934
FXUS02 KWBC 261900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 PM EST THU DEC 26 2024
VALID 12Z SUN DEC 29 2024 - 12Z THU JAN 02 2025
...HEAVY RAIN AND MOUNTAIN SNOW LIKELY FOR THE WEST COAST/NORTHWEST
EARLY NEXT WEEK...
..OVERVIEW
PROGRESSIVE FLOW WITH MULTIPLE SHORTWAVES ROTATING AROUND THE
SOUTHERN SIDE OF A SOUTH-CENTRAL CANADA UPPER LOW WILL BE IN PLACE
NEXT WEEK, PRODUCING MANY LOW PRESSURE/FRONTAL SYSTEMS ACROSS THE
LOWER 48. IN THE WEST ON SUNDAY, AMPLE MOISTURE AHEAD OF AN EASTERN
PACIFIC UPPER TROUGH SHOULD PRODUCE POSSIBLY HEAVY RAIN CONTINUING
ALONG THE WEST COAST AND SNOW OVER THE NORTHERN HALF OR SO OF THE
ROCKIES, ALONG WITH SOME AREAS OF STRONG WINDS. THE NORTHWEST LOOKS
TO QUIET DOWN SOMEWHAT DURING THE NEXT WORKWEEK BUT MODEST RAIN
AND SNOW WILL STILL BE POSSIBLE, WITH MOST OF ANY FOCUSED ACTIVITY
TENDING TO BE OVER WESTERN WASHINGTON. FARTHER EAST, RAIN THAT
COULD BE HEAVY IS LIKELY OVER THE APPALACHIANS TO EASTERN SEABOARD
EARLY NEXT WEEK, WITH FREEZING RAIN POSSIBLE IN NORTHERN NEW
ENGLAND. ANOTHER ROUND OF PRECIPITATION (INCLUDING POSSIBLE SNOW IN
THE NORTH-CENTRAL PLAINS) SHOULD PUSH FROM THE NORTH-CENTRAL TO
EASTERN U.S. MONDAY-WEDNESDAY. EXPECT A TREND TOWARD A LARGER
SCALE WESTERN RIDGE/EASTERN TROUGH PATTERN BY MID-LATE WEEK.
WARMER THAN NORMAL TEMPERATURES FOR MOST OF THE LOWER 48 EARLY NEXT
WEEK WILL MODERATE CLOSER TO NORMAL, IF NOT SLIGHTLY BELOW IN SOME
SPOTS, FROM WEST TO EAST DURING THE PERIOD.
..GUIDANCE/PREDICTABILITY ASSESSMENT
WITHIN THE GENERALLY AGREEABLE PATTERN EVOLUTION THERE ARE SOME
PERSISTENT DETAIL ISSUES WITH INDIVIDUAL SYSTEMS/FEATURES, WITH A
RELATIVE MAJORITY OF GUIDANCE FAVORING MAINTENANCE OF A SOLUTION
THAT TILTS TOWARD A COMPROMISE OR ECMWF/ENSEMBLE MEAN SCENARIO
OVERALL. THIS PROVIDES FAIRLY GOOD CONTINUITY FROM PREVIOUS
FORECAST, WITH ONLY MODEST REFINEMENTS TO REFLECT LATEST CONSENSUS
IDEAS/TRENDS. THE UPDATED FORECAST STARTED WITH A 60/25/15 WEIGHT
OF THE 00Z ECMWF/GFS/UKMET FOR THE FIRST HALF OF THE PERIOD AND
THEN TRANSITIONED TOWARD THE 00Z ECMWF AND 06Z GEFS/00Z ECENS-
CMCENS MEANS.
