE SURFACE AND ALOFT OVER OR NEAR THE GULF OF MEXICO VERSUS  
SOME GFS/ECMWF RUNS.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE MOST PROMINENT FOCUS FOR HEAVY RAINFALL DURING THE PERIOD WILL  
BE FROM THE NORTHERN PLAINS INTO UPPER GREAT LAKES DURING FRIDAY-  
SATURDAY AS UPPER DYNAMICS EMERGING FROM WESTERN NORTH AMERICA  
ENCOURAGE NORTHERN PLAINS INTO GREAT LAKES/ONTARIO SURFACE  
DEVELOPMENT. THIS SYSTEM AND ASSOCIATED FRONTS WILL INTERACT WITH  
ANOMALOUS MOISTURE THAT HAS BEEN PERSISTING OVER THE REGION FOR  
MULTIPLE DAYS. THE DAY 4 EXCESSIVE RAINFALL OUTLOOK FOR FRIDAY  
MAINTAINS A SLIGHT RISK AREA FROM PARTS OF THE DAKOTAS INTO THE  
U.P. OF MICHIGAN (CLOSE TO CONTINUITY) WHILE THE DAY 5 ERO FOR  
SATURDAY PROPOSES A SLIGHT RISK AREA FROM NORTHEASTERN  
MINNESOTA/NORTHERN WISCONSIN INTO NORTHERN MICHIGAN. SURROUNDING  
MARGINAL RISK AREAS ACCOUNT FOR LESS EXTREME BUT STILL LOCALLY  
SIGNIFICANT RAINFALL POTENTIAL OVER THE NORTHERN PLAINS (DAY 4) AND  
OVER THE MIDWEST ALONG THE TRAILING COLD FRONT (DAY 5). SOME AREAS  
ACROSS THE NORTHERN TIER WILL BE SENSITIVE TO ADDITIONAL RAINFALL  
DUE TO ALREADY WET GROUND HEADING INTO FRIDAY.  
 
ELSEWHERE, THE NEW DAY 4 ERO MAINTAINS CONTINUITY WITH A MARGINAL  
RISK AREA OVER PARTS OF THE SOUTHERN ROCKIES/FOUR CORNERS REGION  
DUE TO THE ABUNDANT MOISTURE ARRIVING FROM THE EAST AND SOME  
FORECAST INSTABILITY, ALONG WITH SOME MODEL SIGNALS FOR LOCALLY  
ENHANCED RAINFALL. GUIDANCE SUGGESTS THAT THE GREATEST MOISTURE  
ANOMALIES SHOULD GET PUSHED TO THE SOUTH AND WEST DURING SATURDAY,  
WITH RAINFALL OVER THE REGION LIKELY TO BE LIGHTER AND MORE  
SCATTERED--THUS NOT MERITING A RISK AREA FOR DAY 5. MEANWHILE THE  
NORTHEAST CONTINUES TO MERIT MONITORING FROM FRIDAY INTO THE  
WEEKEND. THE COMBINATION OF MOISTURE AND INSTABILITY ALONG WITH AN  
EAST-WEST FRONT SETTLING OVER THE REGION COULD SUPPORT SOME  
LOCALLY ENHANCED RAINFALL RATES AND TRAINING/REPEAT ACTIVITY.  
HOWEVER ANTECEDENT CONDITIONS SHOULD BE RATHER DRY BY THAT TIME,  
STREAM FLOWS ARE ALREADY NEAR TO BELOW NORMAL, AND MODEL QPF IS  
NOT EXCEPTIONALLY HEAVY THUS FAR. THEREFORE THE OUTLOOKS CONTINUE  
TO DEPICT NO RISK AREA AT THIS TIME.  
 
CONTINUED PROGRESSION OF THE SYSTEM THAT SHOULD BE NEAR THE GREAT  
LAKES AS OF EARLY SUNDAY WILL BRING THE TRAILING COLD FRONT THROUGH  
THE EASTERN U.S. AND TRAILING BACK INTO THE PLAINS BY THE START OF  
NEXT WEEK, BRINGING AREAS OF SHOWERS AND THUNDERSTORMS OF VARYING  
INTENSITY. THE FRONT REACHING THE NORTHWEST THIS WEEKEND MAY BRING  
SOME LIGHT/SCATTERED RAINFALL OVER NORTHERN AREAS. CONTINUED  
PROGRESS OF THIS FRONT MAY GENERATE SOME SHOWERS/THUNDERSTORMS OVER  
THE CENTRAL U.S. BY THE FIRST HALF OF NEXT WEEK. SOME SCATTERED  
DIURNAL CONVECTION MAY PERSIST OVER THE SOUTHERN ROCKIES.  
OCCASIONAL SHOWERS/STORMS ARE POSSIBLE ALONG THE GULF COAST,  
BUT WITH INCREASING UNCERTAINTY OVER SPECIFICS OF THE GULF OF  
MEXICO PATTERN AT THE SURFACE AND ALOFT BY EARLY NEXT WEEK KEEPING  
CONFIDENCE LOW REGARDING ANY POTENTIAL INCREASE IN MOISTURE AT THAT  
TIME.  
 
WITH A COLD FRONT REACHING NEW ENGLAND BY FRIDAY AND PROVIDING A  
COOLING TREND THERE, THE GREATEST TEMPERATURE ANOMALIES FROM LATE  
THIS WEEK INTO THE WEEKEND SHOULD EXTEND FROM THE MIDWEST AND OHIO  
VALLEY/LOWER GREAT LAKES INTO THE MID-ATLANTIC AND NORTHEAST, WITH  
READINGS TENDING TO BE 10-15F ABOVE NORMAL. THIS WOULD TRANSLATE  
TO HIGHS WELL INTO THE 90S, ALONG WITH LOWS IN THE UPPER 60S TO  
UPPER 70S PROVIDING LITTLE OVERNIGHT HEAT RELIEF. DAILY RECORDS FOR  
MAX/WARM MIN TEMPERATURES WILL BE POSSIBLE WITHIN THE ABOVE AREAS,  
UP THROUGH WARM MINS EARLY MONDAY (IF THEY HOLD ON THROUGH THE  
CALENDAR DAY). MEANWHILE MUCH OF THE WEST WILL TREND WARMER/HOTTER  
LATE THIS WEEK INTO THE WEEKEND WITH HIGHS REACHING 10F OR MORE  
ABOVE NORMAL FOR A COUPLE DAYS OR SO. THE FRONT REACHING THE  
NORTHWEST SHOULD BRING COOLER AIR TO THAT REGION BY SUNDAY-MONDAY,  
WHILE SOME OF THE LEADING WESTERN HEAT SHOULD REACH THE NORTHERN-  
CENTRAL HIGH PLAINS AT THAT TIME--CONNECTING WITH LINGERING HEAT  
OVER PARTS OF THE PLAINS INTO SOUTHERN HALF OF THE EAST. CLOUDS  
AND RAINFALL WILL SUPPORT BELOW NORMAL HIGHS OVER THE SOUTHERN  
ROCKIES/HIGH PLAINS LATE THIS WEEK.  
 
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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