535
FXUS02 KWBC 291853
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
253 PM EDT THU AUG 29 2024
VALID 12Z SUN SEP 01 2024 - 12Z THU SEP 05 2024
..OVERVIEW
INITIALLY, AN UPPER TROUGH WILL BE CROSSING THE GREAT
LAKES/NORTHEAST AND RIDGING WILL BE IN PLACE ACROSS THE CENTRAL
U.S. AND MUCH OF THE SOUTHWEST. THE UPPER TROUGH WILL GRADUALLY
MOVE EAST AND HEIGHTS WILL RISE IN IT'S WAKE AS THE CENTRAL U.S.
RIDGE MOVES INTO THE MIDWEST. MEANWHILE, A SMALLER UPPER
TROUGH/LOW WILL APPROACH THE WEST COAST AND MOVE INTO THE
NORTHWEST. THIS FEATURE SHOULD BEGIN TO WEAKEN AS IS MOVES ACROSS
THE NORTHERN ROCKIES INTO THE HIGH PLAINS, AND RIDGING IS EXPECTED
TO BUILD BACK INTO THE NORTHWEST MID-NEXT WEEK. THIS UPPER-LEVEL
PATTERN IS EXPECTED TO PUSH A COUPLE OF FRONTAL BOUNDARIES INTO
THE SOUTH-CENTRAL AND SOUTHEASTERN U.S., WHICH WILL LIKELY RESULT
IN A PERIOD OF RAINY/STORMY WEATHER. A PERSISTENT SURFACE TROUGH
AND EMBEDDED WAVES NEAR THE WESTERN GULF COAST SHOULD PROVIDE A
FOCUS FOR SOME OF THE HEAVIER RAINFALL. PRECIPITATION MAY ALSO
ACCOMPANY THE UPPER LOW IN THE NORTHWEST AND NORTHERN PLAINS, BUT
THIS FEATURE WILL BE RELATIVELY WEAK, LEADING TO LIGHTER RAINFALL
TOTALS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE HAS COME INTO FAIRLY GOOD AGREEMENT ON THE PATTERN
THROUGH THE FIRST HALF OF THE PERIOD. THE LATEST RUNS OF THE ECMWF
AND GFS BOTH SHOW THE UPPER LOW MOVING INTO THE NORTHWEST EARLY
THEN GRADUALLY PROGRESSING TOWARDS THE NORTHERN PLAINS THROUGH
MID-NEXT WEEK. THERE IS STILL SOME UNCERTAINTY WITH HOW THE ENERGY
FROM THE UPPER LOW WILL INTERACT WITH NORTHERN STREAM ENERGY, WHICH
WILL HAVE IMPACTS ON SURFACE FRONTS AND SENSIBLE WEATHER AS THE
SYSTEM MOVES PAST THE ROCKIES. THERE IS ALSO SOME UNCERTAINTY
SURROUNDING THE EVOLUTION OF A SOUTHERN STREAM WAVE THAT IS
FORECAST TO MOVE INTO THE SOUTHERN PLAINS, AND THERE IS A LARGE
AMOUNT OF UNCERTAINTY CONCERNING THE PROGRESSION OF THIS FEATURE.
BY LATE IN THE PERIOD THIS LEADS TO A RANGE OF SOLUTIONS FROM A
PROGRESSIVE SOUTHERN TIER SHORTWAVE THAT BRINGS A LOT OF MOISTURE
THROUGH THE SOUTHEASTERN QUADRANT OF THE LOWER 48 AND PERHAPS
EXTENDING EVEN A LITTLE NORTHWARD TO GREATER NORTHERN STREAM
DOMINANCE OVER THE EAST AND A MUCH MORE SUPPRESSED RAINFALL
PATTERN.
WPCS AFTERNOON FORECAST BLEND CONSISTED OF THE 06Z/00Z OPERATIONAL
MODEL COMPOSITE THROUGH THE FIRST HALF OF THE PERIOD, AND ENSEMBLE
MEANS WERE INTRODUCED TO THE BLEND FOR THE SECOND HALF OF THE
PERIOD, MAKING UP 40% OF THE BLEND FOR DAYS 6 AND 7.
