831
FXUS02 KWBC 021859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 PM EDT MON SEP 2 2024
VALID 12Z THU SEP 05 2024 - 12Z MON SEP 09 2024
...HAZARDOUS HEAT LIKELY LATE WEEK ACROSS PARTS OF THE SOUTHWEST
AND CALIFORNIA WITH HEAVY RAINFALL THREATS ACROSS THE LOWER
MISSISSIPPI VALLEY TO THE SOUTHEAST...
..OVERVIEW
EXPECT THE LARGE SCALE PATTERN TO BECOME MORE BLOCKY WITH TIME, AS
A WESTERN U.S. RIDGE DRIFTS TOWARD THE ROCKIES AND AN UPPER GREAT
LAKES/ONTARIO UPPER LOW CLOSES OFF WITHIN A DEEPENING EASTERN
TROUGH. THE WESTERN RIDGE WILL PRODUCE ABOVE NORMAL TEMPERATURES
OVER PARTS OF THE WEST, AND IN PARTICULAR HEAT THREATS OVER
PORTIONS OF THE SOUTHWEST AND CALIFORNIA, WHILE BELOW NORMAL
TEMPERATURES WILL SPREAD ACROSS THE EASTERN HALF OF THE COUNTRY.
THE MAJORITY OF RAINFALL DURING THE PERIOD WILL BE ACROSS THE SOUTH
AND EAST. WEAK ENERGY NEAR THE GULF COAST WILL INITIALLY INTERACT
WITH A PERSISTENT WAVY SURFACE BOUNDARY TO FOCUS HEAVY RAINFALL
OVER THE LOWER MISSISSIPPI VALLEY AND GULF COAST, FOLLOWED BY
STILL-UNCERTAIN INTERACTION WITH AN APPROACHING FRONT AHEAD OF THE
DEEPENING EASTERN TROUGH/LOW. AN INCREASINGLY LARGE PART OF THE
COUNTRY SHOULD SEE DRY/QUIET WEATHER BY SUNDAY AND BEYOND.
..GUIDANCE/PREDICTABILITY ASSESSMENT
WITH THE MODELS AND ENSEMBLES MAINTAINING GOOD AGREEMENT IN THE
LARGE SCALE PATTERN, THE UPDATED FORECAST STARTED WITH A 00Z/06Z
OPERATIONAL MODEL COMPOSITE EARLY IN THE PERIOD AND EVENTUALLY
TRANSITIONED TOWARD 40 PERCENT TOTAL ENSEMBLE MEAN (06Z GEFS/00Z
ECENS) INPUT TO TEMPER LOWER-CONFIDENCE DETAIL DIFFERENCES. SOME
SPREAD DEVELOPS WITH THE PATH/TIMING OF THE UPPER GREAT LAKES OR
SOUTHERN CANADA UPPER LOW, WITH THE 00Z/06Z GFS LEANING SOMEWHAT ON
THE NORTHEASTERN SIDE BY THE LATTER HALF OF THE PERIOD. THE 12Z GFS
HAS TRENDED BACK TO OTHER GUIDANCE, THOUGH THE 12Z ECMWF WOBBLES A
BIT EASTWARD OF CONTINUITY BY NEXT MONDAY. INTERESTINGLY, THE 00Z
ECMWF-INITIALIZED MACHINE LEARNING (ML) MODELS OVERALL LEANED
SOMEWHAT FASTER WITH THE UPPER LOW ALONG WITH THE LEADING SURFACE
FRONT CROSSING THE NORTHEAST. THERE IS ADDITIONAL SPREAD WITH HOW
EAST COAST/WESTERN ATLANTIC SURFACE WAVINESS MAY EVOLVE AND TRACK.
ML MODELS GENERALLY FAVORED A TRACK CLOSER TO THE COAST
(INCREASING RAINFALL POTENTIAL OVER THE NORTHEAST) WHILE LATEST
DYNAMICAL RUNS SHOW A LOT OF EAST-WEST SPREAD AND THUS SIGNIFICANT
DIFFERENCES FOR HOW MUCH MOISTURE REACHES THE NORTHEAST.
..WEATHER/HAZARDS HIGHLIGHTS
THE MAIN HEAVY RAINFALL FOCUS DURING THE PERIOD WILL BE ACROSS THE
SOUTH LATE THIS WEEK AS SHORTWAVE ENERGY MOVES INLAND FROM THE
WESTERN GULF AND A SURFACE FRONT LINGERS ALONG/NEAR THE COAST.
