850
FXUS02 KWBC 030646
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
246 AM EDT TUE SEP 3 2024
VALID 12Z FRI SEP 06 2024 - 12Z TUE SEP 10 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 HEAT THREATS OVER PORTIONS
OF THE SOUTHWEST AND CALIFORNIA LATE THIS WEEK, WITH BELOW NORMAL
TEMPERATURES PREVAILING ACROSS THE MIDWEST TO THE EAST. THE
MAJORITY OF RAINFALL DURING THE PERIOD WILL BE ACROSS THE SOUTH AND
EAST. WEAK ENERGY NEAR THE GULF COAST WILL INTERACT WITH A
PERSISTENT WAVY SURFACE BOUNDARY TO FOCUS HEAVY RAINFALL OVER PARTS
OF THE SOUTHEAST, FOLLOWED BY INTERACTION WITH A NORTHERN STREAM
COLD FRONT INTO THE NORTHEAST. AN INCREASINGLY LARGE PART OF THE
COUNTRY SHOULD SEE DRY/QUIET WEATHER BY SUNDAY AND BEYOND.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODELS CONTINUE TO SHOW GOOD AGREEMENT ON THE LARGE SCALE PATTERN
EVOLUTION DURING THE PERIOD, BUT SOME DIFFERENCES IN THE TIMING OF
SMALLER WAVES, ESPECIALLY AROUND THE DEEP LOW NORTH OF THE GREAT
LAKES. THIS AFFECTS TIMING OF THE COLD FRONT THROUGH THE EAST EARLY
PERIOD, AND INTERACTIONS WITH WEAK ENERGY AND SURFACE LOW AS IT
LIFTS UP/OFF THE EAST COAST. BY EARLY NEXT WEEK, UNCERTAINTY
INCREASES AS TO WHETHER THE LOW LIFTS MORE NORTHWARD INTO EASTERN
CANADA OR PUSHES THROUGH THE NORTHEAST. THE GFS HAS BEEN CONSISTENT
SHOWING IT LIFTING NORTH, BUT THE NEW 00Z RUNS OF THE CMC AND ECMWF
(WHICH WERE AVAILABLE AFTER FORECAST GENERATION TIME) PULL IT
EASTWARD TOWARDS THE NORTHEAST. OUT WEST, SOME LARGER SCALE
DIFFERENCES IN TIMING AND AMPLITUDE EMERGE AS THE NEXT TROUGH MOVES
INLAND MONDAY- TUESDAY, BUT NEW 00Z MODEL RUNS SEEM TO BE TRENDING
SLOWER WITH THIS FEATURE. THE WPC FORECAST FOR TONIGHT USED A
BLEND OF THE DETERMINISTIC MODELS AMIDST GOOD AGREEMENT FOR THE
FIRST HALF OF THE PERIOD, INCORPORATING UP TO 50 PERCENT OF THE
ENSEMBLE MEANS IN THE BLEND BY DAY 7 TO SMOOTH OUT DIFFERENCES WITH
THE EASTERN LOW AND WESTERN TROUGH.
..WEATHER/HAZARDS HIGHLIGHTS
THE MAIN HEAVY RAINFALL FOCUS DURING THE PERIOD WILL BE ACROSS THE
SOUTHEAST ON FRIDAY AS SHORTWAVE ENERGY INTERACTS WITH A LINGERING
SURFACE FRONT ALONG/NEAR THE GULF COAST. PRECIPITATION COVERAGE
AND TOTALS REMAIN SOMEWHAT UNCERTAIN, BUT THE OVERALL DYNAMICS AND
ANOMALOUS MOISTURE SUPPORT A HEAVY TO EXCESSIVE RAINFALL THREAT AS
HIGHLIGHTED BY A BROAD MARGINAL RISK ON THE DAY 4 EXCESSIVE
RAINFALL OUTLOOK (COVERING FRIDAY-FRIDAY NIGHT). MODELS CONTINUE TO
SUGGEST A SOUTHWARD TREND WITH THE OVERALL THREAT AND SO COMPARED
TO YESTERDAYS DAY 5 ERO, TRIMMED THE NORTHERN EXTENT OF THE
MARGINAL A BIT FOR THE NEW UPDATE. THERE MAY BE A NEED FOR AN
EVENTUAL UPGRADE TO A SLIGHT RISK, PARTICULARLY FOR THE CENTRAL TO
EASTERN FL PANHANDLE WHERE THE STRONGER INSTABILITY SHOULD BE, BUT
DECIDED TO HOLD OFF ON THAT WITH THIS CYCLE GIVEN CONTINUED
UNCERTAINTY IN PLACEMENT. THE FRONT SHOULD LINGER ACROSS NORTHERN
FL FOR A FEW DAYS, WITH SOME POCKETS OF HEAVIER RAINFALL POSSIBLE
SO DID MAINTAIN A NARROW MARGINAL ACROSS THIS AREA FOR THE DAY
5/SATURDAY ERO AS WELL. FARTHER NORTH, SOME HEAVIER RAINFALL MAY
ACCOMPANY THE COLD FRONT AS IT PASSES THROUGH THE NORTHEAST, AIDED
BY WRAP BACK MOISTURE FROM A SURFACE LOW ALONG THE COAST. COVERED
THIS WITH A MARGINAL RISK ON THE DAY 5 ERO FOR NOW. AFTER SATURDAY
THOUGH, MUCH OF THE COUNTRY SHOULD DRY OUT, THOUGH 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 ON 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.
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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