678
FXUS02 KWBC 241859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 PM EST TUE DEC 24 2024
VALID 12Z FRI DEC 27 2024 - 12Z TUE DEC 31 2024
...UNSETTLED WEATHER FOR THE WEST COAST WITH PERIODS OF HEAVY RAIN
AND HIGH ELEVATION MOUNTAIN SNOW...
..OVERVIEW
THE LARGE SCALE FLOW PATTERN WILL FEATURE A RAPID SUCCESSION OF
PACIFIC SYSTEMS MOVING INTO/THROUGH THE WEST AND FEEDING INTO A
MEAN UPPER TROUGH OF MODERATE DEPTH ALIGNED OVER THE
PLAINS/MISSISSIPPI VALLEY, WHILE AN UPPER RIDGE INITIALLY NEAR THE
EAST COAST DRIFTS 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. IN ADDITION A
BROAD AREA OF PRECIPITATION (MOSTLY RAIN) WILL SPREAD ACROSS THE
EASTERN HALF OF THE COUNTRY WITH SOME ENHANCED TOTALS POSSIBLE. FOR
THE MOST PART, THIS SHOULD BE A MILD PATTERN FOR THE LOWER 48 WITH
MUCH OF THE COUNTRY EXPERIENCING ABOVE NORMAL TEMPERATURES.
..GUIDANCE/PREDICTABILITY ASSESSMENT
GUIDANCE AGREES FAIRLY WELL FOR THE OVERALL PATTERN THROUGH AT
LEAST THE FIRST HALF OF THE PERIOD, WITH SOME TYPICAL EMBEDDED
DETAIL ISSUES THAT HAVE LOWER PREDICTABILITY A FEW DAYS OUT IN
TIME. OF PARTICULAR NOTE, THE 06Z GFS LOOKED EXCESSIVELY WEAK/SOUTH
FOR THE EASTERN PACIFIC SYSTEM AFFECTING THE PACIFIC NORTHWEST AS
OF EARLY SATURDAY WHILE THE NEW 12Z UKMET IS ON THE DEEP/EASTERN
SIDE OF THE SPREAD FOR LOW PRESSURE REACHING THE GREAT LAKES BY
EARLY SUNDAY. AT THAT TIME THERE IS A FAIR AMOUNT OF SPREAD FOR THE
STRUCTURE OF THE NEXT SYSTEM AFFECTING THE PACIFIC NORTHWEST.
BY LATE IN THE PERIOD, THREE PRIMARY UNCERTAINTIES ARISE. THESE
INVOLVE THE UPPER DYNAMICS/SURFACE SYSTEM REACHING THE MID-
ATLANTIC/NORTHEAST AND RELATIVE STRENGTH VERSUS THE LEADING SURFACE
RIDGE, EVOLUTION OF A TRAILING SYSTEM REACHING THE PLAINS OR EAST-
CENTRAL U.S. BY EARLY TUESDAY, AND CHARACTER OF FLOW OVER THE WEST.
DYNAMICAL AND MACHINE LEARNING (ML) SOLUTIONS ARE MIXED OVER THE
NORTHEAST, WITH ML GUIDANCE ALSO MIXED FOR HOW MUCH UPPER TROUGHING
MAY REACH THE SOUTHWEST (WITH THE GFS AND GEFS ENSEMBLES TENDING
TO BE MOST ENTHUSIASTIC AMONG THE DYNAMICAL SOLUTIONS, THOUGH THE
06Z GFS LOOKS SUSPECT WITH ITS CLOSED LOW). AS FOR THE SYSTEM IN-
BETWEEN VALID EARLY TUESDAY, GFS/ECMWF RUNS ARE RUNNING A BIT ON
THE STRONG SIDE RELATIVE TO THE ML MODEL ENVELOPE WHILE THERE IS A
MINORITY OF DYNAMICAL GUIDANCE THAT IS NOTABLY SLOWER THAN THE
PRIMARY CLUSTERING OF GFS/ECMWF RUNS AND ENSEMBLE MEANS. THE NEW
12Z ECMWF HAS TRENDED A BIT SLOWER THAN PRIOR RECENT RUNS AS WELL,
SO THIS POTENTIAL TREND WILL BE WORTH MONITORING.
A COMPOSITE OF 00Z/06Z OPERATIONAL MODEL GUIDANCE PROVIDED A GOOD
REPRESENTATION OF CONSENSUS OR INTERMEDIATE IDEAS FOR THE FIRST
HALF OF THE PERIOD, WITH SOME 06Z GEFS/00Z ECENS GUIDANCE
INCORPORATED THEREAFTER TO ACCOUNT FOR INCREASING DETAIL
UNCERTAINTIES.
..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. A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN
EFFECT FOR COASTAL WASHINGTON, OREGON, AND NORTHERN CALIFORNIA IN
THE DAY 4 EXCESSIVE RAINFALL OUTLOOK VALID FRIDAY/FRIDAY NIGHT.
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
FAVORS A DAY 5 SLIGHT RISK AREA OVER THAT REGION. THIS WAS
COORDINATED WITH THE MFR/MEDFORD OR AND EKA/EUREKA CA FORECAST
OFFICES FOR THE PREVIOUS ISSUANCE AND SUPPORT REMAINS IN THE LATEST
CYCLE OF GUIDANCE.
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 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.
DOWNSTREAM, MULTIPLE FRONTAL SYSTEMS WILL BRING RAIN AND
THUNDERSTORM CHANCES TO THE PLAINS, MISSISSIPPI VALLEY, AND
SOUTHEAST, WITH MOISTURE SPREADING INTO THE EAST LATE THIS WEEKEND
INTO EARLY NEXT WEEK. 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. CURRENTLY
THE COMBINATION OF GUIDANCE SIGNALS AND NEUTRAL TO DRY ANTECEDENT
GROUND CONDITIONS OVER THE EAST-CENTRAL STATES DO NOT APPEAR TO
SUPPORT A RISK AREA IN THE DAYS 4-5 EROS BUT WILL CONTINUE TO
MONITOR FOR ANY CHANGES. ANY WINTRY WEATHER SHOULD BE LIMITED TO
LOCATIONS CLOSE TO THE CANADIAN BORDER.
A MAJORITY OF THE CENTRAL/EASTERN U.S. SHOULD SEE WELL ABOVE NORMAL
TEMPERATURES FROM LATE THIS WEEK INTO EARLY NEXT WEEK. EXPECT THE
WARMEST ANOMALIES TO BE FOR MORNING LOWS ACROSS THE EAST-CENTRAL
PART OF THE COUNTRY FRIDAY-SUNDAY, IN THE PLUS 20-30F RANGE AND
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 HIGHS REACH 15-25F ABOVE NORMAL ON MONDAY
IN THE WARM SECTOR AHEAD OF A SYSTEM REACHING THE HIGH PLAINS BY
EARLY IN THE DAY. READINGS CLOSER TO CLIMATOLOGY WILL BROADEN
EASTWARD FROM THE WEST COAST EARLY NEXT WEEK, REACHING THE PLAINS
BY TUESDAY/NEW YEAR'S EVE. AT THAT TIME SOME HIGHS MAY DROP TO
SLIGHTLY BELOW NORMAL OVER AND NEAR THE ROCKIES.
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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