153
FXUS02 KWBC 200651
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
250 AM 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
THE LEADING EDGE OF AMPLIFYING/BROADENING NORTHEAST PACIFIC
TROUGHING EXTENDS INTO THE PACIFIC NORTHWEST AND WESTERN CANADA BY
THE WEEKEND. AN OMEGA BLOCK REMAINS DOWNSTREAM FROM CENTRAL NORTH
AMERICA EASTWARD INTO THE WESTERN ATLANTIC. A FRONTAL WAVE/UPPER
LEVEL SHORTWAVE AROUND THE SOUTHEAST SIDE OF THE OMEGA BLOCK AND
HAS THE POTENTIAL FOR HEAVY COASTAL RAINS TO PARTS OF THE
MID-ATLANTIC AND COASTAL NEW ENGLAND AS IT MOVES UP/OFFSHORE OF
THE COAST.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THE GUIDANCE, IN GENERAL, AGREES ON THE DEEP CYCLONE OFFSHORE THE
WEST COAST MIGRATES EASTWARD AND BRINGS A COLD FRONT THROUGH
PORTIONS OF THE WEST. OTHERWISE, THE 12-00Z CANADIAN RUNS BECAME
AN OUTLIER NEAR THE US/CANADIAN BORDER, RETROGRADING A CLOSED LOW
OUT OF NEW ENGLAND INTO SOUTHEAST CANADA WHERE ALL THE OTHER
GUIDANCE HAD RIDGING. THE 12-00Z CANADIAN WERE NOT JUDGED USEFUL
BEYOND SUNDAY ACROSS THE GREAT LAKES AND EAST. FOR THE
POTENTIALLY CONVECTIVE, POSSIBLY OCCLUDED, LOW MOVING UP THE
COAST, USED TO THE TYPICAL OVERLAP OF THE 12Z ECMWF/GEFS MEMBERS
(FAST SIDE OF THE ECMWF AND SLOW SIDE OF THE GEFS) AND TRIED TO
GUIDE THE LOW PARALLEL TO HEIGHT LINES DUE TO ITS POTENTIALLY
CONVECTIVE NATURE. THIS KEPT GOOD CONTINUITY EARLY ON AND ENDED
UP FAIRLY CLOSE TO THE 00Z ECMWF SOLUTION. HOWEVER, THIS LED TO
FASTER PROGRESSION MID PERIOD ONWARD WHICH LED TO CHANGES.
..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. ON DAY 4/SATURDAY INTO EARLY
SUNDAY, THE EXCESSIVE RAINFALL RISK AREAS MAINTAINED REASONABLE
CONTINUITY. THE NEW DAY 5/SUNDAY INTO EARLY MONDAY OUTLOOK, A
SLIGHT RISK WAS PLACED WHERE THE BEST OVERLAP IN HEAVY RAINFALL
WAS SEEN IN THE GUIDANCE, WHICH SHOULD BE NEAR WHERE THE BETTER
MOISTURE TRANSPORT AND INSTABILITY RESIDES.
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. AMOUNTS AND INLAND EXTENT OF THE HEAVIER RAINS
CONTINUE TO BE FAIRLY UNCERTAIN AT THIS POINT, SO JUST A MARGINAL
RISK WAS MAINTAINED FOR THE DAYS 4/SATURDAY INTO EARLY SUNDAY ERO
WITH ONLY MODEST ADJUSTMENTS. ACTIVITY SHOULD BE MORE CONVECTIVE
NEAR THE COAST WITH MORE FRONTOGENETIC/COMMA HEAD RAINS EXTENDING
SOMEWHAT INLAND
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.
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. ELSEWHERE, TEMPERATURES COULD BE MODERATELY
ABOVE NORMAL ACROSS THE UPPER MIDWEST/GREAT LAKES AND PARTS OF THE
SOUTHERN PLAINS FRIDAY INTO SATURDAY. BY EARLY NEXT WEEK THOUGH,
MUCH OF THE COUNTRY SHOULD BE NEAR NORMAL OR WITHIN A FEW DEGREES
OF NORMAL. THE ONE EXCEPTION MAY BE ALONG THE WEST COAST WHERE
INCOMING RAINFALL MAY KEEP DAYTIME HIGHS AT LEAST 5-10 DEGREES
BELOW NORMAL.
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
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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