121
FXUS02 KWBC 191900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 PM 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
SOUTHERN/EAST-CENTRAL 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. MODELS AND ENSEMBLES
DEVELOP MORE SPREAD THAN USUAL FOR THE OVERALL CENTRAL/EASTERN
NORTH AMERICA PATTERN, SO THERE CONTINUES TO BE A LOT OF
UNCERTAINTY FOR THE ULTIMATE PATH OF THE PLAINS UPPER LOW, AS WELL
AS HOW EASTERN CANADA/NORTHEASTERN U.S. SHORTWAVE ENERGY MAY
EVOLVE AND ITS POSSIBLE INFLUENCE ON THE EAST COAST
SYSTEM/ASSOCIATED RAINFALL.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
GIVEN THE DRAMATIC DIVERGENCE IN SOME ASPECTS OF GUIDANCE, THE
UPDATED FORECAST BASED ON 00Z/06Z RUNS STAYED CLOSEST TO THE 06Z
GEFS/00Z ECENS-CMCENS MEANS IN PRINCIPLE WITH SOME INCORPORATION
OF OPERATIONAL GUIDANCE WHERE FEASIBLE--ESPECIALLY EARLY IN THE
PERIOD WHEN COMPONENTS OF THE 00Z GFS/ECMWF/UKMET MADE UP 60
PERCENT OF THE OVERALL WEIGHT. MEANS WERE TWO-THIRDS OF THE BLEND
BY LATE IN THE PERIOD. THIS PROVIDED THE BEST COMBINATION OF
CONTINUITY IN THE OVERALL FORECAST EVOLUTION WHILE INCORPORATING
SOME MODEST TRENDS.
FOR THE UPPER LOW TRACKING FROM THE WEST INTO THE NORTH-CENTRAL
U.S. THIS WEEKEND, THERE IS A DECENT MAJORITY CLUSTER UP THROUGH
THE POINT OF REACHING THE NORTHERN HIGH PLAINS ON DAY 4 SATURDAY.
THERE IS STILL A SMALL MINORITY (REPRESENTED BY THE 00Z ECMWF, AND
IN EASTWARD/DELAYED FASHION BY THE NEW 12Z CMC) SUGGESTING
POTENTIAL FOR INITIAL SHORTWAVE ENERGY EXTENDING SOUTH/SOUTHWEST
FROM HUDSON BAY TO PULL OFF AN UPPER LOW JUST NORTH OF THE PLAINS
OR GREAT LAKES AND BECOME DOMINANT RELATIVE TO THE PLAINS LOW.
ANOTHER MINORITY SCENARIO, MOST RECENTLY DEPICTED IN THE 06Z GFS
AND NEW 12Z UKMET, WOULD BE FOR THE PLAINS UPPER LOW TO OPEN UP
AND LIFT NORTHWARD AROUND THE WESTERN SIDE OF THE UPPER RIDGE
TRAILING THE AFOREMENTIONED CANADA SHORTWAVE. ADDING TO THE
FORECAST DIFFICULTY, SOME SOLUTIONS ARE SHOWING THE POTENTIAL FOR
THE EASTERN CANADA SHORTWAVE ENERGY TO AMPLIFY INTO THE
NORTHEAST--POSSIBLY WHISKING AWAY THE EAST COAST SYSTEM INTO THE
ATLANTIC (06Z/12Z GFS, 12Z ECMWF) OR EVEN RETROGRADING UNDERNEATH
THE CANADIAN RIDGE AND REACHING A POSITION CLOSE TO THE UPPER
MIDWEST/WESTERN GREAT LAKES LOW POSITION SHOWN BY THE MEANS AND
THE 00Z GFS. THE AVERAGE OF GUIDANCE SHIFTED A LITTLE NORTHWARD
WITH THE UPPER LOW TRACK, SO THE PREFERRED BLEND REFLECTED THIS
ADJUSTMENT ALBEIT WITH FAIRLY LOW CONFIDENCE. MINUS THE UKMET, THE
12Z MODEL AVERAGE IS A LITTLE FARTHER SOUTH AGAIN.
