193
FXUS02 KWBC 202020
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
419 PM EDT WED SEP 20 2023
VALID 12Z SAT SEP 23 2023 - 12Z WED SEP 27 2023
...HEAVY RAINFALL POTENTIAL ACROSS PARTS OF THE PLAINS/UPPER
MIDWEST AS WELL AS THE EAST COAST LATE THIS WEEK AND 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. OVER THE SOUTHEAST, AN UPPER LOW AND DEVELOPING
COASTAL LOW (POTENTIALLY SUB-TROPICAL) WILL LIKELY LIFT
NORTH/NORTHEASTWARD THIS WEEKEND WITH A RAINFALL SHIELD OVER MUCH
OF THE I-95 CORRIDOR FROM VA INTO NEW ENGLAND. ACROSS THE NORTH
PACIFIC, A STRONG UPPER JET ARCING ACROSS THE ALEUTIANS WILL
PROMOTE DIGGING TROUGHING SOUTHEAST OF THE GULF OF ALASKA AND WEST
OF THE PAC NW THIS WEEKEND BEFORE TRANSLATING EASTWARD/INLAND
EARLY NEXT WEEK.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
ASIDE FROM THE EAST COAST, THE 00Z-06Z GUIDANCE SUITE WAS IN GOOD
AGREEMENT OVERALL WITH SOME EXCEPTIONS NOTED IN THE 06Z GFS OFF
THE PAC NW (FARTHER NORTH WITH THE DEEP LOW) BUT OTHERWISE MOSTLY
WITHIN TOLERANCE. OVER CANADA, SOME MODELS FINALLY RELENTED ON
SHOWING A CLOSED LOW MOVING SOUTHWESTWARD ACROSS THE GREAT LAKES
AND NOW THE STRONG MAJORITY FAVOR A MORE PROGRESSIVE TROUGH INTO
THE NORTHEAST THIS WEEKEND, WHICH SHOULD HELP KICK OUT THE COASTAL
SYSTEM. REGARDING THE EVOLUTION OF SAID SYSTEM, GUIDANCE GENERALLY
AGREES ON LIFTING THE LOW NORTHWARD OR NORTH-NORTHEASTWARD ACROSS
THE OUTER BANKS AND MAYBE SE VA BEFORE TURNING NORTHEASTWARD AND
MOVING OUT TO SEA. WILL MAINTAIN THE NON-TROPICAL STRUCTURE
(OCCLUDED CYCLONE) BUT SOME MODELS DO INDICATE POTENTIAL FOR AT
LEAST SUB-TROPICAL CHARACTERISTICS. REGARDLESS, IMPACTS (HEAVY
RAIN, WIND, CHOPPY WATERS, ETC.) WILL BE THE SAME. SPLIT THE
DIFFERENCE BETWEEN THE FASTER (GENERALLY GFS/GEFS) AND SLOWER
(GENERALLY ECMWF-LED) CAMPS AND FAVORED A MORE COASTAL VS INLAND
TRACK OVER VA. ACROSS THE WEST/CENTRAL CONUS, A CONSENSUS BLEND
SUFFICED AS WELL WITH GOOD AGREEMENT ON THE SLOW TRACK OF THE
UPPER LOW ACROSS THE CORN BELT NEXT TUE OR SO BETWEEN THE RECENT
GFS/ECMWF. PREDICTABILITY WAS NEAR TO BETTER THAN AVERAGE IN THE
WEST BUT PERHAPS A BIT LOWER THAN AVERAGE ALONG THE COAST WHERE
THERE MAY BE A SHARP GRADIENT BETWEEN HEAVIER RAIN AND MUCH
LIGHTER/LESS IMPACTFUL RAIN.
..WEATHER/HAZARDS HIGHLIGHTS
AS THE LOW MOVES EAST INTO THE PLAINS AND THEN SOUTHWARD INTO THE
OHIO VALLEY, RAINFALL SHOULD EXPAND IN COVERAGE ACROSS THE CENTRAL
AND NORTHERN PLAINS INTO THE MISSISSIPPI VALLEY THROUGH SATURDAY.
