024
FXUS02 KWBC 240700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 AM EST SUN DEC 24 2023
VALID 12Z WED DEC 27 2023 - 12Z SUN DEC 31 2023
...SOME AREAS OF ENHANCED RAINFALL POSSIBLY REACHING THE NORTHERN
MID-ATLANTIC/NORTHEAST BY WEDNESDAY...
...TWO EPISODES OF LOCALLY HEAVY PRECIPITATION POSSIBLE FOR THE
CENTRAL WEST COAST MID-LATE WEEK...
..OVERVIEW
MODELS AND ENSEMBLES CONTINUE TO SHOW THE LARGE SCALE PATTERN
SETTLING INTO AN AMPLIFIED AND BLOCKY REGIME DOMINATED BY MEAN
RIDGING OVER THE WESTERN U.S. INTO WESTERN/WEST-CENTRAL CANADA,
WHICH WILL WEAKEN INCOMING PACIFIC SHORTWAVES, AND TROUGHING OVER
THE EASTERN HALF OF NORTH AMERICA. AN INITIAL PLAINS/MIDWEST UPPER
LOW WILL INITIALLY ESTABLISH THE EASTERN UPPER TROUGH BUT THEN THE
DETAILS OF THE OVERALL TROUGH AND ASSOCIATED SURFACE REFLECTION
BECOME VERY UNCERTAIN BY LATE WEEK THROUGH THE WEEKEND. A LEADING
WAVE NEAR THE EAST COAST SHOULD SPREAD RAINFALL NORTHWARD INTO THE
NORTHEAST DURING MID-LATE WEEK WHILE THE UPPER LOW MAY PRODUCE
LINGERING LIGHT SNOW OVER THE NORTH-CENTRAL PLAINS AND THEN A MIX
OF PRECIPITATION TYPES TO THE EAST. OVER THE WEST, MOST
PRECIPITATION SHOULD FOCUS NEAR THE WEST COAST WITH A COUPLE
FRONTAL SYSTEMS TENDING TO PRODUCE THE HIGHEST TOTALS OVER/NEAR
NORTHERN CALIFORNIA.
..GUIDANCE/PREDICTABILITY ASSESSMENT
IN SOME RESPECTS THE FORECAST REMAINS IN A HOLDING PATTERN AS
MULTI-DAY MEANS/TELECONNECTIONS RELATIVE TO STRONG POSITIVE HEIGHT
ANOMALIES/RIDGING OVER CANADA SUPPORT THE EXPECTED EASTERN
TROUGH/WESTERN RIDGE ALONG WITH BROAD TROUGHING OVER THE EASTERN
PACIFIC, BUT A LOT OF DETAIL SPREAD PERSISTS.
GUIDANCE IS STILL HAVING A DIFFICULT TIME RESOLVING HOW NORTHERN
STREAM FLOW WILL EVOLVE AND POSSIBLY INTERACT WITH THE INITIAL
UPPER LOW OVER THE MIDWEST AFTER WEDNESDAY. RECENT ECMWF/ECMWF
MEAN RUNS HAVE GENERALLY BEEN ON THE FAST SIDE TO EJECT THE
INITIAL UPPER LOW AND THEN LEANED ON THE NORTHERN SIDE OF THE
SPREAD WITH AN UPPER LOW THAT DIGGING NORTHERN STREAM FLOW MAY
PRODUCE. THE NEW 00Z ECMWF STILL EXHIBITS SOME OF THESE
CHARACTERISTICS BUT HAS TRENDED SLIGHTLY TOWARD OTHER SOLUTIONS.
MEANWHILE THE GEFS MEAN HAS BEEN FAIRLY CONSISTENT WITH A FARTHER
SOUTH TRACK OF THE UPPER LOW WITH THE NEW 00Z GFS ADJUSTING
SOUTHWARD CLOSE TO THE GEFS PATH. A MINORITY OF SOLUTIONS
INCLUDING A SUBSET OF THE 12Z ECMWF-INITIALIZED MACHINE LEARNING
MODELS AND 00Z UKMET/ICON WOULD OFFER POTENTIAL FOR AN EVEN
FARTHER SOUTH TRACK. IN DIFFERENT WAYS THE 00Z UKMET/CMC
ULTIMATELY DEVELOP A SECOND UPPER LOW OVER THE GREAT LAKES BY
FRIDAY. ADDITIONAL DETAIL SPREAD CONTINUES THROUGH THE WEEKEND.
