146
FXUS02 KWBC 201859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
158 PM EST MON NOV 20 2023
VALID 12Z THU NOV 23 2023 - 12Z MON NOV 27 2023
19Z UPDATE: OVERALL FORECAST CONFIDENCE HAS BEEN BELOW AVERAGE
WITH THE POTENTIAL LOW PRESSURE SYSTEM NEAR THE SOUTHEAST U.S. TO
CLOSE OUT THE WEEK. THERE HAS BEEN POOR MODEL RUN-TO-RUN
CONTINUITY OVER THE PAST FEW CYCLES WITH THE INTENSITY AND DEGREE
OF NORTHWESTERN EXTENT, AND THUS THE RESULTING RAINFALL. COMPARED
TO LAST NIGHT, THE LATEST GUIDANCE SUGGESTS A MORE SUPPRESSED AND
WEAKER LOW, WITH QPF NOW CONFINED CLOSER TO THE COAST. THE
INCOMING SOUTHERN STREAM UPPER TROUGH FROM NORTHERN MEXICO HAS
ALSO TRENDED WEAKER AND THEREFORE THE SURFACE EVOLUTION AS WELL.
THE 00Z CMC WAS STILL A STRONG OUTLIER AT THE TIME OF
FRONTS/PRESSURES COMPILATION, BUT ITS 12Z RUN HAS ALSO TRENDED
WEAKER OVERALL. THE 00Z CMC WAS ALSO OUT OF ALIGNMENT WITH THE
ORIENTATION OF THE UPPER TROUGH OVER CENTRAL CANADA AND THE GREAT
LAKES BY NEXT WEEKEND, AND THEREFORE THE FRONTS/PRESSURES FORECAST
USED A NON-00Z CMC BLEND (THE 12Z CMC HANDLES THIS FEATURE
BETTER). ELSEWHERE, THERE IS A SLIGHTLY SLOWER TREND WITH THE
DEPARTING LOW OFF THE NEW ENGLAND COAST ON THURSDAY COMPARED TO
THE PREVIOUS FORECAST, AND THE ARRIVAL OF THE NEXT COLD FRONT
ACROSS THE DAKOTAS HAS TRENDED SLIGHTLY FASTER. INCORPORATION OF
THE ENSEMBLE MEANS WAS GRADUALLY INCREASED EACH TODAY TO ABOUT 50%
BY NEXT MONDAY. NO EXCESSIVE RAINFALL AREAS ARE WARRANTED IN THE
UPDATED OUTLOOK. THE PREVIOUS FORECAST DISCUSSION IS APPENDED
BELOW FOR REFERENCE. /HAMRICK
------------------------
..PATTERN OVERVIEW
MOST GUIDANCE OFFERS FAIRLY SIMILAR IDEAS FOR THE LARGE SCALE
PATTERN, WITH A MEAN RIDGE OVER THE EASTERN PACIFIC INTO
NORTHWESTERN NORTH AMERICA WHILE A DEEP UPPER LOW OVER OR NEAR
HUDSON BAY ANCHORS A BROAD MEAN TROUGH COVERING MUCH OF CANADA
INTO AT LEAST THE NORTHERN PART OF THE LOWER 48. POSITIVELY
TILTED MEAN TROUGHING SHOULD ALSO EXTEND OVER THE WESTERN U.S., AS
FAVORED BY TELECONNECTIONS WITH THE UPSTREAM RIDGE. THIS REGIME
WILL LIKELY BRING BELOW NORMAL HIGH TEMPERATURES ACROSS A MAJORITY
OF THE COUNTRY (AND COLDEST ANOMALIES TO THE CENTRAL U.S.) WITH AN
AREA OF SNOW DROPPING SOUTHWARD OVER AND NEAR THE NORTHERN-CENTRAL
ROCKIES AND HIGH PLAINS. HOWEVER THE FORECAST IS MUCH MORE
UNCERTAIN REGARDING PRECIPITATION COVERAGE AND AMOUNTS ACROSS THE
SOUTH AND EAST FROM THANKSGIVING INTO THE WEEKEND, CORRESPONDING
TO SHORTWAVE DIFFERENCES THAT LEAD TO CONSIDERABLE DIFFERENCES FOR
SURFACE EVOLUTION.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
LEADING INTO THE START OF THE FORECAST PERIOD EARLY
THANKSGIVING/THURSDAY, THE EXACT CHARACTER OF THE SYSTEM FORECAST
TO BE OFF THE NEW ENGLAND COAST CONTINUES TO BE IN DOUBT. A
NUMBER OF UKMET/GFS RUNS HAVE BEEN DEPICTING A DIFFERENT EVOLUTION
OF SHORTWAVE ENERGY IN THE SHORT TERM VERSUS OTHER GUIDANCE,
LEADING TO A SOMEWHAT SLOWER AND/OR STRONGER SURFACE LOW PRESSURE.
