302
FXUS02 KWBC 260658
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
258 AM EDT MON AUG 26 2024
VALID 12Z THU AUG 29 2024 - 12Z MON SEP 02 2024
..OVERVIEW
EXPECT THE STRONG UPPER RIDGE LINGERING OVER THE EAST INTO
THURSDAY TO GIVE WAY TO MEAN TROUGHING THAT SETTLES OVER THE
NORTHEASTERN QUADRANT OF THE LOWER 48 BY THE WEEKEND AND EARLY
NEXT WEEK, AFTER A STRONG AND SHARP UPPER TROUGH CROSSES THE
NORTHERN PLAINS/UPPER MIDWEST LATE THIS WEEK. MEANWHILE AN UPPER
RIDGE SHOULD DRIFT ACROSS THE WEST WHILE A WEAK UPPER LOW
EVENTUALLY NEARS THE WEST COAST. A COLD FRONT PUSHING ACROSS THE
EASTERN HALF OF THE COUNTRY WILL BRING AREAS OF SHOWERS/STORMS LATE
THIS WEEK INTO THE WEEKEND. THE SOUTHERN ROCKIES AND HIGH PLAINS
MAY SEE INCREASING RAINFALL AFTER THURSDAY AND EPISODES OF RAINFALL
WILL BE POSSIBLE THROUGH THE PERIOD NEAR THE WEST-CENTRAL GULF
COAST AND OVER FLORIDA. THE FORECAST PATTERN WILL FAVOR MODERATION
AND SUPPRESSION OF MID-LATE WEEK EASTERN U.S. HEAT BY THE WEEKEND
WHILE THE PLAINS/ROCKIES WILL TEND TO SEE NEAR TO BELOW NORMAL
HIGHS. UPPER RIDGING WILL SUPPORT ABOVE NORMAL HIGHS OVER THE
PACIFIC NORTHWEST INTO FAR NORTHERN ROCKIES LATE WEEK THROUGH THE
WEEKEND.
..GUIDANCE/PREDICTABILITY ASSESSMENT
FROM THURSDAY INTO SATURDAY THE GUIDANCE IS FAIRLY WELL CLUSTERED
AS THE INITIAL TROUGH/UPPER LOW TRACKING NEAR THE CANADIAN BORDER
OPENS UP AND UPSTREAM ENERGY BEGINS TO BROADEN THE MEAN TROUGH,
WHILE THE ASSOCIATED COLD FRONT PROGRESSES EASTWARD FROM THE
CENTRAL U.S. A 12Z/18Z OPERATIONAL MODEL COMPOSITE OFFERS A GOOD
REPRESENTATION OF CONSENSUS FOR THIS PART OF THE FORECAST.
VARIOUS AREAS OF SPREAD ARISE IN THE GUIDANCE BY SUNDAY-MONDAY,
WHILE MOST GUIDANCE ON AVERAGED HAS NUDGED A LITTLE EASTWARD WITH
THE OVERALL EASTERN NORTH AMERICAN TROUGH. WITHIN THIS MEAN TROUGH,
DURING THE WEEKEND SOLUTIONS DIVERGE FOR THE CHARACTER OF SHORTWAVE
ENERGY DROPPING INTO THE WESTERN SIDE. 12Z MACHINE LEARNING (ML)
MODELS SUGGEST A LITTLE STRONGER SHORTWAVE BRUSHING THE GREAT LAKES
THAN SOME DYNAMICAL GUIDANCE, AND ESPECIALLY DISCOUNT LATEST CMC
RUNS THAT STRAY TO THE FLAT/PROGRESSIVE SIDE WITH SOUTHERN CANADA
FLOW. AT THE SAME TIME THE GFS BECOMES STRONGER THAN NEARLY ALL
OTHER DYNAMICAL AND ML MODELS WITH POTENTIAL SHORTWAVE ENERGY
EVOLVING OVER THE SOUTHERN ROCKIES AND VICINITY. REMAINING
SOLUTIONS GENERALLY LEAN TOWARD GREATER PERSISTENCE OF RIDGING OVER
THE REGION AND LESS NORTHWARD EXTENT OF MOISTURE OVER THE PLAINS.
THE 12Z ECENS MEAN DOES OFFER THE POTENTIAL FOR RAINFALL TO EXTEND
INTO THE CENTRAL PLAINS BY LATE IN THE PERIOD, BUT CONSENSUS WOULD
AT LEAST SUGGEST THAT GFS MAX AMOUNTS THERE ARE OVERDONE. FINALLY,
THE ENSEMBLE MEANS AND ML MODELS AGREE UPON SLOWER PROGRESSION OF
THE EAST PACIFIC UPPER LOW THAN SEEN IN THE 18Z GFS/12Z ECMWF. THIS
COMBINATION OF CONSIDERATIONS LED TO TRENDING THE LATE PART OF THE
FORECAST MORE TOWARD THE ENSEMBLE MEANS WITH SOME LINGERING INPUT
FROM THE ECMWF (SPLIT BETWEEN MORE 00Z/25 RUN THAN THE 12Z/25
VERSION).
