461  
FXUS02 KWBC 191857  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
257 PM EDT FRI JUL 19 2024  
 
VALID 12Z MON JUL 22 2024 - 12Z FRI JUL 26 2024  
 
***HEAT WAVE CONTINUES ACROSS THE WEST, AND SHOWERS/STORMS WITH  
HEAVY RAINFALL EXPECTED FROM THE FOUR CORNERS TO PORTIONS OF THE  
EAST***  
 
19Z UPDATE: THE 12Z MODEL GUIDANCE SUITE REMAINS IN ABOVE AVERAGE  
AGREEMENT ON THE SYNOPTIC SCALE THROUGH THE END OF NEXT WEEK, WITH  
A MULTI-DETERMINISTIC MODEL BLEND WORKING WELL THROUGH WEDNESDAY.  
THE GFS DOES BECOME MORE PROGRESSIVE WITH THE TROUGH OVER THE GREAT  
LAKES REGION BY FRIDAY, AND IS ALSO STRONGER WITH THE WESTERN U.S.  
RIDGE DURING THAT TIME. SOME ENSEMBLE MEANS WERE INCLUDED BY  
WEDNESDAY NIGHT, AND GRADUALLY INCREASED TO ABOUT 30-40% BY THE END  
OF NEXT WEEK. THE MAIN UNCERTAINTIES INVOLVE THE TIMING AND  
COVERAGE OF INDIVIDUAL MCS EVENTS THAT WILL BE THE MAIN DRIVERS FOR  
HEAVY CONVECTIVE RAINFALL ACROSS THE SOUTH-CENTRAL U.S. NEXT WEEK.  
THE NBM APPEARED TO BE BOTH TOO LIGHT AND ALSO DELAYED WITH  
CONVECTIVE INITIATION ACROSS THE FOUR CORNERS REGION WITH THE  
MONSOONAL MOISTURE IN PLACE, SO MORE OF THE DETERMINISTIC ECMWF AND  
GFS WAS INCORPORATED TO BOOST EXPECTED QPF. THE PREVIOUS FORECAST  
DISCUSSION IS APPENDED BELOW FOR REFERENCE. /HAMRICK  
------------------------  
 
   
..GENERAL OVERVIEW
 
 
LATEST GUIDANCE SHOWS AN AMPLIFIED PATTERN THAT WILL BE FAIRLY  
SLOW TO EVOLVE WITH TIME. A STRONG RIDGE EXTENDING FROM THE  
SOUTHWEST U.S. INTO WESTERN CANADA AS OF EARLY MONDAY WILL PRODUCE  
DANGEROUS AND POTENTIALLY RECORD-SETTING HEAT INTO NEXT WEEK, WITH  
THE NORTHWEST SEEING ONLY GRADUAL COOLING AS A NORTHEAST PACIFIC  
UPPER LOW WOBBLES INTO WESTERN CANADA. MORE PERSISTENT RIDGING OVER  
THE SOUTHWEST WILL ACCOMPANY MONSOON CONDITIONS ACROSS THE  
SOUTHERN ROCKIES AND VICINITY. MEANWHILE, A WAVY FRONT WILL BE ON  
THE LEADING SIDE OF GREAT LAKES INTO SOUTHERN PLAINS MEAN TROUGHING  
ALOFT, LEADING TO MULTIPLE DAYS OF RAIN/THUNDERSTORMS WITH AREAS  
OF HEAVY RAINFALL FROM THE SOUTHERN PLAINS INTO THE MID-ATLANTIC  
AND PARTS OF NEW ENGLAND. AN AREA OF TROPICAL MOISTURE WILL PASS  
THROUGH FLORIDA EARLY NEXT WEEK TO ENHANCE DAILY SHOWER AND  
THUNDERSTORM ACTIVITY, FOLLOWED BY THE ATLANTIC UPPER RIDGE  
BUILDING INTO THE SOUTHEAST, PEAKING IN STRENGTH AROUND WEDNESDAY-  
THURSDAY.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
AVERAGES OF LATEST DYNAMICAL GUIDANCE AND MACHINE LEARNING (ML)  
MODELS COMPARE FAIRLY WELL TO EACH OTHER FOR THE WESTERN RIDGE  
THROUGH THE PERIOD AND NORTHEAST PACIFIC UPPER LOW THAT REACHES  
INTO BRITISH COLUMBIA AFTER MIDWEEK (ALONG WITH THE TROUGH TO THE  
SOUTH). THERE ARE SOME DETAIL DIFFERENCES WITH THESE FEATURES BUT  
THESE HAVE LOW PREDICTABILITY SEVERAL DAYS OUT IN TIME.  
 
