411
FXUS02 KWBC 090700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 AM EST MON DEC 9 2024
VALID 12Z THU DEC 12 2024 - 12Z MON DEC 16 2024
..OVERVIEW
BEHIND THE DEEP EASTERN NORTH AMERICAN UPPER TROUGH THAT SUPPORTS
DEEP LOW PRESSURE LIFTING THROUGH EASTERN CANADA AS OF EARLY
THURSDAY, MOST GUIDANCE CONTINUES TO SHOW A FAIRLY PROGRESSIVE MEAN
PATTERN ACROSS THE LOWER 48. HOWEVER THE MODELS HAVE BEEN
CHALLENGED IN RESOLVING THE DETAILS OF INDIVIDUAL EMBEDDED
SHORTWAVES AND THEIR REFLECTION AT THE SURFACE ALONG WITH
CORRESPONDING SENSIBLE WEATHER EFFECTS. THE OVERALL PATTERN WILL
FAVOR EPISODES OF RAIN AND HIGHER ELEVATION SNOW OVER PORTIONS OF
THE WEST AND THEN DEVELOPMENT OF PRECIPITATION OVER THE
PLAINS/MISSISSIPPI VALLEY AS SHORTWAVES REACH THE CENTRAL U.S.
ASIDE FROM A BRIEF PERIOD OF COLD TEMPERATURES FROM THE UPPER
MIDWEST THROUGH MOST OF THE EAST LATE THIS WEEK, MUCH OF THE LOWER
48 SHOULD SEE NEAR TO ABOVE TEMPERATURES THROUGH THE PERIOD WITH
WARMEST ANOMALIES CENTERED OVER THE PLAINS FROM THE WEEKEND INTO
EARLY NEXT WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
PREDICTABILITY IN THE ADVERTISED LOW AMPLITUDE MEAN PATTERN (FROM A
MULTI-DAY MEAN PERSPECTIVE) HAS BEEN BELOW AVERAGE OVER RECENT DAYS,
WITH OPERATIONAL RUNS SHOWING VARYING IDEAS FOR INDIVIDUAL
SHORTWAVES AND WHAT SURFACE PATTERN THEY WILL PRODUCE. THERE HAVE
ALSO BEEN DISCREPANCIES BETWEEN SOME GENERAL THEMES OF MACHINE
LEARNING (ML) GUIDANCE AND DYNAMICAL SOLUTIONS. ON THE POSITIVE
SIDE, LATEST GUIDANCE SEEMS TO BE STARTING TO CLUSTER A LITTLE
BETTER FOR INDIVIDUAL FEATURES, AND INCORPORATING A COMBINATION OF
IDEAS FROM EARLIER DYNAMICAL AND ML SOLUTIONS.
WITH CONSENSUS REMAINING GOOD FOR THE UPPER TROUGH LIFTING OUT OF
THE EAST EARLY, THE NEXT FEATURE OF INTEREST IS A SHORTWAVE
REACHING NEAR THE WEST COAST BY EARLY THURSDAY. THERE HAS BEEN
CONSIDERABLE SPREAD FOR HOW ENERGY WILL BE DISTRIBUTED WITHIN THE
SHORTWAVE AS IT CONTINUES EASTWARD, WHILE ML MODELS EARLIER ON HAD
BEEN SHOWING A MORE DEFINED SURFACE SYSTEM REACHING INTO THE
EASTERN HALF OF THE COUNTRY BY THE WEEKEND VERSUS A MAJORITY OF
DYNAMICAL RUNS. LATEST DYNAMICAL MODEL/ENSEMBLE TRENDS SEEM TO BE
TOWARD A SOMEWHAT BETTER DEFINED CENTRAL-EASTERN U.S. SHORTWAVE
WITH MORE PRECIPITATION AND SOME DEGREE OF LOW PRESSURE. MULTI-DAY
ML TRENDS HAVE BEEN A LITTLE SLOWER TO BRING LOW PRESSURE EASTWARD,
ULTIMATELY LEADING TO A COMPROMISE BETWEEN PRIOR ML AND DYNAMICAL
THEMES. TOWARD NEXT SUNDAY-MONDAY THE GUIDANCE IS HINTING AT THE
POTENTIAL FOR SOME SURFACE WAVINESS TO DEVELOP NEAR THE EAST COAST
BUT THIS WILL BE VERY SENSITIVE TO EXACT DETAILS OF SHORTWAVE
ENERGY ALOFT. THIS WILL LIKELY TAKE A WHILE TO RESOLVE, GIVEN HOW
LONG IT HAS TAKEN TO FIGURE OUT THE DETAILS FARTHER BACK OVER THE
EAST-CENTRAL U.S. (WHERE SPREAD STILL EXISTS, WITH ML RUNS LEANING
SOMEWHAT MORE TOWARD GFS/UKMET RUNS VERSUS THE SLOWER ECMWF).
