795
FXUS02 KWBC 251857
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
157 PM EST WED DEC 25 2024
VALID 12Z SAT DEC 28 2024 - 12Z WED JAN 01 2025
...UNSETTLED WEATHER FOR THE WEST COAST WITH PERIODS OF HEAVY RAIN
AND HIGH ELEVATION MOUNTAIN SNOW...
..OVERVIEW
GUIDANCE CONTINUES TO SHOW A LARGE SCALE PATTERN THAT FEATURES A
RAPID SUCCESSION OF PACIFIC SYSTEMS MOVING INTO/THROUGH THE WEST
AND FEEDING INTO A MEAN UPPER TROUGH OF MODERATE DEPTH ALIGNED
NEAR THE MISSISSIPPI VALLEY. A LEADING UPPER RIDGE INITIALLY NEAR
THE EAST COAST WILL DRIFT INTO THE WESTERN ATLANTIC. THIS PATTERN
WILL LEAD TO AREAS OF HEAVY PRECIPITATION ALONG THE NORTHERN-
CENTRAL WEST COAST THROUGH THE WEEKEND AND SNOW OVER THE NORTHERN
HALF OR SO OF THE ROCKIES, ALONG WITH SOME AREAS OF STRONG WINDS.
AFTER A BRIEF LULL, PRECIPITATION SHOULD REBOUND AGAIN OVER THE
PACIFIC NORTHWEST TOWARD MIDWEEK. IN ADDITION A COUPLE SYSTEMS
SHOULD SPREAD PRECIPITATION (MOSTLY RAIN) ACROSS THE EASTERN HALF
OF THE COUNTRY. SOME OF THIS ACTIVITY MAY BE ON THE HEAVY SIDE.
EXPECT MOST OF THE LOWER 48 TO SEE WELL ABOVE NORMAL TEMPERATURES
FROM THE WEEKEND INTO EARLY MONDAY, FOLLOWED BY A RAPID WEST-TO-
EAST PROGRESSION OF NEAR OR SLIGHTLY BELOW NORMAL HIGHS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE LATEST MODELS AND ENSEMBLE MEANS AGREE WELL FOR MOST ASPECTS OF
THE OVERALL PATTERN BUT THERE ARE PERSISTENT AND IMPORTANT DETAIL
DIFFERENCES, ESPECIALLY AFFECTING SYSTEM EVOLUTION OVER THE
EASTERN U.S. FROM THE WEEKEND INTO EARLY NEXT WEEK AND FOR THE
SECOND SYSTEM OVER THE EAST BY TUESDAY-WEDNESDAY.
LOOKING AT INDIVIDUAL SYSTEMS IN SEQUENCE, THE 00Z CMC WAS A SLOW
EXTREME WITH THE LEADING WAVE FORECAST TO REACH THE UPPER MIDWEST
BY EARLY SATURDAY. THE NEW RUN HAS ADJUSTED FASTER TOWARD
CONSENSUS. THEN GUIDANCE DIVERGES FOR THE EXACT CHARACTER OF THE
NEXT SHORTWAVE REACHING THE PLAINS SATURDAY AND LIFTING
NORTHEASTWARD THEREAFTER. ECMWF RUNS HAVE BEEN ON THE SHARPER AND
SLOWER SIDE AMONG THE OPERATIONAL MODELS, ULTIMATELY LEADING TO A
STRONGER DEEP-LAYER GRADIENT FEEDING MOISTURE INTO THE MID-
ATLANTIC AND NORTHEAST. HOWEVER THE GFS RUNS AND GEFS/ECENS MEANS
FIT INTO THE GENERAL IDEA OF A NEGATIVELY TILTED SHORTWAVE, AND
MOST OF THE 00Z MACHINE LEARNING (ML) MODELS SHOWED A SHARP
SHORTWAVE LIKE THE ECMWF. CMC/UKMET RUNS CONTINUE TO BE MORE
ROUNDED WITH THE OVERALL TROUGH INTO THE 12Z CYCLE BUT THE GFS HAS
NUDGED A BIT IN THE ECMWF DIRECTION AND THE 12Z ICON MADE A
PRONOUNCED SHIFT TO THE ECMWF SCENARIO. ONE OTHER DETAIL OF NOTE IS
THAT BY EARLY MONDAY THE ML MODELS FAVORED THE SURFACE SYSTEM'S
PARENT LOW TRACKING INTO OR NEAR ONTARIO WHILE A TRIPLE POINT WAVE
REACHES THE MID- ATLANTIC. ECMWF RUNS TRACKED THE PARENT LOW A
LITTLE FARTHER SOUTH BUT THE NEW 12Z VERSION HAS TRENDED IN THE ML
MODEL DIRECTION.
