541
FXUS02 KWBC 251857
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
257 PM EDT SUN AUG 25 2024
VALID 12Z WED AUG 28 2024 - 12Z SUN SEP 1 2024
19Z UPDATE: THE LATEST GFS RUNS ARE NOW IN BETTER AGREEMENT WITH
THE MODEL CONSENSUS WITH THE LEADING TROUGH ACROSS THE GREAT LAKES
REGION FOR THE MIDDLE OF THE WEEK, IN CONTRAST TO THE MORE
PROGRESSIVE RUNS OVER THE PAST FEW DAYS. THERE REMAINS GOOD
OVERALL AGREEMENT ON THE LARGE SCALE UPPER TROUGH/CLOSED LOW NEAR
THE U.S./CANADIAN BORDER, ALTHOUGH THE GFS IS INITIALLY A LITTLE
SLOWER WITH ITS EASTWARD PROGRESSION. MODEL SPREAD INCREASES ACROSS
SOUTHWESTERN CANADA INTO THE NORTHWESTERN U.S. IN THE WAKE OF THAT
TROUGH REGARDING HOW FAR NORTH THE UPPER RIDGE GETS, BUT BETTER
AGREEMENT WITH THE SYNOPTIC SCALE TROUGH OVER THE NORTHEAST U.S. BY
SUNDAY. A GENERAL DETERMINISTIC MODEL BLEND, WITH SLIGHTLY LESS
WEIGHTING TO THE GFS, WAS USED FOR THE BEGINNING OF THE FORECAST
PERIOD, AND THEN TRENDED UP ON THE ENSEMBLE MEANS TO ABOUT 40% BY
NEXT WEEKEND. THE PREVIOUS FORECAST DISCUSSION IS APPENDED BELOW
FOR REFERENCE. /HAMRICK
------------------
..OVERVIEW
AN EASTERN U.S. UPPER RIDGE WILL REMAIN STRONG ENOUGH TO SUPPORT
ANOMALOUS HEAT IN ITS VICINITY MID-LATE WEEK WITH SOME DAILY
RECORDS POSSIBLE, BUT TEMPERATURES SHOULD SLOWLY MODERATE WITH
TIME AS THE RIDGE GRADUALLY WEAKENS AND DRIFTS SOUTHEASTWARD. THE
HEAT SHOULD BE NOTABLY MORE SUPPRESSED BY THE WEEKEND. A STRONG
TROUGH/EMBEDDED UPPER LOW WILL TRACK ALONG THE U.S.-CANADIAN
BORDER MID-LATE WEEK WITH UPSTREAM ENERGY LIKELY TO BROADEN THE
OVERALL CANADA/NORTHERN TIER U.S. MEAN TROUGH BY THE WEEKEND. A
COUPLE COLD FRONTS SUPPORTED BY THE OVERALL TROUGH WILL BRING AREAS
OF SHOWERS/STORMS TO THE CENTRAL/EASTERN U.S., WITH A LEADING
NORTHEAST/MID-ATLANTIC FRONT ALSO PRODUCING RAINFALL OF VARYING
INTENSITY MID-LATE WEEK. AN UPPER RIDGE WILL LIKELY BUILD OVER MORE
OF THE WEST BY LATE WEEK INTO NEXT WEEKEND AFTER THE INITIAL
TROUGH'S DEPARTURE. DIURNALLY FAVORED CONVECTION SHOULD CONTINUE
OVER THE SOUTHERN ROCKIES AND ALONG THE GULF COAST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
RECENT ADJUSTMENTS IN THE GFS/GEFS CLOSER TO REMAINING GUIDANCE
HAVE IMPROVED OVERALL CLUSTERING COMPARED TO SOME RECENT DAYS.
THEREFORE THE UPDATED FORECAST WAS ABLE TO EMPLOY A 12Z/18Z
OPERATIONAL MODEL BLEND EARLY IN THE PERIOD, FOLLOWED BY A
TRANSITION TOWARD 40 PERCENT ENSEMBLE MEAN INPUT BY NEXT SUNDAY.
DETAILS THAT STILL REQUIRE SOME FINE-TUNING INCLUDE SHORTWAVE
ENERGY CROSSING THE NORTHEAST EARLY IN THE PERIOD AND EXACT
SOUTHWARD EXTENT OF THE FRONT DROPPING DOWN THE EAST COAST, LATE
WEEK EVOLUTION OF THE VIGOROUS TROUGH TRACKING ALONG THE CANADIAN
BORDER, AND SPECIFICS OF INCOMING ENERGY THAT SHOULD SERVE TO
BROADEN THE OVERALL MEAN TROUGH (WITH LATEST GFS RUNS LEANING A BIT
TO THE FAST/AMPLIFIED SIDE). BY NEXT SUNDAY THERE IS ALSO A FAIR
AMOUNT OF SPREAD AND VARIABILITY REGARDING WHETHER UPPER RIDGING OR
SHORTWAVE ENERGY/WEAK TROUGHING WILL BE MORE PREVALENT OVER THE
CENTRAL-SOUTHERN ROCKIES AND VICINITY. MOST OF THE MACHINE LEARNING
MODELS FAVOR A TILT TOWARD RIDGING. CLUSTERING IS BETTER THAN
AVERAGE FOR AN UPPER LOW NEARING THE WEST COAST NEXT WEEKEND.