FARTHER WEST, GUIDANCE SHOWS WHAT LOOKS LIKE A CHALLENGING-TO-
FORECAST BUT POTENTIALLY IMPACTFUL EASTERN PACIFIC SYSTEM LATE
THIS WEEK. SUPPORTING DYNAMICS ARISE FROM ZONAL NORTH PACIFIC FLOW
AS OF WEDNESDAY INTO A FAIRLY SHARP TROUGH WITH EMBEDDED UPPER LOW
BY FRIDAY-SATURDAY, WITH GUIDANCE DEVELOPING QUITE A BIT OF SPREAD
FOR STRENGTH AND TRACK/TIMING. FOR FORECASTS VALID 12Z SATURDAY,
THE ENSEMBLE MEANS HAVE BEEN FAIRLY CONSISTENT WITH A TRACK
OFFSHORE WASHINGTON/VANCOUVER ISLAND. SOME RUNS LIKE THE 12Z
GFS/UKMET HAVE BEEN FASTER (AS ARE A COUPLE OF THE 12Z ML MODELS)
WHILE CMC RUNS HAVE BEEN SLOW TO THE EXTENT THAT THEY APPEAR
QUESTIONABLE (AND HAVE A MORE ELONGATED SURFACE EVOLUTION). THE
12Z ECMWF WAS SOMEWHAT SOUTH WHILE THE NEW 00Z RUN HAS ADJUSTED
NORTHWARD SOMEWHAT. STRENGTH IS ANOTHER ISSUE, WITH CONSIDERABLE
SPREAD THERE AS WELL BUT WITH A FAIR SIGNAL FROM OPERATIONAL RUNS
AND ENSEMBLE MEMBERS FOR CENTRAL PRESSURE THAT COULD REACH AS LOW
AS THE 970S OR 980S MB. THE MANUAL FORECAST BASED ON 12Z/18Z DATA
WAS MORE CONSERVATIVE GIVEN THE SPREAD, BUT THE NEW 00Z
GFS/UKMET/ECMWF RUNS WOULD SUPPORT A DEEPER TREND GOING FORWARD.
FOR TRACK, PREFERENCE ALIGNED WITH A COMPROMISE BETWEEN THE 18Z GFS
AND THE ENSEMBLE MEANS AT 12Z SATURDAY.
AFTER EARLY SATURDAY, RECENT ECMWF/CMC RUNS HAVE BEEN STRAYING
SLOWER WITH THE OVERALL UPPER TROUGH AND IN TURN THE TRAILING UPPER
RIDGE VERSUS WHAT THE ML MODELS HAVE BEEN RECOMMENDING (CLOSER TO
AN AVERAGE OF GFS RUNS AND THE GEFS MEAN). THE NEW 00Z ECMWF HAS
MADE A FAVORABLE PARTIAL TREND TOWARD THE ML MODEL AND GFS/GEFS
CLUSTER. THE PROGRESSIVE NATURE OF THE PATTERN SEEMS TO FAVOR
LEANING AWAY FROM THE SLOW SIDE OF THE ENVELOPE. THIS FEATURE
SHOULD REACH THE CENTRAL U.S. BY NEXT MONDAY WHILE FLOW AHEAD OF
ANOTHER PACIFIC SYSTEM SHOULD COME INTO THE PICTURE NEAR THE WEST
COAST AT THAT TIME.
