257
FXUS02 KWBC 211841
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
240 PM EDT THU SEP 21 2023
VALID 12Z SUN SEP 24 2023 - 12Z THU SEP 28 2023
...POTENTIAL TROPICAL CYCLONE SIXTEEN TO BRING HEAVY RAINFALL TO
MID-ATLANTIC AND NORTHEAST THIS WEEKEND...
..OVERVIEW
STRONG UPPER RIDGING IS FORECAST TO MEANDER IN PLACE IN AN
OMEGA/REX BLOCK CONFIGURATION BETWEEN HUDSON BAY AND THE UPPER
MIDWEST/CORN BELT/MID-MS VALLEY. ON ITS SOUTHEAST SIDE, A FRONTAL
COMPLEX WILL MOVE INTO THE MS VALLEY WITH HEAVY RAIN AND SEVERE
WEATHER CHANCES. ON THE EAST COAST, AN UPPER LOW AND DEVELOPING
COASTAL LOW (POTENTIALLY SUB-TROPICAL) WILL BE LIFTING
NORTH/NORTHEASTWARD THIS WEEKEND WITH A RAINFALL SHIELD MAINLY
JUST AFFECTING PARTS OF THE MID-ATLANTIC TO NEW ENGLAND BY THE
START OF THE PERIOD ON SUNDAY. ACROSS THE NORTH PACIFIC, A STRONG
UPPER JET ARCING ACROSS THE ALEUTIANS WILL PROMOTE DIGGING OF
TROUGHING SOUTHEAST OF THE GULF OF ALASKA AND WEST OF THE PACIFIC
NW THIS WEEKEND BEFORE TRANSLATING EASTWARD/INLAND EARLY NEXT
WEEK.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
FORECAST GUIDANCE HAS TRENDED WESTWARD AND GENERALLY SLOWER WITH
THE NEWLY FORMED POTENTIAL TROPICAL CYCLONE SIXTEEN OFF THE
SOUTHEAST COAST. THIS SYSTEM IS NOW FORECAST BY THE NATIONAL
HURRICANE CENTER TO TRACK GENERALLY NORTHWARD THROUGH EASTERN
NORTH CAROLINA INTO THE MID-ATLANTIC SATURDAY INTO SUNDAY. WHILE
GUIDANCE HAS CLUSTERED BETTER OVERALL SINCE YESTERDAY, THERE
REMAINS SOME TIMING AND TRACK DIFFERENCES LATE SUNDAY THROUGH
MONDAY WITH HOW FAST THE SYSTEM GETS PULLED AWAY FROM THE
NORTHEAST COAST. OVERALL, A SLOWER TREND WAS PREFERRED BASED ON
THE LATEST NHC TRACK AND 12Z GUIDANCE.
ELSEWHERE, THERE ARE SOME NOTABLE DIFFERENCES IN THE LARGE SCALE
PATTERN WITH THE BLOCKY PATTERN EVOLVING THAT FEATURES A RIDGE
SOUTH OF HUDSON BAY AND THE APPROACHING SHORTWAVE ENERGY FROM THE
PACNW AND NORTHERN ROCKIES. MOST OF THE 12Z GUIDANCE SLOWS THIS
FEATURE DOWN AND FORMS A CLOSED LOW AT 500 MB OVER THE DAKOTAS
EARLY NEXT WEEK WITH GENERAL TROUGHING SLOWLY MOVING
EAST/SOUTHEAST THROUGH DAYS 7 WHILE A SLOW MOVING FRONT EVENTUALLY
WASHES OUT/STALLS OVER THE MID-SOUTH BY DAYS 6-7. MEANWHILE, THE
LATEST MODEL GUIDANCE IS IN PRETTY GOOD AGREEMENT FOR A STRONG
TROUGH TO PUSH ONSHORE THE PACIFIC NORTHWEST THAT WILL START A
RATHER UNSETTLED AND ACTIVE PERIOD OF PRECIPITATION FOR THE
REGION.
THE WPC FORECAST BLEND THIS CYCLE GENERALLY USED A BLEND OF THE
LATEST EUROPEAN AND GFS FOR DAYS 3-5 FOLLOWED BY HIGHER WEIGHTS OF
THE ECENS AND GEFS MEANS FOR DAYS 6-7 WHICH TRENDED WELL FROM THE
PREVIOUS FORECAST AND WAS LARGELY CONSISTENT WITH THE PTC 16 TRACK
FROM THE NHC.
