074
FXUS02 KWBC 080700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 AM EDT FRI SEP 08 2023
VALID 12Z MON SEP 11 2023 - 12Z FRI SEP 15 2023
...HAZARDOUS HEAT FROM THE SOUTH-CENTRAL U.S TO THE SOUTHWEST TO
MODERATE NEXT WEEK...
..OVERVIEW
THE GUIDANCE FOR NEXT WEEK EXPECTS MEAN TROUGHING ALOFT OVER THE
EASTERN HALF OF THE COUNTRY TO BECOME SOMEWHAT DEEPER THAN IN THE
SHORT RANGE PERIOD, WHILE A FLAT RIDGE OVER THE WEST EARLY-MID
WEEK COULD STRENGTHEN BY THURSDAY OR FRIDAY AS NORTHEASTERN
PACIFIC SHORTWAVE ENERGY PROGRESSING INTO/THROUGH THE NORTHWESTERN
U.S./WESTERN CANADA EVENTUALLY STARTS FEEDING INTO THE DOWNSTREAM
MEAN TROUGH. IN ASSOCIATION WITH THE DEEPENING EASTERN TROUGH,
GREAT LAKES INTO NEW ENGLAND/SOUTHEASTERN CANADA LOW PRESSURE AND
FRONTAL SYSTEM WILL PROMOTE AN ORGANIZED AREA OF RAINFALL WITH
HIGHEST AMOUNTS TENDING TO BE OVER THE NORTHEAST. MEANWHILE A
COUPLE FRONTS OVER THE SOUTHERN ROCKIES AND PLAINS WILL PROVIDE
SOME FOCUS FOR WHAT COULD BE A FAIRLY WET PERIOD OVER THAT REGION.
SOUTHERN TIER HEAT SHOULD GET SUPPRESSED AFTER EARLY IN THE WEEK
WHILE COOL DAYTIME TEMPERATURES WILL LIKELY SPREAD EASTWARD FROM
THE CENTRAL U.S. OVER THE COURSE OF NEXT WEEK.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
BROADLY SPEAKING, THERE IS DECENT AGREEMENT WITH THE FORECAST
EVOLUTION DURING THE PERIOD. A POSITIVELY TILTED SHORTWAVE
DROPPING INTO THE UPPER MIDWEST EARLY NEXT WEEK SHOULD CLOSE OFF
AN UPPER LOW AROUND MIDWEEK, WITH THIS UPPER LOW OPENING UP AND
EJECTING NORTHEASTWARD WITH THE APPROACH OF NORTHEAST PACIFIC
ENERGY CROSSING THE NORTHERN TIER U.S. AND SOUTHERN CANADA. SOME
DIFFERENCES EXIST FOR EXACTLY WHEN THE UPPER LOW CLOSES OFF AND
HOW DEEP IT IS. THE PAST COUPLE UKMET RUNS ARE PROMINENT EXTREMES
ON THE DEEP SIDE, LEADING TO A SURFACE LOW THAT STRAYS WELL
NORTHWARD OF OTHER GUIDANCE BY NEXT WEDNESDAY. AMONG THE MEANS,
THE ECENS HAS BEEN CLOSEST TO THE OPERATIONAL CONSENSUS AT THE
SURFACE AND ALOFT THUS FAR WHILE THE GEFS/CMCENS HAVE BEEN LAGGING
A BIT--BEING WEAKER ALOFT AND FLATTER AT THE SURFACE. THE NEW 00Z
GEFS MEAN SEEMS TO BE TRENDING IN THE RIGHT DIRECTION. SOME
OPERATIONAL MODEL RUNS SUGGEST THAT UPSTREAM ENERGY REACHING
SOUTHERN CANADA AND THE NORTHERN TIER OF THE U.S. COULD BE
SOMEWHAT DEEPER THAN THE MEANS BY THE END OF THE PERIOD, WITH AN
INTERMEDIATE BLEND PROVIDING A REASONABLE STARTING POINT GIVEN
TYPICALLY LOWER PREDICTABILITY IN SPECIFICS AT THAT TIME FRAME.
ALSO BY NEXT FRIDAY, MOST GUIDANCE SHOWS HURRICANE LEE TRACKING
NORTHWARD BETWEEN THE EAST COAST AND BERMUDA, NEAR 70W LONGITUDE.
ANY POSSIBLE EFFECTS ON NEW ENGLAND AND THE CANADIAN MARITIMES ARE
LIKELY TO BE JUST AFTER THE CURRENT MEDIUM RANGE FORECAST PERIOD.
BASED ON THE 12Z/18Z GUIDANCE AVAILABLE FOR THE LATEST UPDATE, THE
STARTING BLEND USED THE ECMWF/GFS/CMC/UKMET IN ORDER FROM MORE TO
LESS WEIGHT (GFS SPLIT BETWEEN 12Z/18Z RUNS GIVEN DETAILS THAT
WERE MORE OR LESS FAVORABLE DEPENDING ON THE REGION) EARLY IN THE
PERIOD AND THEN INCORPORATED SOME 18Z GEFS/00Z ECENS MEANS BY DAYS
6-7 THURSDAY-FRIDAY GIVEN DECREASING CONFIDENCE IN SOME DETAILS AT
THAT TIME.