THE NEXT SIGNIFICANT SYSTEM IN THE SERIES REACHES THE WEST COAST BY
EARLY SUNDAY (WITH A SOMEWHAT ELONGATED AREA OF SURFACE LOW
PRESSURE), WITH THE 00Z CMC ALREADY ON THE SLOW SIDE AT THAT TIME.
THEREAFTER AN AVERAGE AMONG THE GFS/ECMWF AND THE ENSEMBLE MEANS
HAS HELD UP RATHER WELL FOR TIMING OVER THE PAST DAY OR SO.
MEANWHILE THE NEW 12Z CMC COMPARES A LOT BETTER WITH THIS CLUSTER
VERSUS THE SLOW 00Z RUN. BY EARLY TUESDAY THERE IS CONTINUED NORTH-
SOUTH SPREAD, RANGING FROM THE OHIO VALLEY TO TENNESSEE VALLEY
(GFS LEANING NORTH, ECMWF LEANING SOUTH). THEN THE SYSTEM REACHES
THE EASTERN GREAT LAKES/NORTHEAST BY WEDNESDAY, WITH GUIDANCE SPLIT
REGARDING INLAND OR COASTAL SURFACE LOW DOMINANCE. DURING THE
TUESDAY-WEDNESDAY TIME FRAME THE ML MODELS ARE NOTICEABLY WEAKER
WITH THIS SYSTEM VERSUS THE DYNAMICAL GUIDANCE. THERE HAVE BEEN AT
LEAST A COUPLE CASES OF A WEAK/SUPPRESSED BIAS IN THE ML MODELS FOR
WAVES NEAR THE EAST COAST, SO PREFERENCE SIDES WITH THE DYNAMICAL
CLUSTER FOR THE TIME BEING.
THE NEXT POTENTIAL SHORTWAVE REACHING THE WEST BY MONDAY-TUESDAY IS
WEAKER THAN ITS PREDECESSORS, WITH GFS/GEFS RUNS TENDING TO BE MOST
ENTHUSIASTIC FOR ITS STRENGTH OVER THE SOUTHWEST AND BEYOND. ML
MODELS HAVE BEEN MIXED BUT LATEST RUNS FAVOR LEANING TOWARD A MORE
SUBDUED PRESENTATION OF THIS FEATURE. YET ANOTHER SYSTEM OR TWO
SHOULD REACH OR APPROACH THE PACIFIC NORTHWEST BY TUESDAY-
WEDNESDAY, WITH TYPICAL DETAIL/TIMING DIFFERENCES.
GUIDANCE PREFERENCES LED TO AN EARLY PERIOD BLEND CONSISTING OF 60
PERCENT ECMWF (SPLIT BETWEEN THE 00Z/25 AND 12Z/24 RUNS) AND 40
PERCENT GFS (00Z/06Z). THEN THE FORECAST STARTED INCLUDING ENSEMBLE
MEANS, MORE 00Z ECENS RELATIVE TO 06Z GEFS, WITH TOTAL ENSEMBLE
WEIGHT REACHING 40 PERCENT BY NEXT WEDNESDAY.
..WEATHER/HAZARDS HIGHLIGHTS
MULTIPLE ROUNDS OF MODERATE TO HEAVY RAINFALL WILL LIKELY IMPACT
THE WEST COAST THROUGH THE REMAINDER OF THIS WEEK AND INTO THE
WEEKEND AS MULTIPLE FRONTAL SYSTEMS APPROACH THE COAST, WITH THE
SIGNAL FOR HEAVY RAINFALL BECOMING STRONGEST ON SATURDAY
CONCENTRATING MORE NEAR THE CALIFORNIA/OREGON BORDER WITH TIME.
FLOODING WILL BE POSSIBLE IN RIVERS AND STREAMS, WITH MORE FLASHY
RESPONSES POSSIBLE WHERE BURN SCARS EXIST AND POSSIBLY NEAR THE
STEEP TERRAIN OF THE COASTAL MOUNTAIN RANGES AND INTO THE FOOTHILLS
OF THE CASCADES. THE COMBINATION OF RAINFALL EXPECTED THROUGH
FRIDAY NIGHT AND A MORE CONCENTRATED HEAVY QPF SIGNAL IN SOME
GUIDANCE FOCUSED OVER FAR SOUTHWEST OREGON AND FAR NORTHWEST
CALIFORNIA ON SATURDAY CONTINUES TO FAVOR A SLIGHT RISK AREA OVER
THAT REGION IN THE DAY 4/SATURDAY EXCESSIVE RAINFALL OUTLOOK. WHILE
QPF IS LESS THE FOLLOWING DAY, INCREASED SENSITIVITY DUE TO PRIOR
RAINFALL SUPPORTS MAINTAINING THE DAY 5/SUNDAY MARGINAL RISK FOR
WESTERN OREGON AND NORTHWEST CALIFORNIA AS INTRODUCED IN THE
PREVIOUS CYCLE.