..WEATHER/HAZARDS HIGHLIGHTS
THE DAYS 4-5 EXCESSIVE RAINFALL OUTLOOKS COVERING THE SUNDAY
THROUGH MONDAY NIGHT PERIOD FOCUS ON THREE PRIMARY REGIONS OF
INTEREST, ALL WITH MARGINAL RISK AREAS ON BOTH DAYS. ONE AREA
STRETCHES FROM THE SOUTHERN MID-ATLANTIC INTO THE APPALACHIANS
WHERE A COUPLE OF FRONTS WILL BE SAGGING SOUTHWARD OVER THE COURSE
OF THE PERIOD, WITH ASSOCIATED INSTABILITY AND MODERATELY ANOMALOUS
MOISTURE. MEANWHILE RAINFALL SHOULD CONTINUE ALONG PARTS OF THE
LOUISIANA/TEXAS COAST, WITH PRIOR DAYS OF RAINFALL PROVIDING WET
GROUND CONDITIONS HEADING INTO THE DAYS 4-5 PERIOD. THERE IS STILL
UNCERTAINTY IN THE COVERAGE OF SHOWERS AND THUNDERSTORMS OVER THE
SOUTHERN HIGH PLAINS AND VICINITY, BUT THERE IS STILL ENOUGH OF A
SIGNAL TO FAVOR MAINTENANCE OF THE MARGINAL RISK. LATEST MODEL
TRENDS HAVE BEEN SHOWING INCREASING QPF IN THIS AREA ON DAY
5/MONDAY INTO MONDAY NIGHT, WHICH MAY BE THE BETTER FOCUS FOR HEAVY
RAIN/FLASH FLOODING.
PRECIPITATION COVERAGE AND TOTALS ACROSS THE SOUTHERN TIER AFTER
EARLY TUESDAY REMAIN UNCERTAIN DUE TO SIGNIFICANT DIFFERENCES IN
UPPER FLOW DETAILS. SOME AREAS OF LOCALLY HEAVY RAINFALL APPEAR
LIKELY GIVEN THE FRONTS SETTLING OVER THE SOUTH HELPING TO SERVE AS
A FOCUS, BUT WITH VERY LOW CONFIDENCE IN RESOLVING THE DETAILS OF
COVERAGE AND TOTALS. EXPECT THE NORTHWEST TO SEE SCATTERED RAINFALL
EARLY NEXT WEEK AS THE EASTERN PACIFIC UPPER LOW EJECTS INLAND.
SOME SCATTERED RAINFALL COULD REACH THE NORTHERN PLAINS BY MIDWEEK
OR SO.
UPPER RIDGING WILL SUPPORT AN AREA OF WELL ABOVE NORMAL
TEMPERATURES DRIFTING FROM THE NORTHWEST INTO THE NORTHERN HIGH
PLAINS SUNDAY THROUGH TUESDAY, WITH SOME LOCATIONS SEEING HIGHS
10-15 DEGREES ABOVE NORMAL. THIS WARM AIR SHOULD QUICKLY MODERATE
FARTHER EASTWARD. A RETURN OF UPPER RIDGING OVER THE WEST COAST
STATES BY WEDNESDAY/THURSDAY SHOULD PROMOTE ABOVE NORMAL
TEMPERATURES WITH ANOMALIES FOR HIGHS AGAIN REACHING 10-15 DEGREES
ABOVE AVERAGE AT SOME LOCATIONS. BELOW NORMAL HIGHS OVER THE
CENTRAL U.S. ON SUNDAY WILL EXPAND TOWARD THE EAST COAST EARLY NEXT
WEEK, SUPPRESSING WHAT HEAT LINGERS OVER THE SOUTHEAST TO START
THE WEEK. MOST OF THE CENTRAL/EASTERN U.S. SHOULD TREND CLOSER TO
NORMAL BY NEXT THURSDAY BUT THE SOUTHERN MID-ATLANTIC/SOUTHEAST MAY
REMAIN BELOW NORMAL FOR HIGHS.
DOLAN/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 (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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