636
FXUS02 KWBC 250700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 AM EST MON DEC 25 2023
VALID 12Z THU DEC 28 2023 - 12Z MON JAN 01 2024
...LOCALLY HEAVY PRECIPITATION POSSIBLE FOR THE CENTRAL WEST COAST
LATE THIS WEEK...
..OVERVIEW
UPPER RIDGING OVER WESTERN CANADA, AS WELL AS THE WESTERN U.S.
THROUGH AT LEAST SATURDAY, WILL SUPPORT A MEAN TROUGH OVER THE
EASTERN U.S. DURING THE PERIOD. WITHIN THE EASTERN TROUGH A
LEADING UPPER LOW SHOULD TRACK OUT OF THE LOWER OHIO VALLEY WITH
CONTINUED UNCERTAINTY OVER HOW NORTHERN STREAM FLOW MAY INTERACT.
ANOTHER SHORTWAVE/SURFACE FRONT MAY DROP INTO THE MEAN TROUGH
SATURDAY ONWARD. MEANWHILE WEAKENING SHORTWAVE ENERGY WILL
QUICKLY BRUSH THE NORTHWEST EARLY THURSDAY AND THEN A
SHARPENING/NEGATIVELY TILTING TROUGH WILL LIKELY REACH THE WEST
COAST BY FRIDAY NIGHT/EARLY SATURDAY, BRINGING A BRIEF EPISODE OF
ENHANCED PRECIPITATION TO THE CENTRAL WEST COAST. AFTER EARLY
SATURDAY THE DETAILS BECOME VERY UNCLEAR FOR WHAT WILL HAPPEN TO
THIS SHORTWAVE ENERGY AS WELL AS FOR POTENTIAL UPSTREAM ENERGY.
EXPECT CHILLY TEMPERATURES FOR MOST OF THE PERIOD OVER THE SOUTH
AND NEAR TO ABOVE NORMAL READINGS OVER MOST OTHER AREAS OF THE
LOWER 48.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MULTI-DAY MEANS AND TELECONNECTIONS RELATIVE TO THE STRONG
POSITIVE HEIGHT ANOMALIES OVER CENTRAL AND THEN WESTERN CANADA
OFFER CONTINUED SUPPORT FOR THE GENERAL MEAN TROUGH OVER THE
EASTERN U.S. BUT EMBEDDED DETAILS CONTINUE TO BE AN ISSUE. BY THE
LATTER HALF OF THE PERIOD THERE IS ALSO A WIDE ARRAY OF
POSSIBILITIES FOR HOW INCOMING PACIFIC SHORTWAVE ENERGY ON
SATURDAY MAY ULTIMATELY PASS THROUGH THE LONG TERM WESTERN U.S.
MEAN RIDGE AND FOR DEPICTIONS OF POTENTIAL ENERGY BEHIND IT.
THE 12Z/18Z RUNS AND NEW 00Z SOLUTIONS ARE FINALLY STARTING TO
CLUSTER BETTER FOR THE TRACK OF THE UPPER LOW FORECAST TO BE OVER
THE LOWER OHIO VALLEY AS OF EARLY THURSDAY. THE LATEST CONSENSUS
FALLS WITHIN THE SOUTHERN HALF OF THE PRIOR ENVELOPE, WITH GEFS
MEAN RUNS AND ONE OR MORE GFS RUNS APPEARING TO PROVIDE THE
EARLIEST LEAD ON SUCH A PATH. SOME ECMWF-INITIALIZED MACHINE
LEARNING MODELS ALSO ADVERTISED A FAIRLY SOUTHWARD TRACK AND IN A
GREATER PROPORTION RELATIVE TO RECENT OPERATIONAL MODEL/ENSEMBLE
SPREAD. GUIDANCE ARRIVES AT THE CURRENT CLUSTER BY WAY OF DELAYED
INTERACTION OF NORTHERN STREAM FLOW, BUT WITH THE FORECAST FROM
FRIDAY ONWARD STILL PLAGUED BY A WIDE SPREAD FOR HOW EASTERN
CANADA TROUGH ENERGY WILL EVOLVE--AFFECTING THE EJECTING EASTERN
U.S. UPPER LOW AND THEN DETAILS OVER THE NORTHEAST BEHIND IT (12Z
ECMWF AND 00Z UKMET/CMC MUCH DEEPER WITH NORTHEAST TROUGHING
RELATIVE TO THE GFS/GEFS BY SATURDAY). AT THE SURFACE, EXPECT A
LEADING WAVE TO DEPART FROM NEW ENGLAND EARLY AND THEN A NEW
SYSTEM TO FORM NEAR THE MID-ATLANTIC IN RESPONSE TO THE UPPER LOW,
WITH A TRACK MOST LIKELY FAR ENOUGH OFFSHORE NOT TO HAVE A
SIGNIFICANT EFFECT ON NEW ENGLAND.
