915
FXUS02 KWBC 010700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 AM EDT SUN SEP 1 2024
VALID 12Z WED SEP 04 2024 - 12Z SUN SEP 08 2024
..OVERVIEW
AN UPPER RIDGE OVER THE SOUTHERN TIER TO START THE PERIOD WILL
STRENGTHEN OVER THE WEST MID TO LATE WEEK AS A SHORTWAVE SHIFTS
INTO THE NORTHERN PLAINS. THIS LOOKS TO INTERACT WITH ENERGY
DROPPING OUT OF CANADA TO PRODUCE A FAIRLY AMPLIFIED TROUGH OVER
THE GREAT LAKES, MOVING INTO THE NORTHEAST NEXT WEEKEND. THIS
SHOULD PUSH A MEAN BOUNDARY THROUGH THE NORTHERN TIER DURING THE
PERIOD. MEANWHILE, EMBEDDED ENERGY WITHIN THE LARGER SCALE UPPER
RIDGE AND A PERSISTENT SURFACE BOUNDARY DRAPED NEAR THE GULF COAST
SHOULD PROVIDE FOCUS FOR HEAVY RAINFALL THAT SHIFTS SLOWLY FROM THE
LOWER MISSISSIPPI VALLEY/GULF COAST INTO THE SOUTHEAST AND
POSSIBLY POINTS NORTH.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODELS CONTINUE TO SHOW REASONABLE AGREEMENT THE FIRST HALF OF THE
PERIOD AS A SHORTWAVE SHIFTS FROM THE NORTHWEST INTO THE NORTHERN
PLAINS. BY NEXT WEEKEND THOUGH, THERE IS SIGNIFICANT UNCERTAINTY IN
THE EVOLUTION OF THE PATTERN AS AN AMPLIFIED TROUGH AND ADDITIONAL
ENERGY DROP SOUTHWARD. THROUGH YESTERDAYS 12Z/18Z RUNS, THERE WAS
UNCERTAINTY IN HOW AMPLIFIED THIS TROUGH MIGHT GET NEAR THE GREAT
LAKES, BUT NOW MOST OF THE NEW 00Z GUIDANCE (AVAILABLE AFTER THE
FORECAST WAS GENERATED) HAVE TRENDED TOWARDS SOME SORT OF DEEP
CLOSED LOW DEVELOPMENT BUT HUGE DISTANCE DIFFERENCES IN LOCATION.
THE GFS DROPS THIS LOW SOUTH INTO THE MIDWEST, WHILE THE ECMWF
PULLS IT EAST TOWARDS EASTERN CANADA OR THE NORTHEAST, WITH OTHER
MODELS SOMEWHERE IN BETWEEN. THIS EVOLUTION IS OBVIOUSLY VERY FLUID
AND WILL CHANGE SEVERAL TIMES BETWEEN NOW AND THEN.
THE WPC FORECAST FOR TONIGHT USED A GENERAL BLEND OF THE
DETERMINISTIC MODELS (12Z/18Z RUNS) FOR THE FIRST HALF OF THE
PERIOD. TRENDED TOWARDS 60 PERCENT ENSEMBLE MEANS FOR NEXT WEEKEND
GIVEN THE UNCERTAINTY IN THE NORTHERN STREAM PATTERN.
..WEATHER/HAZARDS HIGHLIGHTS
THE MAIN HEAVY RAINFALL FOCUS DURING THE PERIOD WILL BE ACROSS THE
SOUTH AS SHORTWAVE ENERGY MOVES INLAND FROM THE WESTERN GULF AND A
SURFACE FRONT LINGERS ALONG/NEAR THE COAST. PRECIPITATION COVERAGE
AND TOTALS REMAIN HIGHLY UNCERTAIN, BUT THE OVERALL DYNAMICS AND
ANOMALOUS MOISTURE SUPPORT A HEAVY TO EXCESSIVE RAINFALL THREAT
WITH SLOW- MOVING STORMS. THIS IS COVERED BY FAIRLY BROAD MARGINAL
RISKS ON THE DAYS 4 AND 5 EROS (COVERING WEDNESDAY TO THURSDAY
NIGHT), WITH A FOCUS FOR AN POSSIBLE EVENTUAL SLIGHT UPGRADE ACROSS
THE GULF COAST AND FARTHER INLAND ACROSS THE LOWER MISSISSIPPI
VALLEY ON WEDNESDAY AND TOWARDS THE TENNESSEE VALLEY/ALABAMA ON
THURSDAY. BEYOND, THE LINGERING FRONTAL BOUNDARY ACROSS THE SOUTH
WILL PROMOTE ADDITIONAL RAIN AND LOCALLY HEAVY DOWNPOURS WITH
AMOUNTS AND LOCATIONS STILL VERY UNCERTAIN, BUT THE TREND IS FOR
INCREASED RAIN TOTALS ALONG THE GULF COAST AND INTO PARTS OF THE
SOUTHEAST. BY THE WEEKEND, SOME MOISTURE MAY SPREAD INTO THE OHIO
VALLEY AND/OR EASTERN SEABOARD. MEANWHILE, SOME LIGHT TO MODEST
PRECIPITATION SHOULD ACCOMPANY A SHORTWAVE MOVING THROUGH THE
INTERIOR WEST, AND RAIN CHANCES COULD SPREAD INTO THE NORTHERN
PLAINS AND MIDWEST/GREAT LAKES LATER NEXT WEEK ALONG AND AHEAD OF
THE NORTHERN FRONT.
INCREASING HEAT IS EXPECTED ACROSS MUCH OF THE WEST, WITH MODERATE
TO MAJOR HEATRISK THREATS ACROSS THE SOUTHWEST AND CENTRAL VALLEYS
OF CALIFORNIA. ELSEWHERE, FROM THE CENTRAL U.S. INTO THE EAST,
TEMPERATURES WILL BE BELOW OR NEAR NORMAL AS SYSTEMS MOVE THROUGH.
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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