GUIDANCE IS STILL HAVING SOME TROUBLE IN RESOLVING A MISSISSIPPI
VALLEY INTO NORTHEAST SHORTWAVE SUNDAY-MONDAY, WITH RESULTING
SPREAD FOR STRUCTURE AND TIMING OF THE ASSOCIATED SURFACE LOW
PRESSURE/FRONTAL SYSTEM. MACHINE LEARNING (ML) MODELS HAVE BEEN
CONSISTENT IN FAVORING A MODERATELY SHARP AND SLOW SHORTWAVE--
THOUGH NOT AS SLOW AS THE 12Z/25 ECMWF RUN, WITH THE 00Z ECMWF AND
NEARBY SOLUTIONS PROVIDING THE BEST STARTING POINT. THE 00Z GFS WAS
A BIT SLOWER THAN THE 06Z RUN SO IT AND THE UKMET PROVIDED A MODEST
COMPROMISE WITH THE ECMWF TO ACCOUNT FOR ONGOING TIMING SPREAD. THE
00Z CMC'S SYSTEM STRUCTURE COMPARED POORLY TO OTHER GUIDANCE AND
THE NEW 12Z RUN AT LEAST LOOKS BETTER.
DYNAMICAL AND ML GUIDANCE AGREE ON A SOMEWHAT ELONGATED SYSTEM
REACHING THE PACIFIC NORTHWEST ON SUNDAY, WITH RAPID PROGRESSION OF
THE SHORTWAVE LEADING TO PLAINS LOW PRESSURE BY MONDAY. LOW
PRESSURE SHOULD CONTINUE RAPIDLY ALONG AND REACH THE NORTHEAST
U.S./CANADIAN MARITIMES BY WEDNESDAY-THURSDAY. AN INTERMEDIATE
SOLUTION LOOKS GOOD OVER THE EAST-CENTRAL U.S. WITH CONTINUED
NORTH-SOUTH SPREAD OVER THE OHIO/TENNESSEE VALLEYS AND SOME
OPPOSING CLUSTERING/TRENDS THAT INCLUDE ML MODELS LEANING TO THE
CENTRAL-SOUTHERN PART OF THE SPREAD BUT THE 12Z ECMWF ADJUSTING A
LITTLE NORTHWARD FROM PRIOR RUNS. THEREAFTER, GFS RUNS STILL LEAN A
BIT FAST WITH THE SYSTEM WHILE GUIDANCE IS STILL SPLIT OVER THE
RELATIVE EMPHASIS ON EASTERN GREAT LAKES VERSUS COASTAL LOW
PRESSURE AS OF EARLY WEDNESDAY. DUE TO THE FAVORED OVERALL
TIMING/EVOLUTION OF RECENT ECMWF RUNS THE BLEND REFLECTED A LITTLE
MORE COASTAL LOW EMPHASIS, BUT THE NEW 12Z ECMWF HAS ADJUSTED TO A
MORE EVEN EMPHASIS. BY EARLY THURSDAY THERE IS FAIRLY GOOD SUPPORT
FROM THE ML MODELS AND A DECENT NUMBER OF DYNAMICAL MODELS/MEANS
FOR THE SURFACE LOW TO REACH A POSITION OVER OR A LITTLE
NORTH/EAST OF MAINE. ML MODELS HAVE TRENDED STRONGER WITH THIS
SYSTEM OVER THE EASTERN HALF OF THE COUNTRY VERSUS 24 HOURS AGO AS
A WHOLE, SUPPORTING OPERATIONAL RUNS IN PRINCIPLE.
GFS RUNS/GEFS ENSEMBLES CONTINUE TO BE MORE PRONOUNCED THAN OTHER
GUIDANCE WITH SHORTWAVE ENERGY THAT REACHES THE WEST BY AROUND
TUESDAY. THIS DIFFERENCE RESULTS IN THE GFS/GEFS PRODUCING MUCH
MORE SOUTHERN TIER PRECIPITATION OVER THE CENTRAL/EASTERN STATES BY
LATE IN THE FORECAST PERIOD. ML MODELS STRONGLY SUPPORT THE NON-
GFS/GEFS GUIDANCE WHICH SHOWS MORE EASTERN U.S. UPPER TROUGHING AND
MOSTLY DRY CONDITIONS ACROSS THE SOUTHERN TIER, ASIDE FROM PERHAPS
SOME LIGHT WESTERN GULF COAST ACTIVITY PER THE 00Z ECENS MEAN.