GUIDANCE CONSIDERATIONS LED TO UPDATING THE FORECAST WITH THE
12Z/18Z GFS AND 12Z ECMWF/UKMET EARLY IN THE PERIOD. THEN THE BLEND
TILTED MORE TOWARD GFS/GEFS GUIDANCE MID-LATE PERIOD (MORE 18Z GFS
VERSUS 12Z SATURDAY-SUNDAY) WITH SOME ECMWF MEAN AS A SLIGHT
COMPROMISE. THE ENSEMBLE MEANS COMPRISED 50-60 PERCENT OF THE
FORECAST LATE IN THE PERIOD.
..WEATHER/HAZARDS HIGHLIGHTS
COLD CYCLONIC FLOW BEHIND DEEP EASTERN CANADA LOW PRESSURE WILL
SUPPORT SNOW TO THE LEE OF THE GREAT LAKES CONTINUING INTO
THURSDAY. SNOW SHOULD TAPER OFF THEREAFTER AS THE STORM AND
SUPPORTING UPPER TROUGH RAPIDLY LIFT AWAY.
A SHORTWAVE REACHING THE WEST COAST BY EARLY THURSDAY WILL
CONTINUE EASTWARD WITH TIME. THIS FEATURE SHOULD PRODUCE SOME RAIN
AND HIGHER ELEVATION SNOW BUT WITH GENERALLY LIGHT TO LOCALLY
MODERATE AMOUNTS. THE SHORTWAVE AND ASSOCIATED SURFACE WAVE/FRONTAL
SYSTEM MAY SPREAD PRECIPITATION ACROSS THE EAST-CENTRAL U.S. BY
FRIDAY-SATURDAY AND CONTINUING EAST THEREAFTER BUT WITH
CONSIDERABLE UNCERTAINTY FOR MAGNITUDE AND LOCATION OF HIGHEST
TOTALS. CURRENTLY EXPECT SUFFICIENT PROGRESSION TO KEEP TOTALS
BELOW EXCESSIVE THRESHOLDS THROUGH FRIDAY, SO THE DAY 5 EXCESSIVE
RAINFALL OUTLOOK DEPICTS NO RISK AREA FOR THIS ACTIVITY. THE
NORTHERN PART OF THE PRECIPITATION SHIELD MAY CONTAIN SOME WINTRY
WEATHER AT TIMES.
THE STORM LIKELY NEARING THE PACIFIC NORTHWEST BY SATURDAY WILL
LIKELY PRODUCE HEAVIER PRECIPITATION AND POSSIBLY A PERIOD OF BRISK
TO STRONG WINDS. HOWEVER GUIDANCE HAS SHOWN ENOUGH SPREAD FOR
STRENGTH/TRACK/TIMING TO LOWER CONFIDENCE ENOUGH IN RAINFALL
SPECIFICS NOT TO MERIT A RISK AREA IN THE DAY 5/FRIDAY ERO YET.
SUFFICIENT IMPROVEMENT IN CLUSTERING MAY EVENTUALLY LEAD TO
INTRODUCTION OF A RISK AREA DEPENDING ON SYSTEM DETAILS. AFTER
EARLY SATURDAY EXPECT THIS SYSTEM'S MOISTURE TO CONTINUE ACROSS THE
WEST, WITH THE SHORTWAVE/SURFACE SYSTEM LIKELY PRODUCING ANOTHER
AREA OF PRECIPITATION OVER THE EAST-CENTRAL STATES BY AROUND THE
START OF NEXT WEEK.
WITHIN THE COLD PATTERN OVER THE EAST LATE THIS WEEK, THE MOST
EXTREME ANOMALIES WILL EXTEND FROM THE UPPER MIDWEST INTO THE
CENTRAL APPALACHIANS ON THURSDAY WITH READINGS 15-25F BELOW NORMAL.
AREAS CLOSER TO THE EAST COAST WILL GENERALLY SEE TEMPERATURES
5-15F BELOW NORMAL THURSDAY-FRIDAY. THEN THE EAST WILL TREND CLOSER
TO NORMAL WHILE ABOVE NORMAL TEMPERATURES EXPAND FROM THE INTERIOR
WEST AND ROCKIES INTO THE PLAINS/MISSISSIPPI VALLEY. SOME AREAS
OVER THE CENTRAL U.S. MAY REACH GREATER THAN 10F ABOVE NORMAL FROM
THE WEEKEND INTO MONDAY.
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 (QPF), EXCESSIVE RAINFALL
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:
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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