..WEATHER/HAZARDS HIGHLIGHTS
THE STRONG UPPER RIDGE OVER THE EASTERN U.S. AS OF WEDNESDAY WILL
SUPPORT ANOMALOUS HEAT FROM THE LOWER GREAT LAKES/NORTHERN MID-
ATLANTIC INTO THE SOUTH, WITH DECENT COVERAGE OF PLUS 10-15F
ANOMALIES FOR HIGHS. FORECAST TRENDS OVER THE PAST COUPLE DAYS
VALID ON WEDNESDAY HAVE RAISED TEMPERATURES IN THE NORTHERN PART OF
THIS AREA AND INCREASED THE NUMBER OF LOCATIONS WHERE READINGS MAY
APPROACH OR REACH DAILY RECORDS. AFTER MIDWEEK, EXPECT THE SLOW
WEAKENING AND SOUTHEASTWARD PROGRESSION OF THE UPPER RIDGE TO YIELD
A GRADUAL MODERATING TREND, ESPECIALLY BY NEXT WEEKEND AS AN
APPROACHING FRONTS REACHES CLOSER TO THE EAST COAST. DURING
WEDNESDAY-THURSDAY IN PARTICULAR, EXPERIMENTAL HEATRISK CONTINUES
TO SHOW A BROAD AREA OF MODERATE TO MAJOR LEVEL IMPACTS WITH SOME
EMBEDDED POCKETS OF EXTREME. SOME MODERATE/MAJOR COVERAGE PERSISTS
INTO FRIDAY BUT THEN BECOMES MUCH MORE SUPPRESSED BY SATURDAY.
ELSEWHERE, THE NORTHERN ROCKIES INTO PARTS OF THE PLAINS WILL BE
THE MAIN FOCUS FOR BELOW NORMAL TEMPERATURES DURING THE PERIOD. THE
SYSTEM EXITING THE NORTHWEST AROUND MIDWEEK WILL BRING COOL HIGHS
THROUGH THE NORTHERN ROCKIES/NORTHERN HIGH PLAINS WEDNESDAY-
THURSDAY. A TRAILING COLD FRONT MAY REINFORCE BELOW NORMAL HIGHS
OVER THE NORTHERN AND THEN CENTRAL PLAINS NEXT WEEKEND. MEANWHILE
UPPER RIDGING THAT BUILDS INTO THE PACIFIC NORTHWEST WILL BRING
ABOVE NORMAL TEMPERATURES TO THE REGION LATE WEEK INTO THE WEEKEND.
THE DAY 4 EXCESSIVE RAINFALL OUTLOOK VALID WEDNESDAY-WEDNESDAY
NIGHT HAS MAINTAINED THE MARGINAL RISK AREA EXTENDING FROM THE
EASTERN GREAT LAKES/UPPER OHIO VALLEY EASTWARD INTO SOUTHERN NEW
ENGLAND AND THE NORTHERN MID-ATLANTIC. WHILE GUIDANCE QPFS ARE
STILL NOT PARTICULARLY WELL CLUSTERED, THERE IS NOW A MORE COHERENT
SIGNAL FOR AN AXIS OF INSTABILITY AND ANOMALOUS MOISTURE TO EXIST
AHEAD OF A WEST-EAST COLD FRONT SAGGING THROUGH THE REGION. THIS
SETUP APPEARS FAVORABLE FOR LOCALLY HIGH RAIN RATES AND POSSIBLY
SOME AREAS OF WEST-EAST TRAINING. MARGINAL RISK AREAS OVER THE
NORTHERN PLAINS/UPPER MIDWEST (CONVECTION AHEAD OF AN ADVANCING
NORTHERN TIER FRONT) AND OVER THE SOUTHERN ROCKIES (ADEQUATE
MOISTURE/INSTABILITY, GFS/ECMWF QPF SIGNALS, AND LIKELY EXISTENCE
OF WEAK SHORTWAVE IMPULSES ALOFT) REMAIN SIMILAR FROM THE OVERNIGHT
ISSUANCE. THE DAY 5 ERO COVERING THURSDAY-THURSDAY NIGHT DEPICTS
A MARGINAL RISK AREA OVER THE UPPER MIDWEST, WITH CONTINUED
EASTWARD PROGRESSION OF THE CENTRAL U.S. FRONT. SOME LOCALLY HEAVY
CONVECTION MAY ALSO BE POSSIBLE IN THE MID-ATLANTIC NEAR THE
SURFACE FRONT THAT SHOULD BE STALLING OVER THE REGION, AND A
MARGINAL RISK AREA IS NOW PLANNED FOR THE PIEDMONT OF VIRGINIA AND
PORTIONS OF ADJACENT STATES.
CONTINUED SOUTHWARD/EASTWARD PROGRESSION OF THE CENTRAL U.S. COLD
FRONT AFTER EARLY FRIDAY SHOULD BRING AREAS OF RAIN AND
THUNDERSTORMS INTO THE EASTERN PART OF THE COUNTRY BY LATE WEEK
INTO THE WEEKEND, AS WELL AS EXTENDING SOUTHWEST INTO THE SOUTHERN
ROCKIES/HIGH PLAINS WHERE AT LEAST A BRIEF PERIOD OF LOW LEVEL
UPSLOPE FLOW COULD HELP TO ENHANCE RAINFALL. ANOTHER FRONT DROPPING
INTO THE PLAINS NEXT WEEKEND COULD EVENTUALLY PRODUCE SOME
SHOWERS/STORMS AS WELL. PERIODS OF CONVECTION WILL BE POSSIBLE
ALONG THE GULF COAST THROUGH THE PERIOD, WITH GUIDANCE HINTING AT
RELATIVELY BETTER POTENTIAL OVER WEST-CENTRAL AREAS. ALSO FLORIDA
MAY SEE SOME INCREASE IN MOISTURE AND RAINFALL BY LATE WEEK OR THE
WEEKEND.
RAUSCH/HAMRICK
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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