337
FXUS02 KWBC 210644
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
243 AM EDT THU SEP 21 2023
VALID 12Z SUN SEP 24 2023 - 12Z THU SEP 28 2023
...HEAVY RAINFALL POTENTIAL ACROSS PARTS OF THE PLAINS/LOWER
MISSISSIPPI VALLEY...
..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
ASIDE FROM THE EAST COAST LOW, THE LATEST GUIDANCE SUITE WAS IN
FAIRLY GOOD AGREEMENT, WITH SOME EXCEPTIONS. THE GFS REMAINS ON
THE SLIGHTLY FARTHER NORTH SIDE OF THE GUIDANCE WITH THE UPPER LOW
OFF THE PACIFIC NORTHWEST COAST AND ALSO MUCH MORE AMPLIFIED WITH
AN INITIAL SHORTWAVE ACROSS THE WEST MID NEXT WEEK. THE ECMWF AND
CMC SEEMED TO HAVE FORMED A BETTER CONSENSUS WITH THE ENSEMBLE
MEANS FOR THE WESTERN TROUGH. ALONG THE EAST COAST, THE NEW 00Z
GUIDANCE (AVAILABLE AFTER FORECAST GENERATION TIME) SEEM TO BE
CONVERGING ON A CONSENSUS NOW, WITH THE EXCEPTION OF THE CMC WHICH
REMAINS ON THE MUCH FASTER SIDE. THROUGH THE 12Z/18Z (SEP 20) RUNS
THOUGH, THE GFS AND CMC WERE OFFERING A MORE INLAND TRACK OVER THE
MID-ATLANTIC WHILE THE ECMWF AND UKMET WERE NOTICEABLY OFFSHORE.
BOTH THE ECMWF AND UKMET WITH THE NEW 00Z RUN HAS TRENDED CLOSER
TO THE COAST/JUST INLAND OVER THE MID-ATLANTIC BEFORE TURNING
NORTHEASTWARD AND RACING OUT TO SEA. MANY OF THE MODELS ALSO
INDICATE POTENTIAL FOR AT LEAST SUB-TROPICAL CHARACTERISTICS,
WHICH THE NHC IS MONITORING. REGARDLESS THOUGH, IMPACTS (HEAVY
RAIN, WIND, WAVES, ETC.) WILL BE THE SAME. THE WPC FORECAST PROGS
FOR TONIGHT SPLIT THE DIFFERENCE BETWEEN THE MORE ONSHORE GFS AND
OFFSHORE ECMWF, AND LUCKILY IS STILL VERY CONSISTENT WITH THE
LATEST 00Z GUIDANCE THAT HAS COME IN SINCE THEN.
ACROSS THE BOARD, THE WPC FORECAST SUITE FOR TONIGHT WAS PRIMARILY
DERIVED FROM A DETERMINISTIC MODEL BLEND FOR THE FIRST HALF OF THE
PERIOD, MOSTLY FOR THE EAST COAST SYSTEM AS THE WEST WAS IN GOOD
AGREEMENT THE FIRST HALF. FOR DAYS 6 AND 7, TRENDED TOWARDS THE
ENSEMBLE MEANS TO TRY AND MITIGATE DIFFERENCES OUT WEST, WHILE
ALSO MAINTAINING SOME DETAIL USING THE ECMWF.
..WEATHER/HAZARDS HIGHLIGHTS
THE SURFACE LOW NEAR THE MID-ATLANTIC BY THE START OF THE PERIOD
ON SUNDAY SHOULD CONTINUE TO TAP TROPICAL MOISTURE, WITH HEAVY
RAINFALL POSSIBLE STILL ACROSS PARTS OF THE MID-ATLANTIC AND
SOUTHERN NEW ENGLAND COAST AS THE DEEP MOISTURE IS WRAPPED
NORTHWARD AND UP AND AROUND THE SURFACE LOW (REMINISCENT OF A COOL
SEASON NOR'EASTER). AMOUNTS AND INLAND EXTENT OF THE HEAVIER RAINS
CONTINUE TO BE SOMEWHAT UNCERTAIN AT THIS POINT, SO A MARGINAL
RISK WAS MAINTAINED FOR THE DAY 5/SUNDAY ERO FROM COASTAL NEW
JERSEY TO CAPE COD. THERE ARE SOME SUGGESTIONS THAT BY SUNDAY,
MUCH OF THE HEAVY RAIN COULD BE OFF THE COAST AND SO EVEN A
MARGINAL RISK MAY BE GENEROUS.
RAINFALL SHOULD FOCUS ALONG A FRONTAL BOUNDARY DROPPING ACROSS THE
PLAINS/MISSISSIPPI VALLEY SUNDAY INTO MONDAY. THE SET UP IS
FAVORABLE FOR HEAVY TO EXCESSIVE RAINFALL CONTINUING INTO SUNDAY,
GIVEN AMPLE MOISTURE AND INSTABILITY, ACROSS PORTIONS OF THE
SOUTHERN PLAINS/LOWER MISSISSIPPI VALLEY. THE DAY 4 EXCESSIVE
RAINFALL OUTLOOK CONTINUES A SLIGHT RISK ACROSS EASTERN
OKLAHOMA/NORTHEAST TEXAS/WESTERN ARKANSAS. RAINFALL WILL CONTINUE
ACROSS THE GENERAL AREA INTO MONDAY, BUT SHOULD BE LESSENING IN
COVERAGE/INTENSITY AS THE FRONT WEAKENS. FOR NOW, ONLY A MARGINAL
RISK WAS DRAWN FOR THE DAY 5 ERO FROM EAST TEXAS INTO THE LOWER
MISSISSIPPI VALLEY BUT THERE ARE INDICATIONS FOR LOCALLY HEAVY TO
EXCESSIVE AMOUNTS, BUT PLENTY OF UNCERTAINTY STILL ON WHERE.
ELSEWHERE, TROUGHING ALONG THE WEST COAST WILL POSSIBLY HEAVY
RAINFALL TO MAINLY COASTAL LOCATIONS AND RANGES, WITH EXTENT
FARTHER INLAND STILL QUITE UNCERTAIN. A MARGINAL RISK WAS ADDED TO
FAR NORTHWEST CALIFORNIA INTO SOUTHWEST OREGON ON DAY 5/MONDAY ERO
WHERE MODELS SEEMED TO HAVE A BETTER CONSENSUS FOR SIGNIFICANT
RAINS. GENERALLY SHOWERY WEATHER SHOULD CONTINUE INTO THE EASTERN
U.S. DURING THE MIDDLE OF NEXT WEEK, WITH SOME LOCAL ENHANCEMENT
ALONG FAVORABLE TERRAIN. TROUGHING ALONG THE SOUTHEAST COAST MAY
BRING ANOTHER PERIOD OF WET WEATHER BY NEXT WEDNESDAY INTO
THURSDAY TO COASTAL LOCATIONS.
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.
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, 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
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page