073
FXUS02 KWBC 190624
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
224 AM EDT TUE SEP 19 2023
VALID 12Z FRI SEP 22 2023 - 12Z TUE SEP 26 2023
...HEAVY RAINFALL POTENTIAL ACROSS PARTS OF THE PLAINS/UPPER
MIDWEST AS WELL AS THE EAST COAST LATE THIS WEEK AND WEEKEND...
..OVERVIEW
A DEEP UPPER LEVEL CLOSED LOW OVER THE WEST ON FRIDAY SHOULD
EVENTUALLY SHIFT EAST AS THE LEADING EDGE OF AMPLIFYING/BROADENING
NORTHEAST PACIFIC TROUGHING EXTENDS INTO THE PACIFIC NORTHWEST AND
WESTERN CANADA BY THE WEEKEND. MOST GUIDANCE SUGGESTS THE UPPER
LOW WILL SLOW DOWN FOR A TIME ACROSS THE NORTH-CENTRAL U.S. AS
UPPER RIDGING BUILDS INTO THE WESTERN ATLANTIC AND PARTS OF
SOUTHERN CANADA, CREATING A BLOCKY PATTERN ACROSS THE CONUS DURING
THE SECOND HALF OF THE PERIOD. ELSEWHERE, A SURFACE LOW FORMING
OFF THE FLORIDA COAST LATE THIS WEEK WILL BRING POTENTIAL FOR
HEAVY COASTAL RAINS TO PARTS OF THE SOUTHEAST AND MID-ATLANTIC AS
IT MOVES SLOWLY UP THE COAST. FROM THE WEEKEND ONWARD THERE IS
INCREASING UNCERTAINTY IN THE TRACK OF THIS SURFACE SYSTEM AND THE
COVERAGE OF ASSOCIATED RAINFALL, CORRESPONDING TO ONGOING
DIFFERENCES THAT ARISE FOR UPPER PATTERN DETAILS OVER SOUTHERN
CANADA AND THE NORTHEASTERN U.S.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THERE REMAINS UNCERTAINTY IN THE EVOLUTION OF THE CLOSED LOW OVER
THE WEST AS IT MOVES INTO THE NORTH-CENTRAL U.S. THIS WEEKEND,
WITH PLENTY OF DETAIL VARIABILITY WITH BIGGER IMPLICATIONS FOR
RAINFALL AMOUNTS/QPF. ISSUES ARISE DUE TO AN INITIAL SOUTHERN
CANADA TROUGH/TRAILING RIDGE, WHERE SOME OF THE GUIDANCE YESTERDAY
SUGGESTED A SECONDARY CLOSED LOW TO FORM OUT OF THIS. THE 18Z/SEP
18 GFS WAS THE GREATEST OUTLIER SUGGESTING THE TWO LOWS ATTEMPT TO
PHASE, BUT THE LATEST 00Z RUN THIS MORNING (AVAILABLE AFTER
FORECAST GENERATION TIME) PULLED BACK ON THIS. THERE IS A LITTLE
BIT BETTER CONSENSUS NOW THAT THE WESTERN LOW SHOULD REMAIN MOSTLY
IN TACT AS IT MOVES EAST, WEAKENING AND POSSIBLY OPENING INTO MORE
OF A WAVE LATER IN THE PERIOD. STILL, THERE REMAINS SOME SPREAD IN
EXACT PLACEMENT OF THIS LOW AS IT MEANDERS ACROSS THE UPPER
MIDWEST REGION SUNDAY-TUESDAY AND ANY POSSIBLE INTERACTION WITH
THE EAST COAST LOW.
THIS SOUTHERN CANADA FLOW UNCERTAINTY ALSO CONTRIBUTES TO DRAMATIC
DIVERGENCE OF SOME GUIDANCE FOR THE SURFACE LOW ALONG THE EAST
COAST AND WHETHER IT EVENTUALLY GETS PULLED INLAND OR STAYS NEAR
THE COAST. LATEST GFS RUNS REMAIN ON THE FASTER SIDE, THOUGH THE
NEW 00Z RUN DOESN'T PULL IT AS FAR WEST AS PREVIOUS RUNS. CMC RUNS
ARE ALSO FAST/FARTHER INLAND, WHILE THE ECMWF AND UKMET (WITH
SUPPORT FROM THE ENSEMBLE MEANS) SUGGEST A MUCH SLOWER PROGRESSION
NORTH ALONG THE COAST. CURRENT PREFERENCE REMAINS IN LINE WITH THE
ECMWF AND ENSEMBLE MEANS WHICH MAINTAINS CONTINUITY FOR NOW
AWAITING ANY DRAMATIC SHIFTS IN GUIDANCE CLUSTERING. THE NATIONAL
HURRICANE CENTER IS MONITORING THIS SYSTEM FOR POTENTIAL
SUB-TROPICAL DEVELOPMENT, BUT REGARDLESS OF CLASSIFICATION, THIS
SURFACE LOW HAS THE POTENTIAL TO BRING SIGNIFICANT RAINFALL TO
PARTS OF THE SOUTHEAST AND MID-ATLANTIC COAST THIS COMING WEEKEND.