FARTHER UPSTREAM, GUIDANCE HAS RAPIDLY BECOME BETTER DEFINED WITH
A SHORTWAVE DROPPING INTO THE UPPER MIDWEST BY SATURDAY. THEN
SOLUTIONS DIVERGE FOR WHAT BECOMES OF THIS TROUGH. THE CURRENT
MAJORITY SUGGESTS LESS SOUTHWESTWARD ELONGATION THAN DEPICTED IN
THE PAST COUPLE OF ECMWF RUNS OR 12Z GFS, WHILE 12Z
ECMWF-INITIALIZED MACHINE LEARNING MODELS FAVOR A FAIRLY
PROGRESSIVE TROUGH NEAR THE EAST COAST BY EARLY NEXT WEEK PER THE
NEW 00Z GFS. THE 18Z GFS AND LATEST CMC RUNS ARE A COMPROMISE
WHILE THE INTERMEDIATE ENSEMBLE MEANS WITH BROAD TROUGHING HAVE
SUPPORT FROM TELECONNECTIONS.
OVER THE WEST, SOME RECENT RUNS HAVE BEEN NUDGING A LITTLE FASTER
WITH THE NEGATIVELY TILTED SHORTWAVE TROUGH FORECAST TO REACH THE
WEST COAST BY SATURDAY. ON THE OTHER HAND, THE NEW 00Z CMC HAS
STRAYED TO THE SLOW AND AMPLIFIED SIDE OF THE SPREAD. AS IS
TYPICALLY THE CASE WITH SHORTWAVE ENERGY PUSHING INTO A MEAN
RIDGE, GUIDANCE VARIES CONSIDERABLY FOR WHAT BECOMES OF THIS
SHORTWAVE AFTER ITS ARRIVAL. A SLIGHT MAJORITY SCENARIO HAS BEEN
FOR SOME SORT OF SPLIT WHILE THE 12Z ECMWF-INITIALIZED MACHINE
LEARNING MODELS GENERALLY MAINTAINED A BETTER DEFINED SHORTWAVE
PUSHING INTO THE INTERIOR WEST BY SUNDAY, CLOSEST TO THE 12Z ECENS
MEAN (PLUS NEW 00Z UKMET THROUGH THE END OF ITS RUN) WITH THE 18Z
GFS/12Z CMC THE CLOSEST IN PRINCIPLE ALBEIT WITH MORE SPLITTING OF
THE ENERGY. THE GEFS MEAN WASHES OUT THE ENERGY QUICKER THAN THE
OTHER MEANS OR OPERATIONAL RUNS. BY NEXT MONDAY/NEW YEAR'S DAY,
THE MOST COMMON THEME IS FOR ANOTHER AREA OF TROUGHING TO APPROACH
THE WEST COAST. THE 12Z MACHINE LEARNING MODELS GENERALLY FAVORED
A SOUTHERN STREAM TROUGH REACHING NEAR CALIFORNIA.
BASED ON 12Z/18Z GUIDANCE, PREFERENCES LED TO AN EARLY PERIOD
BLEND CONSISTING OF MORE 12Z ECMWF AND 12Z/18Z GFS RELATIVE TO THE
UKMET/CMC AND A MID-PERIOD AVOIDANCE OF THE CMC. LATER IN THE
PERIOD THE FORECAST INCREASED 18Z GEFS/12Z ECENS MEAN WEIGHT
TOWARD 50 PERCENT WHILE REDUCING 12Z ECWMF INPUT, MAINTAINING THE
GFS, AND RETURNING SOME CMC INTO THE BLEND.