THE WIDE ARRAY OF GUIDANCE IN THE 12Z/18Z CYCLES FAVORED AN
INTERMEDIATE APPROACH THAT KEPT THE FORECAST FAIRLY CLOSE TO
CONTINUITY. THIS WOULD HAVE A LEADING WAVE TRACK NORTHWARD NEAR
THE MID-ATLANTIC/NEW ENGLAND COAST AND THEN TRAILING LOW PRESSURE
CROSSING NEW ENGLAND. IF TRENDS FOR UPPER LOW TRACK END UP
GRAVITATING TOWARD THE SOUTHERN SIDE OF THE SPREAD, THE TRAILING
SURFACE SYSTEM MAY DEVELOP MORE OVER/OFFSHORE THE MID-ATLANTIC.
OVER THE WEST, A CONSENSUS APPROACH LOOKS GOOD FOR RESOLVING
ONGOING LOW-PREDICTABILITY DETAIL DIFFERENCES FOR THE WEAKENING
SHORTWAVE CROSSING THE NORTHWEST WEDNESDAY-THURSDAY AS WELL AS
SOME TIMING AND DEPTH DIFFERENCES FOR THE NEXT MORE AMPLIFIED
TROUGH LIKELY TO REACH THE WEST COAST BY AROUND EARLY SATURDAY
(WITH THE 18Z GFS/00Z UKMET ON THE SLOW SIDE AND THE 12Z ECMWF
PERHAPS A BIT DEEP). THEN SPREAD RAPIDLY INCREASES BY SUNDAY AS
THE ENVELOPE RANGES BETWEEN RECENT GFS/ECMWF/ECENS RUNS THAT
UNDERCUT THE WESTERN U.S. RIDGE, BRINGING SOME ENERGY INTO THE
SOUTHWEST, AND 00Z GEFS MEAN/CMC THAT HOLD ONTO A RIDGE. FOR NOW
PREFERENCE IS TO LEAN TOWARD A MODEST AMOUNT OF UNDERCUTTING
ENERGY.
THE UPDATED FORECAST STARTED WITH AN OPERATIONAL MODEL COMPOSITE
EARLY, WITH A LITTLE LESS 12Z ECMWF WEIGHT THAN AVERAGE, FOLLOWED
BY INCORPORATION OF SOME 18Z GEFS/12Z ECENS-CMCENS MEANS LATER IN
THE PERIOD.
..WEATHER/HAZARDS HIGHLIGHTS
THE SYSTEM FORECAST TO AFFECT THE CENTRAL AND EASTERN U.S.
MID-LATE WEEK WILL SPREAD RAINFALL FROM THE MID-ATLANTIC INTO
NORTHEAST AND BACK INTO THE GREAT LAKES/UPPER MIDWEST, WITH SNOW
TRENDING LIGHTER AND MORE SCATTERED OVER THE NORTH-CENTRAL PLAINS
AND PARTS OF THE UPPER MIDWEST. RAINFALL MAY BE LOCALLY HEAVY AT
TIMES ALONG THE EAST COAST, WITH THE DAY 4/WEDNESDAY EXCESSIVE
RAINFALL OUTLOOK DEPICTING A MARGINAL RISK AREA FROM PARTS OF THE
MID-ATLANTIC INTO NEW ENGLAND. DIFFERENCES IN UPPER LEVEL AND
SURFACE DETAILS HAVE GENERALLY DECREASED CONFIDENCE IN THIS AREA,
BUT THERE IS STILL SOME AGREEMENT TOWARD AN AXIS OF ANOMALOUS
MOISTURE CROSSING THE REGION WHILE THE 12Z ECMWF INDICATED
POTENTIAL FOR SOME INSTABILITY WITHIN THE MOISTURE SHIELD.