..WEATHER/HAZARDS HIGHLIGHTS
HAZARDOUS HEAT WILL PERSIST OVER PORTIONS OF THE EASTERN U.S. INTO
LATE THIS WEEK, WITH SOME LOCATIONS SEEING HIGHS UP TO 10-15F ABOVE
NORMAL AND POTENTIAL FOR DAILY RECORDS. EXPERIMENTAL HEATRISK SHOWS
A BROAD AREA OF MODERATE TO MAJOR (AND LOCALIZED EMBEDDED EXTREME)
LEVEL IMPACTS THURSDAY WITH COVERAGE OF THESE CATEGORIES NARROWING
ON FRIDAY. WEAKENING OF THE EASTERN U.S. UPPER RIDGE AND
APPROACH/ARRIVAL OF A COLD FRONT FROM THE WEST SHOULD BRING
TEMPERATURES CLOSER TO NORMAL BY THE WEEKEND, CONFINING MOST OF THE
ABOVE NORMAL READINGS OVER THE SOUTH. UPPER RIDGING WILL PRODUCE
ABOVE NORMAL TEMPERATURES (BY UP TO 10-15F) OVER THE PACIFIC
NORTHWEST INTO FAR NORTHERN ROCKIES LATE THIS WEEK INTO THE
WEEKEND, WITH AN EVENTUAL COOLING TREND FROM WEST TO EAST. IN-
BETWEEN, THE PLAINS AND MOST OF THE ROCKIES WILL TEND TO SEE NEAR
TO BELOW NORMAL HIGHS THROUGH THE PERIOD.
DURING THE THURSDAY-FRIDAY PERIOD COVERED BY THE DAYS 4-5 EXCESSIVE
RAINFALL OUTLOOKS, THE COLD FRONT PROGRESSING EASTWARD FROM THE
CENTRAL U.S. WILL BE ONE FEATURE PRODUCING AREAS OF POTENTIALLY
HEAVY CONVECTION. THE DAY 4 OUTLOOK MAINTAINS A MARGINAL RISK AREA
OVER THE UPPER MIDWEST WHERE ACTIVITY MAY BE PROGRESSIVE BUT
SHORT-TERM RAIN RATES COULD BE FAIRLY HIGH. A MODEST INCREASE IN
CLUSTERING/QPF TOTALS COULD OPEN THE DOOR TO AN EMBEDDED SLIGHT
RISK UPGRADE. BY FRIDAY THE BEST SIGNAL FOR RELATIVELY GREATER
RAINFALL EXISTS OVER AND SOUTH OF THE EASTERN GREAT LAKES, FAVORING
A MARGINAL RISK AREA. THE DAY 4 MARGINAL RISK AREA OVER THE MID-
ATLANTIC HAS BEEN MAINTAINED CLOSE TO CONTINUITY AS GUIDANCE STILL
SHOWS POTENTIAL FOR LOCALLY HEAVY AND SLOW-MOVING CONVECTION IN THE
VICINITY OF A STALLING SURFACE FRONT. CONVECTION OVER THE SOUTHERN
ROCKIES AND HIGH PLAINS MAY INCREASE IN COVERAGE AND INTENSITY ON
FRIDAY RELATIVE TO THURSDAY AS LOW LEVEL UPSLOPE FLOW PROVIDES AN
ADDED CONTRIBUTION. THUS THE DAY 5 OUTLOOK DEPICTS A MARGINAL RISK
FOR THIS ACTIVITY. CONVECTION WILL BE POSSIBLE ALONG THE REMAINDER
OF THE MIDWEST/GREAT LAKES INTO SOUTHERN PLAINS FRONT BUT WITH
LESSER CLUSTERING FOR LOCATION AND AMOUNTS IN ADDITION TO THE
CENTRAL PLAINS LOCATIONS HAVING DRY ANTECEDENT CONDITIONS. THE
WEST-CENTRAL GULF COAST WILL BE ANOTHER REGION THAT REQUIRES
MONITORING. FOR MULTIPLE DAYS GUIDANCE HAS PROVIDED VARYING SIGNALS
FOR RAINFALL IN THIS REGION BUT WITH NO CLEAR CONSENSUS FOR
PRECISE AMOUNTS AND HOW MUCH WILL BE OVER LAND VERSUS JUST
OFFSHORE.
BEYOND EARLY SATURDAY, EXPECT EASTERN U.S. RAINFALL TO CONTINUE
PROGRESSING EASTWARD ALONG A LEADING (AND EVENTUALLY WEAKENING)
FRONT WHILE A SECOND DRIER LIKELY ARRIVES BY EARLY NEXT WEEK.
EPISODES OF CONVECTION SHOULD CONTINUE ALONG THE GULF COAST AND
OVER FLORIDA. SOUTHERN ROCKIES AND HIGH PLAINS RAINFALL SHOULD
CONTINUE THROUGH THE WEEKEND, THOUGH WITH INCREASING UNCERTAINTY
OVER COVERAGE AND AMOUNTS BY SUNDAY-MONDAY.
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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