REGARDING THE GREAT LAKES INTO SOUTHERN PLAINS TROUGH, THERE IS  
REASONABLE AGREEMENT THAT ENERGY DROPPING SOUTH FROM CANADA BY  
TUESDAY-WEDNESDAY SHOULD BEGIN TO EJECT AN EMBEDDED WEAK UPPER LOW  
OVER THE MIDWEST AS OF EARLY MONDAY. THEN SOME MORE NOTICEABLE  
DIFFERENCES ARISE WITH THE OVERALL TROUGH AFTER MIDWEEK. THE 12Z  
ECMWF STRAYED TO THE SLOW/SOUTHWESTERN SIDE OF THE SPREAD WITH THE  
CORE OF ITS TROUGH, WHILE THE GFS HAS BEEN LEANING A BIT ON THE  
FASTER SIDE AMONG THE REMAINING MORE PROGRESSIVE MODELS/MEANS.  
INTERESTINGLY, BY NEXT FRIDAY MOST OF THE 12Z ECMWF-INITIALIZED  
ML MODELS BECOME FAIRLY PROGRESSIVE AS WELL, AND WITH SOMEWHAT MORE  
AMPLITUDE THAN DEPICTED IN THE LATEST GFS RUNS. THERE HAVE BEEN A  
COUPLE CASES IN THE PAST FEW MONTHS WHEN THE MLS ULTIMATELY WERE  
TOO AMPLIFIED WITH FORECAST NORTHEAST TROUGHING SO TRENDS ONE WAY  
OR THE OTHER WILL BE WORTH MONITORING. THE NEW 00Z ECMWF HAS  
ADJUSTED SOMEWHAT FASTER, NOW COMPARING BETTER TO THE MEANS AND CMC  
BY NEXT FRIDAY. BY LATE NEXT WEEK THE CLUSTER THAT IS FASTER THAN  
THE 12Z ECMWF AND LESS AMPLIFIED THAN THE MLS WITH THE UPPER TROUGH  
(WITH CORRESPONDING SURFACE EVOLUTION/PROGRESSION) REPRESENTS THE  
BEST INTERMEDIATE SOLUTION.  
 
WITH THE 12Z UKMET NOT COMPARING WELL TO CONSENSUS FOR SOME DETAILS  
FROM THE UPPER MIDWEST NORTHEASTWARD AFTER MONDAY, THE FIRST PART  
OF THE FORECAST BASED ON 12Z/18Z GUIDANCE INCORPORATED THE  
GFS/ECMWF/CMC. THEN THE LATTER HALF OF THE PERIOD GRADUALLY  
INCREASED 18Z GEFS/12Z ECENS WEIGHT, REACHING 50 PERCENT TOTAL BY  
NEXT FRIDAY, WHILE PHASING OUT 12Z ECMWF INPUT DUE TO ITS  
QUESTIONABLY SLOW MIDWEST UPPER TROUGH.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
EXPECT DANGEROUS HEAT OVER THE WEST TO EXTEND INTO NEXT WEEK WITH  
HIGH TEMPERATURES REACHING THE 90S AND 100S AND WARM OVERNIGHT LOWS  
PROVIDING ONLY LIMITED RELIEF. THE INTERIOR NORTHWEST/FAR NORTHERN  
ROCKIES SHOULD SEE THE GREATEST COVERAGE OF MAJOR TO EXTREME HEAT  
RISK VALUES (ALONG WITH DAILY RECORD HIGHS) ON MONDAY, FOLLOWED BY  
GRADUAL COOLING MOVING IN FROM THE PACIFIC NORTHWEST AS AN UPPER  
LOW EVENTUALLY MOVING INTO BRITISH COLUMBIA HELPS TO LOWER HEIGHTS  
ALOFT. ELSEWHERE TEMPERATURES IN THE WEST SHOULD REMAIN WELL ABOVE  
AVERAGE NEXT WEEK ASIDE FROM PERHAPS SOME MODERATION OVER THE GREAT  
BASIN TOWARD THE END OF NEXT WEEK. SOME OF THE HEAT WILL EXTEND  
INTO MONTANA/NORTHERN HIGH PLAINS. THE PROLONGED PERIOD OF DRY  
HEAT COULD ALSO RESULT IN ENHANCED WILDFIRE DANGER.  
 