PRECIPITATION COVERAGE AND TOTALS REMAIN UNCERTAIN, BUT THE OVERALL
DYNAMICS AND ANOMALOUS MOISTURE SUPPORT A HEAVY TO EXCESSIVE
RAINFALL THREAT WHICH SLOWLY SHIFTS ACROSS THE DEEP SOUTH. THE DAY
4 EXCESSIVE RAINFALL OUTLOOK (COVERING THURSDAY-THURSDAY NIGHT)
SHOWS A BROAD MARGINAL RISK ACROSS THE LOWER MISSISSIPPI VALLEY
INTO PARTS OF THE LOWER TENNESSEE VALLEY AND SOUTHEAST. AN EMBEDDED
SLIGHT RISK HAS BEEN MAINTAINED NEAR THE COAST FROM SOUTHEAST
LOUISIANA TO THE FAR WESTERN FLORIDA PANHANDLE WHERE MODELS SHOW
OVERLAP IN HEAVY QPF FROM THE SHORT RANGE INTO THE MEDIUM RANGE
PERIOD. THE OVERALL THREAT SHIFTS EAST ON FRIDAY INTO THE
SOUTHEAST, WITH A MARGINAL RISK AREA DEPICTED. ASIDE FROM
12Z-VERSUS-06Z GFS COMPARISONS, THE NEW 12Z MODELS SHOW A FAIRLY
PRONOUNCED TREND TOWARD LESS NORTHWARD EXTENT OF SOUTHERN TIER
RAINFALL. THE UPDATED DAYS 4-5 EROS HAVE ACCORDINGLY TRIMMED THE
NORTHERN SIDE OF THE MARGINAL RISK AREA EACH DAY WITH SOME LEEWAY
FOR ADDITIONAL ADJUSTMENT PENDING TRENDS OR CONTINUITY IN FUTURE
RUNS. ALTHOUGH CONFIDENCE IN SPECIFICS HAS DECREASED SOMEWHAT,
ENOUGH OF A SIGNAL PERSISTS TO ALLOW KEEPING THE DAY 4 SLIGHT RISK
AREA AS-IS FOR THE TIME BEING.
AFTER EARLY SATURDAY, EXPECT SOME RAINFALL TO REACH THE NORTHEAST
BUT WITH UNCERTAINTY OVER AMOUNTS GIVEN GUIDANCE SPREAD FOR HOW
MUCH INTERACTION THERE MAY BE AMONG THE FRONT PUSHING INTO THE
NORTHEAST FROM THE OHIO VALLEY-GREAT LAKES AND WAVINESS/MOISTURE
OVER THE SOUTHERN TIER INTO EAST COAST OR WESTERN ATLANTIC.
SUFFICIENT MOISTURE INPUT COULD LEAD TO SOME AREAS OF HEAVY
RAINFALL. LIGHTER WRAP-AROUND ACTIVITY MAY PERSIST FOR A TIME GIVEN
THE PROXIMITY OF THE UPPER LOW BUT OTHERWISE RAIN SHOULD CLEAR THE
EAST COAST, ASIDE FROM LOCATIONS OVER/NEAR FLORIDA WHERE A SURFACE
FRONT WILL LINGER. SHORTWAVE ENERGY WITHIN AND AROUND WESTERN U.S.
UPPER RIDGING MAY PRODUCE ISOLATED TO SCATTERED RAINFALL WITH
MOSTLY LIGHT AMOUNTS.
ABOVE NORMAL TEMPERATURES WITH HAZARDOUS HEAT OVER THE WEST SHOULD
BE MOST PRONOUNCED THURSDAY-FRIDAY, WITH HEATRISK SHOWING
WIDESPREAD MAJOR THREATS ACROSS THE SOUTHWEST AND CENTRAL VALLEYS
OF CALIFORNIA AND MODERATE TO LOCALLY MAJOR INTO THE PACIFIC
NORTHWEST. CALIFORNIA HIGHS MAY REACH UP TO 10-15F ABOVE NORMAL
WHILE THE PACIFIC NORTHWEST COULD SEE READINGS UP TO 15-20F OR SO
ABOVE NORMAL. SOME DAILY RECORDS ARE POSSIBLE. HEAT SHOULD PERSIST
TO SOME DEGREE INTO THE WEEKEND OVER THE NORTHWEST AND SLOWLY
EXPAND INTO THE NORTHERN HIGH PLAINS. IN CONTRAST, A MUCH COOLER
AIRMASS WILL FILTER EASTWARD FROM THE NORTHERN PLAINS AND GREAT
LAKES INTO THE EAST THIS WEEKEND AND EARLY NEXT WEEK, WITH DAYTIME
HIGHS 10-20 DEGREES BELOW NORMAL.
RAUSCH/SANTORELLI
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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