NOTABLE QPF ADJUSTMENTS FROM THE 13Z NBM FOR THIS CYCLE INCLUDED A
PARTIAL TREND TO IMPROVE DEFINITION OF THE NORTH-CENTRAL PLAINS
INTO MIDWEST BAND OF RAIN/SNOW PER A COMPOSITE OF OPERATIONAL
MODELS, ADJUSTING LATE-PERIOD SOUTHERN TIER QPF TOWARD A LESS
EXPANSIVE VERSION OF THE 00Z ECENS MEAN (WHICH MAINTAINS NON-GFS
FOCUS PER CONTINUITY), AND MORE OPERATIONAL MODEL INPUT OVER/NEAR
THE GREAT LAKES LATE IN THE PERIOD TO IMPROVE LAKE EFFECT
DEFINITION.
..WEATHER/HAZARDS HIGHLIGHTS
AN ATMOSPHERIC RIVER WILL TAKE AIM AT THE WEST COAST OVER THE
WEEKEND. THE HEAVIEST RAIN IS LIKELY ON SATURDAY (NOW INTO THE
SHORT RANGE), BUT RAIN TOTALS STILL APPEAR SUFFICIENT TO WARRANT A
MARGINAL RISK FOR COASTAL OREGON TO NORTHERN CALIFORNIA IN THE DAY
4/SUNDAY ERO, ESPECIALLY CONSIDERING THE INCREASED SENSITIVITY DUE
TO PRIOR RAINFALL. MOISTURE SPREADING FARTHER INLAND INTO THE
HIGHER TERRAIN WILL ALLOW FOR HEAVY SNOW TOTALS ON SUNDAY IN THE
CASCADES AND NORTHERN ROCKIES, WITH LESSER AMOUNTS IN THE SIERRA
NEVADA AND CENTRAL ROCKIES. SNOW LEVELS SHOULD BE LOWERED BY SUNDAY
COMPARED TO EARLIER DAYS. EXPECT PRECIPITATION TO TREND LIGHTER
ACROSS THE NORTHWEST MONDAY-TUESDAY. PRECIPITATION COULD INCREASE
AGAIN FROM TUESDAY NIGHT ONWARD BUT CURRENTLY THESE EPISODES
GENERALLY DO NOT LOOK TO BE AS HEAVY AS THE WEEKEND SYSTEM. WHAT
AREAS OF ENHANCED ACTIVITY THAT DO OCCUR ARE MOST LIKELY TO BE
OVER WESTERN WASHINGTON.
DOWNSTREAM, THE EASTERN U.S. CAN EXPECT MODERATE TO HEAVY RAIN ON
SUNDAY IN THE VICINITY OF A COUPLE OF FRONTAL SYSTEMS. WHILE THERE
IS PERSISTENT SPREAD FOR SYSTEM EVOLUTION AND TIMING/MAGNITUDE OF
HEAVIEST RAIN, HIGH PROBABILITIES FOR RAINFALL EXCEEDING AN INCH
ON SUNDAY ARE IN GOOD AGREEMENT IN THE GEFS AND EC ENSEMBLES ACROSS
PENNSYLVANIA GIVEN LIKELY GULF AND ATLANTIC MOISTURE INFLOW.
FARTHER NORTH INTO NEW YORK STATE, FROZEN GROUND/SNOW COVER WILL
LEAD TO ADDED SENSITIVITY. MEANWHILE FARTHER SOUTH ACROSS THE
APPALACHIANS TO PIEDMONT, RELATIVELY MORE INSTABILITY MAY OVERCOME
RELATIVELY DRIER TO AVERAGE ANTECEDENT CONDITIONS TO PRODUCE
CONVECTION WITH HEAVY RAIN RATES ON SUNDAY, CONTINUING FROM
SATURDAY-SATURDAY NIGHT'S HEAVY RAIN IN THE MID-SOUTH. HEAVIEST
ML MODEL RAINFALL ALSO FITS THIS PATTERN. THUS THE MARGINAL RISK
IN THE DAY 4/SUNDAY ERO STAYS FAIRLY CLOSE TO THE PRIOR CYCLE'S
CONTINUITY. ANOTHER THREAT WILL BE THE POTENTIAL FOR FREEZING RAIN
ACROSS MAINE IN PARTICULAR WITH THIS SYSTEM SUNDAY INTO MONDAY.
