001
FXUS02 KWBC 031908
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
308 PM 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 DURING THE WEEKEND...
..OVERVIEW
AN AMPLIFIED UPPER-LEVEL PATTERN WITH AN UPPER LOW SLOWLY MOVING
THROUGH THE NORTHEAST WILL SUPPORT A PERIOD OF ENHANCED RAINFALL
OVER PORTIONS OF NEW ENGLAND THIS WEEKEND, WITH A SURGE OF FALL-
LIKE DRY AND COOL AIR MASS BEHIND IT ACROSS MUCH OF THE EASTERN
HALF OF THE COUNTRY. A LINGERING FRONT WILL KEEP DECENT RAIN
CHANCES FROM CENTRAL GULF COAST EASTWARD TO FLORIDA, WITH HEAVY
RAIN MOST LIKELY FRIDAY AND SATURDAY ALONG CENTRAL TO EASTERN GULF
COAST. MEANWHILE, A RIDGE SHIFTING SLOWLY EAST ACROSS THE WESTERN
U.S. WILL INCREASE THE THREAT OF EXCESSIVE HEAT OVER PORTIONS OF
THE SOUTHWEST AND CALIFORNIA LATE THIS WEEK. TROPICAL MOISTURE
MOVING INTO THE SOUTHWESTERN GULF COULD BRING HEAVY RAIN INTO
SOUTHERN TEXAS BY EARLY NEXT WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODELS CONTINUE TO INDICATE THE LIKELIHOOD FOR A DEEP UPPER LOW TO
FORM OVER THE GREAT LAKES REGION BY THIS WEEKEND AS A RIDGE AXIS
SHIFTS SLOWLY EASTWARD ACROSS THE WESTERN U.S., ALLOWING THE NEXT
TROUGH FROM THE GULF OF ALASKA TO APPROACH THE PACIFIC NORTHWEST
EARLY NEXT WEEK. THERE HAS BEEN A TENDENCY FOR THE MODELS TO SLOW
DOWN THE EASTWARD PROGRESS OF THE UPPER LOW OUT OF THE GREAT LAKES
THIS WEEKEND IN RESPONSE TO A TREND FOR A DEEPER AND MORE CLOSED-
OFF UPPER LOW CIRCULATION.
ALONG THE GULF COAST, THERE HAS BEEN A TREND TO SLOW DOWN THE
MOTION OF A FRONTAL WAVE LATE THIS WEEK INTO THE WEEKEND, ALLOWING
HEAVIER RAINFALL TO FALL ALONG THE COAST BUT THIS DID NOT AFFECT
THE TIMING AND LOCATION OF THE HEAVY RAIN JUST OF THE SOUTHEAST
COAST ALL THAT MUCH. ELSEWHERE, MODELS HAVE NOT SHOWN TOO DRASTIC
OF A CHANGE LATELY.
THE WPC MEDIUM-RANGE PRODUCTS WERE DERIVED BASED ON 40% FROM THE
00Z EC/EC MEAN, 40% FROM THE 06Z GFS/GEFS, AND 20% FROM THE CMC/CMC
MEAN, AND LEANING TOWARD THE CONSENSUS OF THE ENSEMBLE MEANS FOR
DAYS 6 AND 7.
..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. A SLIGHT RISK AREA HAS BEEN INTRODUCED ALONG THE
CENTRAL TO EASTERN GULF COAST DUE TO A GENERAL INCREASE OF MODEL
QPFS WITH A SLOWING FRONTAL WAVE. 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, WITH A POSSIBILITY FOR AN
EMBEDDED SLIGHT RISK. FROM SUNDAY INTO EARLY NEXT WEEK, 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. MEANWHILE, TROPICAL MOISTURE
MOVING INTO THE SOUTHWESTERN GULF COULD BRING AN INCREASING THREAT
OF HEAVY RAIN INTO SOUTHERN TEXAS BY EARLY NEXT WEEK.
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.
KONG/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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