MOISTURE SPREADING INLAND ACROSS THE WEST WILL LIKELY PRODUCE
HEAVY SNOW OVER THE HIGHER/HIGHEST TERRAIN OF THE CASCADES AND
ROCKIES, AS WELL AS PORTIONS OF THE SIERRA NEVADA. EXPECT THE HEAVY
SNOW POTENTIAL TO CONTINUE THROUGH SUNDAY. PRECIPITATION OVER THE
PACIFIC NORTHWEST SHOULD TREND LIGHTER EARLY NEXT WEEK, WITH
ROCKIES SNOW TAPERING OFF BY MONDAY OR TUESDAY. TIGHT PRESSURE
GRADIENTS MAY RESULT IN SOME PERIODICALLY GUSTY WINDS FOR SOME
LOCATIONS OVER THE PACIFIC NORTHWEST, INTERIOR WEST, PLUS THE
NORTHERN/CENTRAL ROCKIES AND HIGH PLAINS. PRECIPITATION SHOULD
TREND LIGHTER OVER THE PACIFIC NORTHWEST AROUND MONDAY INTO TUESDAY
BUT THEN INCREASE AGAIN LATE TUESDAY/TUESDAY NIGHT THROUGH
WEDNESDAY.
DOWNSTREAM, MULTIPLE FRONTAL SYSTEMS WILL BRING RAIN AND
THUNDERSTORM CHANCES TO THE EASTERN HALF OF THE COUNTRY. SOME
MODERATE TO HEAVY RAINFALL WILL BE POSSIBLE WITHIN A GENERAL AREA
FROM THE LOWER HALF OF THE MISSISSIPPI VALLEY INTO THE MID-ATLANTIC
AND NORTHEAST. THE WETNESS OF THE SOME MOST RECENT GUIDANCE IN AND
NEAR TENNESSEE AND THE SOUTHERNMOST APPALACHIANS SHOWING LOCAL
AMOUNTS IN THE 3-5" RANGE LED TO THE INTRODUCTION OF A DAY
4/SATURDAY MARGINAL RISK AREA. HEAVIEST RAINFALL SHIFTS INTO THE
VICINITY OF PENNSYLVANIA AND NEW YORK STATE ON DAY 5/SUNDAY WITH A
MARGINAL RISK AREA AGAIN DEPICTED. ACROSS NORTHERN AREAS, FROZEN
GROUND/SNOW COVER WILL LEAD TO ADDED SENSITIVITY. THERE IS STILL A
FAIR AMOUNT OF UNCERTAINTY OVER RAINFALL SPECIFICS, BUT THE OVERALL
PATTERN FAVORING A PERIOD OF ATLANTIC INFLOW SUGGESTS POTENTIAL
FOR SIGNIFICANT TOTALS. THE NEXT SYSTEM MAY BRING A BAND OF
RAIN/SNOW TO THE NORTH-CENTRAL PLAINS EARLY IN THE WEEK AND THEN
SPREAD ANOTHER AREA OF PRECIPITATION ACROSS THE EAST TUESDAY-
WEDNESDAY. AS WITH THE LEADING SYSTEM, WINTRY WEATHER SHOULD BE
CONFINED FAIRLY FAR NORTHWARD. HOWEVER 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.
ABOVE NORMAL TEMPERATURES WILL PREVAIL OVER MUCH OF THE COUNTRY
DURING THE WEEKEND AND INTO EARLY MONDAY. THE WARMEST ANOMALIES OF
PLUS 20F OR GREATER WILL LIKELY BE FOR MORNING LOWS FROM THE UPPER
MIDWEST INTO THE SOUTH/SOUTHEAST DURING THE WEEKEND, POSSIBLY
CHALLENGING DAILY RECORDS FOR WARM LOWS IN SOME AREAS. WARMEST MIN
TEMPERATURE ANOMALIES SHOULD BE MORE CONFINED TO THE UPPER
MISSISSIPPI VALLEY TO NEW ENGLAND EARLY NEXT WEEK. ALSO THE
SOUTHERN PLAINS COULD SEE TEMPERATURES REACH 15-25F ABOVE NORMAL
ON MONDAY IN THE WARM SECTOR OF A SYSTEM TRACKING BY TO THE NORTH.
NEAR TO SLIGHTLY BELOW NORMAL HIGH TEMPERATURES SHOULD SPREAD
ACROSS THE WEST INTO THE NORTHERN PLAINS ON MONDAY, EXPANDING
THROUGH THE REST OF THE PLAINS AND INTO EAST-CENTRAL U.S. TUESDAY-
WEDNESDAY.
RAUSCH/ROTH
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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