THE SET UP IS FAVORABLE FOR HEAVY TO EXCESSIVE RAINFALL, GIVEN
AMPLE MOISTURE AND INSTABILITY AND DESPITE DRY ANTECEDENT
CONDITIONS. SPC NOTED A SEVERE POTENTIAL AS WELL. A SLIGHT RISK
OUTLINE IS NOTED FROM THE UPPER MIDWEST/NORTHERN PLAINS SOUTHWARD
TOWARD THE OZARKS. ON SUNDAY INTO EARLY MONDAY, AS THE FRONT
CONTINUES SOUTHWARD AND EASTWARD, HEAVY RAINFALL WILL BE POSSIBLE
OVER PORTIONS OF TX/OK/AR/LA WHERE A SLIGHT RISK IS SHOWN.
THE SURFACE LOW MOVING OFFSHORE THE CAROLINAS SHOULD TAP TROPICAL
MOISTURE, WITH INCREASING POTENTIAL FOR HEAVY RAINFALL ALONG AT
LEAST THE ATLANTIC COAST FROM THE MID-ATLANTIC COAST INTO SOUTHERN
NEW ENGLAND 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
FAIRLY UNCERTAIN AT THIS POINT, SO A MARGINAL RISK WAS MAINTAINED
FOR THE DAY 4/SATURDAY ERO WITH ROOM FOR ADJUSTMENTS PER FUTURE
GUIDANCE. BY SUNDAY, A QUICKER EXODUS MAY REDUCE THE HEAVY RAIN
RISK OVER SOUTHEASTERN NEW ENGLAND BUT KEPT A SMALL MARGINAL AREA
FOR NOW.
THROUGH THE REST OF THE WEEKEND INTO EARLY NEXT WEEK, RAINFALL
OVER THE CENTRAL U.S. SHOULD SHIFT GRADUALLY EAST AND SOUTHEAST
ALONG WITH THE UPPER LOW AND SURFACE SYSTEM WITH SOME ACTIVITY
LOCALLY HEAVY, WITH LESSER AMOUNTS EXPECTED BY MID NEXT WEEK. THE
PACIFIC NORTHWEST SHOULD SEE GRADUALLY MORE RAINFALL WITH TIME AS
THE COLD FRONT APPROACHES THEN MOVES INLAND. THE AXIS OF MOISTURE
MAY BE FAIRLY NARROW, WITH MINOR CHANGES IN POSITION BEING THE
DIFFERENCE BETWEEN MINIMAL TOTALS OR SIGNIFICANTLY MORE.
BELOW NORMAL TEMPERATURES OUT WEST SHOULD MODERATE BY SUNDAY AS
THE LOW SHIFTS EAST IN THE CENTRAL CONUS. ELSEWHERE, TEMPERATURES
COULD BE MODERATELY ABOVE NORMAL ACROSS THE UPPER MIDWEST/GREAT
LAKES AND PARTS OF THE SOUTHERN PLAINS FRIDAY INTO SATURDAY AND
EVEN APPROACH RECORD HIGHS OVER SOUTH TEXAS. BY EARLY NEXT WEEK
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.
FRACASSO/ROTH/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
HAZARDS:
- HEAVY RAIN ACROSS PORTIONS OF NORTHERN CALIFORNIA, AND THE
PACIFIC NORTHWEST, MON, SEP 25.
- HEAVY RAIN ACROSS PORTIONS OF THE PLAINS, AND THE MISSISSIPPI
VALLEY, SAT-SUN, SEP 23-SEP 24.
- HEAVY RAIN ACROSS PORTIONS OF THE NORTHERN MID-ATLANTIC AND THE
NORTHEAST, SAT, SEP 23.
- SEVERE WEATHER ACROSS PORTIONS OF THE CENTRAL/SOUTHERN PLAINS,
AND THE MIDDLE/LOWER MISSISSIPPI
VALLEY, SAT, SEP 23.
- HAZARDOUS HEAT ACROSS PORTIONS OF THE SOUTHERN PLAINS, SAT-SUN,
SEP 23-SEP 24.
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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