259
FXUS02 KWBC 180700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 AM EST SAT NOV 18 2023
VALID 12Z TUE NOV 21 2023 - 12Z SAT NOV 25 2023
..PATTERN OVERVIEW
CONFIDENCE IN VARIOUS ASPECTS OF THE FORECAST HAS DECREASED
DRAMATICALLY OVER THE PAST DAY AS GUIDANCE HAS DIVERGED FOR
IMPORTANT DETAILS FROM EARLY IN THE FORECAST PERIOD THAT STARTS
EARLY TUESDAY. THIS AFFECTS SPECIFICS OF A SYSTEM EXPECTED TO
TRACK NORTHEASTWARD FROM THE MISSISSIPPI VALLEY FROM TUESDAY
ONWARD, SPREADING A BROAD AREA OF PRECIPITATION OVER THE EAST JUST
AHEAD OF THE THANKSGIVING HOLIDAY, AS WELL AS POTENTIAL TRAILING
WAVINESS DUE TO ANY CENTRAL U.S. INTO NORTHWESTERN MEXICO TROUGH
ENERGY THAT MAY HAPPEN TO CONSOLIDATE AND THEN EJECT
EASTWARD/NORTHEASTWARD. FARTHER WEST, THERE IS LOOSE AGREEMENT
AND CONTINUITY WITH A SHORTWAVE TROUGH REACHING THE WESTERN U.S.
BY LATE WEEK BUT WITH DETAIL DIFFERENCES THAT AFFECT PRECIPITATION
DISTRIBUTION AND AMOUNTS FROM THE WEST INTO THE PLAINS. BY THE
END OF THE PERIOD NEXT SATURDAY, THE AVERAGE OF GUIDANCE SUGGESTS
THAT THE COMBINATION OF THE WESTERN SHORTWAVE AND SOUTHERN
CANADA/NORTHERN TIER U.S. FLOW SHOULD YIELD A MEAN TROUGH WITH AN
AXIS EXTENDING SOUTHWESTWARD FROM THE NORTHERN PLAINS/UPPER
MIDWEST. ASIDE FROM THE SOUTHERN TIER STAYING COOL, SYSTEM
PROGRESSION WILL PROMOTE VARIABLE TEMPERATURES FROM TUESDAY INTO
THANKSGIVING. THEN THE PATTERN SHOULD FAVOR COLDER TEMPERATURES
SPREADING DOWN THROUGH THE CENTRAL U.S.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
IN THE PAST 24 HOURS THE MOST COMMON CHANGE IN THE GUIDANCE HAS
BEEN TOWARD LESS PHASING BETWEEN NORTHERN STREAM FLOW AND THE
TROUGH EXTENDING FROM THE MISSISSIPPI VALLEY SOUTHWESTWARD AS OF
EARLY TUESDAY. THIS HAS SHIFTED THE POTENTIAL RESULTING UPPER LOW
FROM THE UPPER GREAT LAKES (OR EVEN UPPER MIDWEST PER THE 00Z/17
GFS) WELL NORTHWARD, AT LEAST INTO NORTHERN ONTARIO, WHILE
RESULTING IN WEAKER LOW PRESSURE OVER THE EASTERN U.S. IN A
MAJORITY OF SOLUTIONS. LATEST RUNS HAVE OFFERED A HOST OF
POSSIBILITIES FOR WHERE THIS WAVE COULD GO, FROM EASTWARD OFF THE
MID-ATLANTIC COAST (12Z/18Z GFS) TO A NORTHEASTWARD TRACK THROUGH
NEW ENGLAND. TO ADD SOME COMPLICATIONS BEHIND THIS SYSTEM, THE
NEW 00Z GFS IS ON THE EXTREME SIDE OF GUIDANCE WITH HOW MUCH
NORTHERN STREAM ENERGY IT DIGS INTO THE CENTRAL U.S., CREATING A
SECOND STRENGTHENING SURFACE SYSTEM, WHILE OTHER RUNS HAVE TRIED
CONSOLIDATING ENERGY IN THE NORTHWESTERN MEXICO PART OF THE
INITIAL UPPER TROUGH AND THEN EJECTING IT, LEADING TO YET ANOTHER
POSSIBLE SOURCE OF SURFACE WAVE/QPF FORCING. MEANWHILE THE
DETAILS CONTINUE TO VARY FOR THE UPPER TROUGH MOVING INTO THE
WEST, WITH MINIMAL CONFIDENCE IN ANY SPECIFIC SOLUTION BY LATE IN
THE PERIOD. THIS LACK OF CONFIDENCE APPLIES TO THE SURFACE
PATTERN FARTHER EAST, WITH CONSECUTIVE OPERATIONAL RUNS VARYING
CONSIDERABLY BY NEXT SATURDAY.
THE PRONOUNCED MODEL SPREAD AND VARIABILITY SEEN THROUGH THE
PERIOD FAVORED A CONSERVATIVE FORECAST APPROACH CLOSEST TO
CONTINUITY WHILE ASSESSING HOW GUIDANCE MAY TRY TO GRAVITATE
TOWARD ANY OF THE ALTERNATE SOLUTIONS CURRENTLY OFFERED. THIS LED
TO STARTING THE UPDATED FORECAST WITH A BLEND OF THE PAST TWO
ECMWF RUNS THROUGH THE 12Z/17 RUN AND 18Z GEFS MEAN PLUS 12Z
ECENS/CMCENS MEANS.