..WEATHER/HAZARDS HIGHLIGHTS
POTENTIAL TROPICAL CYCLONE SIXTEEN IS FORECAST TO BE ALONG THE
MID-ATLANTIC COAST SUNDAY MORNING AND THE HIGHER TROPICAL MOISTURE
ASSOCIATED WITH IT WILL LIKELY BRING ROUNDS OF MODERATE TO HEAVY
RAINFALL TO PORTIONS OF THE MID-ATLANTIC THROUGH THE NORTHEAST AND
SOUTHERN NEW ENGLAND. THIS HEAVY RAINFALL MAY BRING INSTANCES OF
FLASH FLOODING SUNDAY THROUGH MONDAY MORNING AS THE STORM SYSTEM
TRACKS ALONG THE NORTHEAST COAST. CONTINUE TO MONITOR THE LATEST
INFORMATION ON PTC 16 FROM THE NATIONAL HURRICANE CENTER FOR ALL
ADDITIONAL WEATHER HAZARDS.
A SLOW MOVING FRONTAL SYSTEM ACROSS THE CENTRAL U.S. WILL ACT AS
THE FOCUS FOR AREAS OF SHOWERS AND THUNDERSTORMS LATE THIS WEEKEND
INTO EARLY NEXT WEEK. SOME LOCALLY HEAVY RAINFALL WILL BE
POSSIBLE, PARTICULARLY ACROSS PORTIONS OF THE ARKLATEX REGION AND
SOUTHERN PLAINS WHERE INTERACTIONS WITH GREATER INSTABILITY AND
MOISTURE MAY HELP FUEL STRONGER STORMS. THE DAY 4 EXCESSIVE
RAINFALL OUTLOOK HIGHLIGHTS A SLIGHT RISK ACROSS THE ARKLATEX
WHILE A MARGINAL RISK REMAINS FOR MONDAY FURTHER SOUTH INTO TEXAS
AND LOUISIANA. THE ASSOCIATED FRONTAL BOUNDARY WILL STALL AND WASH
OUT ACROSS THE SOUTHERN U.S. INTO NEXT WEEK AND MAY ACT AS THE
FOCUS FOR ADDITIONAL SHOWERS AND THUNDERSTORMS THOUGH CONFIDENCE
ON DETAILS AND LOCATIONS IS LOW AT THIS POINT.
AN EARLY SEASON STORM SYSTEM WILL BRING UNSETTLED AND ACTIVE
WEATHER TO THE PACIFIC NORTHWEST BEGINNING LATE THIS WEEKEND AND
THROUGH EARLY NEXT WEEK. A FEW ROUNDS OF HEAVIER PRECIPITATION ARE
LIKELY, FAVORED FOR THE COASTAL REGIONS AND UPSLOPE AREAS OF THE
WASHINGTON AND OREGON CASCADES INTO NORTHERN CALIFORNIA WHERE SOME
LOCALIZED FLOODING MAY BE POSSIBLE WITH REPEATING ROUNDS OF
MODERATE RAINFALL.
BELOW NORMAL TEMPERATURES OUT WEST SHOULD MODERATE BY SUNDAY AS AN
INITIAL UPPER LOW SHIFTS EAST IN THE CENTRAL CONUS. ELSEWHERE,
TEMPERATURES COULD BE MODERATELY ABOVE NORMAL ACROSS PARTS OF THE
SOUTHERN PLAINS SUNDAY INTO MONDAY. BY TUESDAY AND BEYOND THOUGH,
MUCH OF THE COUNTRY SHOULD BE NEAR NORMAL OR WITHIN A FEW DEGREES
OF NORMAL. THE ONE EXCEPTION WILL LIKELY BE ALONG THE WEST COAST
(ESP NORCAL NORTHWARD INTO THE NORTHERN GREAT BASIN) WHERE
INCOMING RAINFALL MAY KEEP DAYTIME HIGHS AT LEAST 5-10 DEGREES
BELOW NORMAL.
TAYLOR
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, EXCESSIVE RAINFALL OUTLOOK,
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY
MESSAGES ARE AT:
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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