A LOW PRESSURE SYSTEM WILL DEVELOP IN THE PLAINS BY MONDAY WITH A
SHORTWAVE EJECTING FROM THE WEST. STRONG AND GUSTY WINDS ARE
LIKELY BEHIND THIS LOW ACROSS PARTS OF THE ROCKIES AND HIGH PLAINS.
MEANWHILE NORTH OF THE LOW TRACK, SNOW IS POSSIBLE ACROSS THE
NORTH-CENTRAL PLAINS MONDAY WITH PROBABILITIES FOR AT LEAST 0.25
INCH LIQUID IN THE FORM OF SNOW MAXIMIZING AROUND 30-35 PERCENT
OVER PARTS OF SOUTHERN SOUTH DAKOTA. A BROADER AREA OF
PRECIPITATION WILL SPREAD ACROSS THE EAST TUESDAY-WEDNESDAY. WINTRY
WEATHER SHOULD BE CONFINED FAIRLY FAR NORTHWARD AS WITH THE PRIOR
SYSTEM, BUT PRECIPITATION TOTALS OVER THE EAST SHOULD BE LIGHTER
WITH THIS STORM. THE EXTENT OF SNOW/ICE OVER THE NORTHEAST AROUND
MIDWEEK WILL BE SENSITIVE TO SURFACE LOW TRACK DETAILS THAT WILL
TAKE MORE TIME TO BE RESOLVED. COLD CYCLONIC FLOW BEHIND THE STORM
SYSTEM WILL LIKELY PRODUCE A PERIOD OF LAKE EFFECT SNOW.
MUCH OF THE COUNTRY WILL BE WELL ABOVE AVERAGE TEMPERATURE-WISE
EARLY NEXT WEEK. LOW TEMPERATURES OF 20-30 DEGREES ABOVE NORMAL
WILL BE COMMON IN THE MIDWEST TO EAST ON SUNDAY, WITH 50S FOR LOWS
REACHING ALL THE WAY INTO THE OHIO VALLEY. HIGH TEMPERATURES SHOULD
BE MAXIMIZED ACROSS THE SOUTHERN PLAINS ON SUNDAY AND ESPECIALLY
MONDAY, WITH HIGHS IN THE 70S AND 80S ACROSS TEXAS THAT COULD BREAK
DAILY RECORD HIGHS (ESPECIALLY IN SOUTHERN PARTS OF THE STATE). AS
MULTIPLE COLD FRONTS TRACK ACROSS THE NATION, MORE SEASONABLE COOL
AIR SHOULD GRADUALLY TAKE OVER AS THE WEEK PROGRESSES. EXPECT THE
NORTHWESTERN TO CENTRAL U.S. TO SEE NEAR NORMAL TEMPERATURES
MONDAY-TUESDAY, AND COOLER TEMPERATURES WILL SPREAD ACROSS THE
EASTERN U.S. WEDNESDAY-THURSDAY. UPPER TROUGHING MAY SUPPORT HIGHS
UP TO 5-10F BELOW NORMAL IN A FEW AREAS BETWEEN THE NORTHERN PLAINS
AND APPALACHIANS BY THURSDAY. ON THE OTHER HAND, BY THAT TIME THE
UPPER RIDGING THAT BUILDS INTO THE WEST WILL LIKELY RAISE THAT
REGION'S TEMPERATURES UP TO 5-10F ABOVE NORMAL AND ESPECIALLY OVER
THE SOUTHWEST.
RAUSCH/TATE
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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