..WEATHER/HAZARDS HIGHLIGHTS
IN SPITE OF UNCERTAINTY WITH SPECIFICS, CONTINUE TO EXPECT A BROAD
AREA OF PRECIPITATION TO AFFECT THE EASTERN U.S. TUESDAY INTO
WEDNESDAY WITH SOME POSSIBLE ADVERSE EFFECTS ON HOLIDAY TRAVEL.
DURING THE PERIOD COVERED BY THE DAYS 4-5 EXCESSIVE RAINFALL
OUTLOOKS (TUESDAY THROUGH WEDNESDAY NIGHT), THE BEST POTENTIAL FOR
HEAVIEST RAINFALL SHOULD EXTEND FROM NEAR THE CENTRAL GULF COAST
NORTHEASTWARD THROUGH THE MID-ATLANTIC AND PARTS OF
SOUTHERN/EASTERN NEW ENGLAND. THE NEW DAY 4 OUTLOOK MAINTAINS THE
PRIOR MARGINAL RISK, WITH A MODEST SOUTHWESTWARD EXTENSION INTO
SOUTHERN MISSISSIPPI BASED ON LATEST GUIDANCE SUGGESTING INCREASED
POTENTIAL FOR AN AXIS OF HEAVY RAIN TO EXTEND INTO THAT AREA.
INITIALLY DRY SOIL CONDITIONS SHOULD TEMPER RUNOFF ISSUES SOMEWHAT
WITHIN MOST OF THE MARGINAL RISK AREA, BUT STILL EXPECT SOME
LOCATIONS WHERE DRAINAGE IS IMPAIRED (DUE TO
URBANIZATION/LEAVES/ETC) TO HAVE SOME SENSITIVITY TO PERIODS OF
HEAVIER RAIN. THE DAY 5 OUTLOOK INTRODUCES A MARGINAL RISK AREA
OVER EASTERN NEW ENGLAND, WITH THE MOST LIKELY FORECAST SCENARIO
HAVING A FRONTAL WAVE PROMOTING A PERIOD OF STRONG ONSHORE FLOW
AHEAD OF IT TO ENHANCE RAINFALL TOTALS. AS WITH AREAS TO THE
SOUTH THE PRIOR DAY, RELATIVELY DRY GROUND CONDITIONS MAY HELP TO
MITIGATE RUNOFF ISSUES TO SOME DEGREE BUT LOCALIZED SENSITIVITY
MAY STILL EXIST.
ELSEWHERE, SNOW WILL BE POSSIBLE OVER PARTS OF INTERIOR NEW
ENGLAND AT THE START OF THE TUESDAY-WEDNESDAY EVENT. A PERIOD OF
LAKE EFFECT SNOW WILL BE POSSIBLE AFTER SYSTEM PASSAGE, WITH
DURATION AND TOTALS DEPENDENT ON HOW QUICKLY THE SYSTEMS LIFTS
AWAY. WINDS SHOULD BE BRISK BEHIND THE SYSTEM BUT NOT AS STRONG
AS THEY WOULD HAVE BEEN BASED ON SOME EARLIER FORECASTS OF A
STRONGER GREAT LAKES INTO CANADA STORM. WITH VERY UNCERTAIN
DETAILS, ONE OR MORE WAVES COULD PRODUCE ADDITIONAL AREAS OF
RAINFALL ACROSS THE SOUTHERN/EASTERN U.S. AFTER MIDWEEK. OVER THE
WEST, A SYSTEM MOVING INTO THE REGION LATE IN THE WEEK MAY BRING
MOSTLY LIGHT TOTALS ACROSS NORTHERN AREAS, POSSIBLY EXTENDING INTO
THE PLAINS BY FRIDAY. THERE IS SOME POTENTIAL FOR LOCALLY HEAVIER
SNOWFALL IN PARTS OF THE NORTHERN ROCKIES AROUND THANKSGIVING, BUT
STILL WITH A LOT OF UNCERTAINTY IN THE DETAILS.
THE MOST PROMINENT TEMPERATURE ANOMALIES DURING THE PERIOD WILL BE
WITH COLDER AIR DROPPING SOUTH THROUGH THE PLAINS/EASTERN SLOPES
OF THE ROCKIES AROUND NEXT FRIDAY-SATURDAY, WITH AN EXPANDING AREA
OF HIGHS 10-15F BELOW NORMAL. OTHERWISE, MODERATELY BELOW NORMAL
READINGS WILL PROGRESS EASTWARD THROUGH THE SOUTHERN TIER AND
EASTERN U.S. FROM TUESDAY INTO THANKSGIVING, WHILE AN INITIAL
UPPER RIDGE OVER THE WEST WILL SUPPORT ABOVE NORMAL TEMPERATURES
INTO MIDWEEK BEFORE A GRADUAL COOLING TREND.
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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