..WEATHER/HAZARDS HIGHLIGHTS
A LEADING DEPARTING WAVE SHOULD BRING SOME PRECIPITATION TO THE
NORTHEAST ON THURSDAY, WITH SNOW OVER FAR NORTHERN NEW ENGLAND AND
RAIN TO THE SOUTH. THE UPPER LOW TRACKING OUT OF THE LOWER OHIO
VALLEY SHOULD SUPPORT AN AREA OF RAIN/SNOW ALONG ITS PATH LATE
THIS WEEK, WITH BEST POTENTIAL FOR HIGHEST SNOW ACCUMULATIONS OVER
FAVORED TERRAIN IN THE APPALACHIANS. INTERIOR NORTHEAST LOCATIONS
COULD SEE ANOTHER EPISODE OF SNOW DEPENDING ON DETAILS OF EASTERN
CANADA TROUGH ENERGY BY THE WEEKEND, WHILE SOME MOSTLY LIGHT
PRECIPITATION MAY BE POSSIBLE WITH A WAVE/FRONT OVER THE
CENTRAL/EASTERN U.S. FROM SATURDAY ONWARD. RAINFALL COULD BE
SOMEWHAT HEAVIER IN THE EVENT OF A LESS PROBABLE SLOWER UPPER
TROUGH IN THAT TIME FRAME.
OVER THE WEST, A LATE WEEK PACIFIC SHORTWAVE/WEAKENING FRONTAL
SYSTEM WILL PROVIDE THE BEST FOCUS FOR MEANINGFUL PRECIPITATION
ALONG THE CENTRAL WEST COAST. THE DAY 5/FRIDAY EXCESSIVE RAINFALL
OUTLOOK PROPOSES A MARGINAL RISK AREA COVERING PORTIONS OF
NORTHERN CALIFORNIA AS WELL AS THE CENTRAL CALIFORNIA COAST AND
FAR SOUTHWEST CORNER OF OREGON. MODEL QPFS VARY OVER THIS REGION
BUT THERE IS AN INDICATION OF SOME INSTABILITY REACHING THE
CALIFORNIA COAST AND THE SYSTEM'S ORIENTATION COULD ALLOW FOR SOME
TRAINING BANDS OF ENHANCED RAINFALL. ANOTHER EVENT A COUPLE DAYS
EARLIER SHOULD LEAD TO DAMP ANTECEDENT CONDITIONS AS WELL. SOME
ACTIVITY MAY LINGER INTO SATURDAY, WHILE SNOW IS POSSIBLE AT HIGH
ELEVATIONS. CONFIDENCE REMAINS LOW REGARDING COVERAGE OF ANY
LIGHTER PRECIPITATION THAT COULD EXTEND FARTHER INTO THE WEST.
THE PACIFIC NORTHWEST SHOULD ALSO SEE SOME PRECIPITATION FROM THIS
EVENT BUT WITH LOWER TOTALS THAN FORECAST ALONG THE CENTRAL WEST
COAST.
THE FORECAST PATTERN WILL FAVOR MULTIPLE DAYS OF CHILLY WEATHER
ACROSS THE SOUTH, WITH TEMPERATURES 5-15F BELOW NORMAL. FLORIDA
SHOULD SEE THE BEST POTENTIAL FOR DOUBLE-DIGIT ANOMALIES. THE
SOUTHEAST SHOULD BEGIN TO MODERATE BY SUNDAY-MONDAY BUT WITH
FLORIDA LIKELY STAYING BELOW NORMAL THROUGH THAT TIME. ON THE
OTHER HAND, PARTS OF THE FAR NORTHERN PLAINS AND UPPER MIDWEST
WILL BE CONSISTENTLY ABOVE NORMAL BY 10F OR MORE. MORNING LOWS
FROM THE UPPER MIDWEST INTO NORTHEAST MAY BE AT LEAST 20F ABOVE
NORMAL ON THURSDAY, WHICH COULD LEAD TO SOME DAILY RECORDS FOR
WARM LOWS IN THE NORTHEAST IF THEY HOLD FOR THE CALENDAR DAY.
MUCH OF THE WEST WILL SEE MODERATELY ABOVE NORMAL READINGS DURING
THE PERIOD BUT WITH A SLIGHT COOLING TREND IF UPPER RIDGING
WEAKENS A LITTLE AS CURRENTLY FORECAST.
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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