..WEATHER/HAZARDS HIGHLIGHTS
A PRIMARY REGION OF FOCUS IN THE DAYS 4-5 EXCESSIVE RAINFALL
OUTLOOKS (EROS) THAT COVER DAYTIME MONDAY THROUGH TUESDAY NIGHT
WILL BE OVER AND EVENTUALLY WEST OF THE SOUTHERN PLAINS WHERE
MULTIPLE EPISODES OF CONVECTION ARE POSSIBLE. CONTRIBUTING
INGREDIENTS INCLUDE SHORTWAVE IMPULSES PROGRESSING WEST TO EAST
AROUND UPPER RIDGING CENTERED OVER MEXICO AND A COUPLE SURFACE
FRONTS SAGGING SOUTHWARD OVER THE SOUTHERN PLAINS. A PERIOD OF
EASTERLY UPSLOPE FLOW WILL BE POSSIBLE OVER THE HIGH PLAINS AS
WELL. FORECAST RAINFALL AMOUNTS AND GUIDANCE SCATTER WITHIN A
BACKGROUND OF MODERATELY POSITIVE MOISTURE ANOMALIES AND SOME
INSTABILITY, OFFSET SOMEWHAT BY RELATIVELY DRY ANTECEDENT
CONDITIONS, GENERALLY SUPPORT A MARGINAL RISK AREA EACH DAY. THE
DAY 5 AREA DROPS A BIT SOUTHWARD VERSUS DAY 4 AS WELL AS EXTENDING
FARTHER WEST (INTO ARIZONA VERSUS JUST INTO THE HIGH PLAINS).
REFINEMENTS IN FUTURE MODEL RUNS MAY ULTIMATELY REVEAL AREAS OF
MORE PRONOUNCED FLASH FLOOD POTENTIAL. MEANWHILE, A MARGINAL RISK
AREA HAS BEEN MAINTAINED OVER PARTS OF NEW ENGLAND FOR DAY 4 AS A
WAVY FRONT AND ITS ASSOCIATED MOISTURE SLOWLY MOVE THROUGH. THE
DEVELOPING UPPER/SURFACE SYSTEM REACHING THE GREAT LAKES COULD
PRODUCE ONE OR MORE POCKETS OF MODERATE TO HEAVY RAINFALL BY THE
DAY 5 TIME FRAME BUT CURRENT GUIDANCE SIGNALS APPEAR TOO
LOCALIZED/DIFFUSE FOR A RISK AREA AT THIS TIME.
RAIN AND THUNDERSTORMS OVER THE SOUTHERN PLAINS AND FOUR CORNERS
STATES MAY CONTINUE INTO THE LATTER HALF OF THE WEEK, WITH A
LIGHTER AND MORE SCATTERED TREND OVER ARIZONA AND UTAH. THE
SYSTEM REACHING THE GREAT LAKES BY MIDWEEK SHOULD PRODUCE SOME
RAINFALL OVER PARTS OF THE MIDWEST EARLY IN THE WEEK AND THEN
SPREAD AN ORGANIZED AREA OF RAINFALL FROM THE GREAT LAKES INTO THE
NORTHEAST/MID-ATLANTIC, WITH HIGHEST TOTALS CURRENTLY EXPECTED
OVER THE NORTHERN MID-ATLANTIC/APPALACHIANS INTO NEW ENGLAND.
HAZARDOUS LATE SUMMER HEAT WILL LINGER ACROSS THE FAR SOUTHERN
TIER OF THE U.S. AND THE SOUTHWEST INTO MONDAY. HIGH DEWPOINTS AND
HUMIDITY ESPECIALLY NEAR THE WESTERN GULF COAST WILL LEAD TO
DANGEROUS HEAT INDICES AS HIGH AS 115F WITH ACTUAL AIR
TEMPERATURES UP TO 10-15F ABOVE NORMAL. SOME RECORD HIGH MINIMUM
AND MAXIMUM TEMPERATURES WILL BE POSSIBLE. EXPECT A COOLER TREND
AFTER MONDAY AS SUPPORTING UPPER LEVEL RIDGING GETS SUPPRESSED.
BELOW NORMAL HIGH TEMPERATURES WILL PREVAIL OVER THE REST OF THE
CENTRAL U.S., SPREADING EASTWARD WITH TIME. SOME HIGHS MAY BE AS
COOL AS 10-15F BELOW NORMAL, ESPECIALLY OVER THE CENTRAL PLAINS
EARLY IN THE WEEK. TEMPERATURES IN THE NORTHWEST MAY TREND
MODERATELY ABOVE NORMAL BY LATE WEEK UNDER STRONGER RIDGING ALOFT.
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, EXCESSIVE RAINFALL OUTLOOK,
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY
MESSAGES ARE AT:
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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