MONSOONAL MOISTURE WILL LINGER OVER THE FOUR CORNERS REGION AND  
PROMOTE DAILY EPISODES OF SHOWERS/THUNDERSTORMS. ISOLATED TO  
SCATTERED INSTANCES OF FLASH FLOODING WILL BE POSSIBLE, ESPECIALLY  
NEAR STEEP TERRAIN AND BURN SCARS. THE DAYS 4-5 EXCESSIVE RAINFALL  
OUTLOOKS COVERING MONDAY-TUESDAY CONTINUED TO SHOW MARGINAL RISK  
AREAS, WITH THE DAY 4 OUTLOOK CONTINUING A SLIGHT RISK AREA OVER  
PARTS OF NORTHERN NEW MEXICO AND SOUTH-CENTRAL COLORADO GIVEN  
SUFFICIENTLY FAVORABLE GUIDANCE SIGNALS, PERSISTENCE FROM THE DAY 3  
TIME FRAME, AND ALREADY WET GROUND CONDITIONS, ALONG WITH  
VULNERABLE BURN SCAR AREAS.  
 
A SOUTHERN PLAINS INTO NORTHEAST WAVY FRONT WILL PROVIDE A MULTI-  
DAY FOCUS FOR AREAS OF HEAVY SHOWERS AND THUNDERSTORMS NEXT WEEK.  
THE SOUTHERN PLAINS PART OF THE BOUNDARY MAY DISSIPATE BY MIDWEEK  
BUT ANOTHER FRONT PUSHING SOUTH FROM THE NORTHERN TIER SHOULD  
MAINTAIN A MEAN FRONT OVER THE EASTERN HALF OF THE COUNTRY INTO  
LATE WEEK. WITH AMPLE MOISTURE AND INSTABILITY IN PLACE, LOCALLY  
HEAVY RAINFALL WILL BE POSSIBLE AND MAY CAUSE INSTANCES OF FLASH  
FLOODING, ESPECIALLY IN URBAN AND POOR DRAINAGE AREAS WHERE  
CONVECTION REPEATS/TRAINS. DURING DAY 4/MONDAY, GUIDANCE IS  
SIGNALING THE BEST POTENTIAL FOR HEAVY RAINFALL OVER PARTS OF  
SOUTH-CENTRAL/EAST-CENTRAL TEXAS, AND THE EXISTING SLIGHT RISK  
AREA HAS BEEN EXPANDED SOME OVER THIS REGION. A BROAD SURROUNDING  
MARGINAL RISK AREA REMAINS IN PLACE FROM THE SOUTHERN PLAINS AND  
CENTRAL GULF COAST TO THE MID- ATLANTIC. THE DAY 5/TUESDAY ERO  
CONTINUES A BROAD MARGINAL RISK AREA FROM THE SOUTHERN  
PLAINS/CENTRAL GULF COAST INTO THE NORTHERN MID-ATLANTIC/SOUTHERN  
NEW ENGLAND, WITH GUIDANCE SUGGESTING THAT FRONTAL WAVINESS AND  
ASSOCIATED MOISTURE WILL LIFT NORTHEASTWARD OVER THE EASTERN U.S.  
THE SOUTHERN PORTION OF THE EXISTING MARGINAL ACROSS THE SOUTHEAST  
AND SOUTHERN MID-ATLANTIC WAS TRIMMED BACK SOME, WHILE THE NORTHERN  
PORTION WAS EXTENDED SLIGHTLY MORE. THERE IS ALSO A MARGINAL RISK  
AREA ON TUESDAY OVER PARTS OF THE UPPER MIDWEST WHERE SHORTWAVE  
ENERGY DROPPING SOUTH FROM CANADA MAY PRODUCE LOCALLY HEAVY  
CONVECTION.  
 
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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