OVER THE PACIFIC NORTHWEST THE GUIDANCE SPREAD FOR THE LEADING
EDGE OF NORTHEAST PACIFIC UPPER TROUGHING APPEARS FAIRLY TYPICAL,
WITH SHORTWAVE DIFFERENCES HAVING LOW PREDICTABILITY FOR THE TIME
FRAME IN QUESTION. HOWEVER FAIRLY MINOR DIFFERENCES DO HAVE A MORE
SIGNIFICANT INFLUENCE ON EXACTLY HOW MUCH MOISTURE REACHES THE
REGION LATE IN THE PERIOD.
THE WPC BLEND FOR TONIGHT LEANED TOWARDS THE BETTER CLUSTERED
ECMWF/CMC/UKMET THE FIRST HALF OF THE PERIOD WITH THE WESTERN
UPPER LOW, TRENDING QUICKLY TOWARDS THE ENSEMBLE MEANS AND ECMWF
THE SECOND HALF OF THE PERIOD AS SPREAD INCREASES ACROSS BOTH THE
NORTH-CENTRAL U.S. AND THE EAST COAST. OVERALL, THIS MAINTAINED
GOOD CONTINUITY WITH THE PREVIOUS WPC FORECAST.
..WEATHER/HAZARDS HIGHLIGHTS
ANOMALOUS MOISTURE ASSOCIATED WITH THE UPPER LOW IN THE WEST LATE
THIS WEEK WILL ALLOW FOR THE POTENTIAL FOR HEAVY RAINFALL FROM THE
NORTHERN ROCKIES INTO THE HIGH PLAINS. THERE COULD ALSO BE SOME
SNOW IN THE HIGHEST ELEVATIONS DEPENDING ON THE DEPTH OF THE UPPER
LOW. AS THE LOW MOVES EAST INTO THE PLAINS, RAINFALL SHOULD BEGIN
TO 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. FOR THE DAY 4/FRIDAY EXCESSIVE RAINFALL OUTLOOK, A
SLIGHT RISK REMAINS ACROSS PARTS OF THE HIGH PLAINS FROM SOUTHEAST
MONTANA/NORTHEAST WYOMING INTO WESTERN SOUTH DAKOTA, WITH A BROAD
MARGINAL RISK SURROUNDING THIS FROM THE NORTHERN ROCKIES INTO THE
NORTH-CENTRAL PLAINS. ON DAY 5, INTRODUCED A SLIGHT RISK FROM
SOUTHERN MINNESOTA TO NORTHERN MISSOURI WHERE QPF CONSENSUS WAS
GREATEST FOR POTENTIALLY HAZARDOUS/EXCESSIVE RAINS, WITH A BROAD
MARGINAL SURROUNDING AND BACK INTO THE NORTHERN PLAINS.
THE SURFACE LOW DEVELOPING OFF THE NORTHEAST FLORIDA COAST BY
FRIDAY SHOULD TAP TROPICAL MOISTURE, WITH INCREASING POTENTIAL FOR
HEAVY RAINFALL ALONG AT LEAST THE ATLANTIC COAST FROM FLORIDA TO
SOUTHERN NEW ENGLAND. AMOUNTS AND INLAND EXTENT OF THE HEAVIER
RAINS CONTINUE TO BE EXTREMELY UNCERTAIN AT THIS POINT, SO JUST A
MARGINAL RISK WAS DRAWN FOR THE DAYS 4 AND 5 EROS. EXTENSION OF
THIS FARTHER INLAND AND AN UPGRADE TO A SLIGHT WITH TIME ARE
POSSIBLE, BUT HIGHLY DEPENDENT ON HOW CLOSE TO THE COAST THE LOW
EVENTUALLY TRACKS AND HOW FAR INLAND THE QPF EXTENDS (WHICH SOME
OF THE GUIDANCE IS SUGGESTING).
THROUGH THE REST OF THE WEEKEND INTO EARLY NEXT WEEK, RAINFALL
OVER THE CENTRAL U.S. SHOULD CONTINUE GRADUALLY EASTWARD ALONG
WITH THE UPPER LOW AND SURFACE SYSTEM WITH SOME ACTIVITY LOCALLY
HEAVY. THE PACIFIC NORTHWEST SHOULD SEE GRADUALLY MORE RAINFALL
WITH TIME, BUT TO WHAT EXTENT IS STILL A QUESTION MARK THAT WILL
TAKE ADDITIONAL TIME TO RESOLVE. THE AXIS OF MOISTURE MAY BE
FAIRLY NARROW, WITH MINOR CHANGES IN POSITION BEING THE DIFFERENCE
BETWEEN MINIMAL TOTALS OR SIGNIFICANTLY MORE.
TEMPERATURES AS MUCH AS 10-20 DEGREES BELOW NORMAL ASSOCIATED WITH
THE DEEP WESTERN UPPER LOW FROM CALIFORNIA TO MONTANA 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.
SANTORELLI/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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