PRECIPITATION SPECIFICS BECOME EVEN MORE UNCERTAIN AFTER
WEDNESDAY. AT THE VERY LEAST THERE SHOULD BE SOME LAKE
EFFECT/TERRAIN ENHANCED SNOW LATER IN THE WEEK AND POSSIBLY INTO
THE WEEKEND AS TEMPERATURES GRADUALLY DECLINE AND ONE OR MORE
UPPER FEATURES MOVE OVERHEAD.
OVER THE WEST, THE FORECAST REMAINS ON TRACK FOR TWO PRIMARY
PRECIPITATION EVENTS TO BE FOCUSED OVER THE CENTRAL WEST COAST
AROUND WEDNESDAY AND FRIDAY WITH THE ARRIVAL OF UPPER SHORTWAVES
AND ASSOCIATED SURFACE FRONTS. THE DAY 4/WEDNESDAY EXCESSIVE
RAINFALL OUTLOOK MAINTAINS THE MARGINAL RISK AREA DEPICTED IN
YESTERDAY'S DAY 5 OUTLOOK, INCLUDING PARTS OF NORTHERN CALIFORNIA
AND FAR SOUTHWESTERN OREGON. ASIDE FROM LATEST GFS RUNS TRENDING
LIGHTER, REMAINING MODEL/ENSEMBLE GUIDANCE MAINTAINS REASONABLE
AGREEMENT TOWARD A BRIEF EPISODE OF LOCALLY HEAVY RAINFALL WITHIN
THE OUTLOOK AREA. CURRENTLY THE FRIDAY EVENT LOOKS LIKE IT MAY
PRODUCE SIMILAR TOTALS. IN BOTH EVENTS EXPECT SNOW OVER HIGHER
ELEVATIONS. THE PACIFIC NORTHWEST WILL LIKELY SEE SOME
PRECIPITATION FROM THESE EVENTS BUT WITH LIGHTER AMOUNTS, WHILE
THE FRIDAY EVENT (PERHAPS LINGERING INTO SATURDAY WITH LIGHTER
AMOUNTS) MAY SPREAD A LITTLE MOISTURE INTO THE SOUTHERN HALF OF
CALIFORNIA BUT WITH LOWER CONFIDENCE. THERE IS ALSO NOT MUCH
CONFIDENCE IN COVERAGE OF ANY MOSTLY LIGHT PRECIPITATION THAT
COULD REACH FARTHER INLAND DURING THE WEEKEND.
TEMPERATURES WILL BE UNSEASONABLY WARM ON WEDNESDAY FROM THE UPPER
MIDWEST INTO NORTHEAST AND SOUTH INTO THE MID-ATLANTIC, WITH
MORNING LOWS UP TO 20-25 DEGREES ABOVE NORMAL AND DAYTIME HIGHS UP
TO 10-15 DEGREES ABOVE NORMAL. LATER IN THE WEEK, THE GREATEST
ANOMALIES WILL BECOME MORE CONFINED TO NEW ENGLAND AND GRADUALLY
DECLINE. SOME DAILY RECORDS FOR WARM LOWS WILL BE POSSIBLE
WEDNESDAY INTO THURSDAY FROM THE GREAT LAKES EASTWARD.
ESTABLISHMENT OF UPPER TROUGHING OVER THE EAST AFTER MIDWEEK WILL
FAVOR PERSISTENT CHILLY CONDITIONS WITH COOLEST ANOMALIES OF 5-10
DEGREES BELOW NORMAL OVER THE SOUTHEAST AND INTO THE CENTRAL
APPALACHIANS. CONTINUE TO EXPECT MODERATELY ABOVE NORMAL
TEMPERATURES OVER THE WEST FOR MOST OF THE PERIOD, THOUGH PERHAPS
TRENDING CLOSER TO NORMAL BY NEXT SUNDAY.
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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