AS FOR OTHER ASPECTS OF THE EAST COAST SYSTEM, THERE IS A SLIGHT
MAJORITY AMONG NEW RUNS LEANING TOWARD SOMEWHAT FASTER NORTHWARD
PROGRESSION THAN 00Z/12Z ECMWF AND 00Z ECENS RUNS AND 06Z GFS,
WITH A COMPROMISE PROVIDING A REASONABLE DETERMINISTIC SOLUTION
FOR NOW. THE 00Z ECMWF TRACKED THE SYSTEM A LITTLE FARTHER INLAND
FROM THE EAST COAST (ADJUSTING BACK TO THE EAST IN THE NEW 12Z
RUN), WHILE THE CMC IS NOW THE ONLY HOLDOUT DEPICTING A SURFACE
LOW TRACK WELL NORTHWARD OF THE MID-ATLANTIC. OTHERWISE THERE IS
FAIRLY OVERWHELMING SUPPORT FOR WHAT HAS BEEN THE ENSEMBLE MEAN
IDEA OF THE SURFACE WAVE STAYING FAIRLY SUPPRESSED, UNDERNEATH
STRONG EASTERN CANADA HIGH PRESSURE. THE NATIONAL HURRICANE CENTER
CONTINUES TO MONITOR 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. BY LATE IN THE PERIOD 00Z/06Z GFS RUNS APPEARED
TO BE ON THE FAST SIDE WITH THE PROGRESSION OF LEADING HEIGHT
FALLS BUT THE NEW 12Z RUN SEEMS TO COMPARE BETTER TO OTHER
GUIDANCE. THERE IS FAIRLY GOOD MODEL AGREEMENT THAT A STRONG
SYSTEM WILL DEVELOP A WAYS OFFSHORE THE PACIFIC NORTHWEST COAST
AROUND SUNDAY-MONDAY. THE GREATER MAJORITY SIDES WITH THE ENSEMBLE
MEANS FOR THE LATITUDE OF THIS STORM, WHILE THE 00Z ECMWF/12Z
UKMET WERE FARTHER SOUTH. SOME DIFFERENCES ALONG THE COAST WOULD
APPEAR MINOR BUT HAVE A MORE SIGNIFICANT INFLUENCE ON EXACTLY HOW
MUCH MOISTURE REACHES THE REGION AT TIMES.
..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. LATEST GUIDANCE SHOWS MORE OF A SPLIT
BETWEEN NORTHERN PLAINS MOISTURE AND ANOTHER BAND WITH A SOUTHERN
CANADA FRONT, FAVORING REMOVAL OF SOME LOCATIONS NEAR THE CANADIAN
BORDER FROM THE MARGINAL RISK AREA. ON DAY 5, THE SLIGHT RISK AREA
FROM SOUTHERN MINNESOTA THROUGH NORTHERN MISSOURI WAS EXTENDED
SOUTHWARD SOME FOR THE AFTERNOON UPDATE, IN ACCORDANCE WITH
GUIDANCE TRENDS FOR CONVECTIVE POTENTIAL. GUIDANCE AGREES ON
FAIRLY HIGH VALUES OF INSTABILITY OVER THIS AREA, ADDING TO THE
POTENTIAL FOR HAZARDOUS/EXCESSIVE RAINS. A SURROUNDING BROAD
MARGINAL RISK NOW EXTENDS FARTHER BACK INTO THE HIGH PLAINS PER
LATEST MODEL TRENDS. SOME LOCATIONS OVER THE NORTHERN PLAINS COULD
EVENTUALLY SEE AN UPGRADE IN RISK LEVEL IF GUIDANCE BECOMES
CLUSTERED BETTER. SOME OF THE CONVECTION OVER THE PLAINS MAY ALSO
BE STRONG TO SEVERE. CHECK THE LATEST OUTLOOKS FROM THE STORM
PREDICTION CENTER FOR MORE INFORMATION ON SEVERE THREATS.
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 MAINTAINED FOR THE DAYS 4 AND 5 EROS WITH ONLY
MODEST ADJUSTMENTS. EXTENSION OF THIS AREA FARTHER INLAND AND AN
UPGRADE TO AN EMBEDDED SLIGHT RISK ARE EVENTUALLY POSSIBLE, BUT
HIGHLY DEPENDENT ON HOW CLOSE TO THE COAST THE LOW ULTIMATELY
TRACKS AND HOW FAR INLAND THE QPF EXTENDS.
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. THE ONE EXCEPTION MAY BE ALONG
THE WEST COAST WHERE INCOMING RAINFALL MAY KEEP DAYTIME HIGHS AT
LEAST 5-10 DEGREES BELOW NORMAL.
RAUSCH/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
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