THE 18Z GFS STILL HAD A VARIATION OF THIS LOWER CONFIDENCE DETAIL
ALOFT BUT WAS CLOSER TO THE MAJORITY CLUSTER THAN MOST OTHER
RECENT GFS RUNS. HOWEVER THE NEW 00Z ECMWF HAS SWITCHED TO
SOLUTION CLOSER TO THE 00Z GFS, RAISING THE POSSIBILITY OF THIS
ALTERNATE SCENARIO.
BEHIND THIS SYSTEM, DIFFERENCES IN HOW AN UPPER LOW EJECTS FROM
THE RIO GRANDE VALLEY PLUS VARYING INFLUENCE FROM NORTHERN TIER
FLOW AND ENERGY EMERGING FROM THE WESTERN U.S. LEAD TO A WIDE
VARIETY OF POSSIBILITIES FOR POTENTIAL LOW PRESSURE BETWEEN THE
GULF OF MEXICO AND EAST COAST/WESTERN ATLANTIC. RECENT CMC RUNS
HAVE BEEN THE MOST PRONOUNCED BUT ALSO CONSISTENT WITH THIS WAVE
AND ITS FARTHER NORTHWESTWARD EXTENT OF MOISTURE WHILE ECMWF RUNS
HAVE BEEN TRENDING STRONGER WITH AN EAST COAST TRACK BY SATURDAY.
THE GFS HAS TAKEN LONGER TO START REFLECTING THIS WAVE AND THE 18Z
RUN HAD ENDED UP JUST A LITTLE SLOWER THAN THE 12Z ECMWF. HOWEVER
THE NEW 00Z GFS/ECMWF HAVE LOWERED HEIGHTS OVER NEW ENGLAND,
SIGNIFICANTLY SUPPRESSING THE OVERALL SURFACE PATTERN. REFLECTING
THE BROAD SPREAD AMONG INDIVIDUAL MEMBERS, THE ENSEMBLE MEANS LEAN
WEAK AND OFFSHORE WITH WHAT SURFACE REFLECTION THERE MAY BE BY THE
WEEKEND. TRENDS OBSERVED THROUGH THE 12Z/18Z CYCLES SEEMED TO
FAVOR SOMEWHAT BETTER DEFINITION THAN THE MEANS BUT WITH WEAKER
LOW PRESSURE THAN THE GFS/ECMWF/CMC WERE SHOWING. HOWEVER THE 00Z
GFS/ECMWF ADJUSTMENTS SIGNIFICANTLY LOWER WHAT LOW-MODERATE
CONFIDENCE THERE HAD BEEN IN A CERTAIN EVOLUTION.
MEANWHILE THE GUIDANCE IS GRADUALLY COMING INTO BETTER AGREEMENT
FOR THE SHAPE OF WESTERN U.S. TROUGHING INTO LATE WEEK, WITH
INCREASING SUGGESTION OF AN UPPER LOW BRIEFLY CLOSING OFF BY
AROUND EARLY FRIDAY AND THEN OPENING UP AS SOME OF THE TROUGH
ENERGY CONTINUES EASTWARD. TOWARD THE END OF THE PERIOD THE
12Z/18Z GFS SHOWED WEAKER TROUGHING LEFT OVER THE WEST VERSUS MOST
OTHER MODELS AND ENSEMBLE MEANS, WHILE THE 12Z CMC WAS A DEEP
EXTREME WITH ITS UPPER LOW. 00Z RUNS OF BOTH THE GFS AND CMC
FAVORABLY TRENDED CLOSER TO THE MEANS (AND TELECONNECTION-FAVORED
PATTERN RELATIVE TO THE EAST PACIFIC RIDGE) BY THE START OF NEXT
WEEK.
CYCLONIC FLOW EXTENDING INTO THE NORTHERN TIER OF THE LOWER 48
WILL LIKELY BRING AT LEAST ONE COLD FRONT INTO THE REGION BY
SATURDAY-MONDAY. SPECIFICS WILL DEPEND ON LOWER PREDICTABILITY
SHORTWAVE DETAILS, SO THE TIMING AND SOUTHWARD EXTENT OF THE
FRONT(S) MAY VARY NOTICEABLY FROM RUN TO RUN.
PREFERENCES TOWARD A MODERATE/INTERMEDIATE SOLUTION WHERE
DIFFERENCES EXISTED LED USING 70 PERCENT TOTAL WEIGHT OF THE 18Z
GFS/12Z ECMWF AND THE REST 18Z GEFS/12Z ECMWF MEANS FOR THE FIRST
HALF OF THE PERIOD, FOLLOWED BY A RAPID TRANSITION TOWARD 50-60
PERCENT ENSEMBLE WEIGHT BY DAYS 6-7 SUNDAY-MONDAY.
..WEATHER/HAZARDS HIGHLIGHTS
THE RELATIVELY MORE CONFIDENT PART OF THE FORECAST IS FOR AN AREA
OF SNOW TO DROP SOUTHWARD OVER THE NORTHERN-CENTRAL ROCKIES AND
HIGH PLAINS LATE THIS WEEK, WITH SOME LOCALLY ENHANCED TOTALS
POSSIBLE, INCLUDING SOME OF THE LOWER ELEVATIONS IN EASTERN
WYOMING. THEN THE SNOW SHOULD TREND LIGHTER AND MORE SCATTERED AS
ENERGY WITHIN UPPER TROUGH (WITH POSSIBLE EMBEDDED LOW) PROGRESSES
ONWARD. ON THE OTHER HAND, CONFIDENCE IS QUITE LOW FOR THE
COVERAGE AND AMOUNTS OF PRECIPITATION OVER THE SOUTH AND EAST FROM
LATE THIS WEEK ONWARD. POSSIBILITIES RANGE BETWEEN A FAIRLY WET
GULF/EAST COAST SURFACE SYSTEM WHICH COULD ALSO PRODUCE SNOW IN
THE NORTHWESTERN PERIPHERY OF THE MOISTURE SHIELD, AND MUCH
LIGHTER/SCATTERED PRECIPITATION ASIDE FROM WHAT ACTIVITY WOULD
CROSS FLORIDA WITH A MORE SUPPRESSED SURFACE SYSTEM. CONTINUE TO
MONITOR FORECASTS FOR LATEST TRENDS REGARDING THE SOUTHERN/EASTERN
U.S. SURFACE PATTERN AND EXPECTED PRECIPITATION. THERE ARE NO
EXCESSIVE RAINFALL AREAS CURRENTLY DEPICTED ON BOTH THE DAY 4 AND
DAY 5 OUTLOOKS WITH NO STRONG SIGNALS FOR ANY INLAND HEAVY RAIN.
EXPECT MOST OF THE COUNTRY TO SEE NEAR TO BELOW NORMAL HIGH
TEMPERATURES FROM THANKSGIVING INTO THE START OF NEXT WEEK, WITH
HIGHS UP TO 10-20F BELOW CLIMATOLOGY MOST LIKELY OVER THE
CENTRAL-SOUTHERN HIGH PLAINS FRIDAY THROUGH THE WEEKEND AND
BECOMING MORE CONFINED TO THE SOUTHERN PLAINS BY MONDAY. THERE
MAY BE SOME FROST/FREEZE CONDITIONS FOR PARTS OF TEXAS THAT HAVE
NOT EXPERIENCED THIS YET SO FAR THIS SEASON. ALSO THE
CENTRAL-WESTERN GULF COAST REGION MAY SEE HIGHS OF 10-15F BELOW
NORMAL ON THANKSGIVING. ALSO, A COMBINATION OF MORNING LOWS AND
BREEZY CONDITIONS MAY BRING NOTICEABLY COLD WIND CHILLS TO VALLEY
LOCATIONS SUCH AS LAS VEGAS. ONE EXCEPTION TO THE CHILLY THEME
MAY BE LOCATIONS ALONG THE CENTRAL-NORTHERN WEST COAST WHERE
PACIFIC RIDGING ALOFT COULD BE JUST CLOSE ENOUGH TO KEEP HIGHS A
LITTLE ABOVE NORMAL.
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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