940
FXUS02 KWBC 240658
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
158 AM 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...
..GENERAL OVERVIEW
THE GENERAL FLOW PATTERN WILL DEAMPLIFY/BECOME MORE ZONAL WITH
TIME AS RIDGING IN THE EAST MOVES INTO THE WESTERN ATLANTIC BY
NEXT MONDAY AS A RELENTLESS SERIES OF PACIFIC SYSTEMS SHOVES THE
DOWNSTREAM RIDGE ALONG, WITH THE FLOW PATTERN EVOLVING INTO A VERY
BROAD BUT CLIMATOLOGICALLY MODEST UPPER LEVEL TROUGH ACROSS A MUCH
OF SOUTHERN CANADA AND THE LOWER 48 BY NEXT TUESDAY. THIS WILL
ALLOW PROGRESSIVE SYSTEMS TO MOVE FAIRLY QUICKLY FOR THE MOST PART,
WITH THE MAIN EXCEPTION BEING A LOW PRESSURE SYSTEM MOVING NORTH
INTO THE UPPER MIDWEST AHEAD OF A BROAD BUT NEGATIVELY TILTED UPPER
TROUGH EARLY ON. 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
THE LATEST GUIDANCE CONTINUES TO SHOW FAIRLY GOOD AGREEMENT ON THE
LARGE SCALE PATTERN, UNCERTAINTY IN THE DETAILS REMAINS. THE WINDS,
PRESSURES, FRONTS, QPF, AND TO SOME DEGREE THE POPS USED A BLEND
OF THE 12Z/18Z DETERMINISTIC GUIDANCE EARLY ON, BEFORE BLENDING IN
SOME OF THE 12Z NAEFS/12Z ECMWF ENSEMBLE MEANS TO DEAL WITH THE
DETAIL DIFFERENCES. OTHER GRIDS RELIED MORE HEAVILY ON THE 01Z
NBM. OVERALL, THIS MAINTAINED REASONABLE CONTINUITY WITH THE
PREVIOUS WPC FORECAST.
..WEATHER/HAZARDS HIGHLIGHTS
MULTIPLE ROUNDS OF MODERATE TO HEAVY RAINFALL ARE EXPECTED TO
IMPACT THE WEST COAST THROUGH THE REMAINDER OF THIS WEEK 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 ON FRIDAY AND SATURDAY
FOR FRIDAY/FRIDAY NIGHT. WHEN COMBINED WITH THE RAINFALL EXPECTED
THROUGH FRIDAY NIGHT, ENOUGH OF A QPF SIGNAL APPEARS TO BE THERE
THROUGH SATURDAY FOR A DAY 5 SLIGHT RISK AREA FOR FAR SOUTHWEST OR
AND FAR NORTHWEST CA, WHICH WAS COORDINATED WITH THE MFR/MEDFORD OR
AND EKA/EUREKA CA FORECAST OFFICES.
MOISTURE SPREADING INLAND IN THE WEST WILL LIKELY PRODUCE HEAVY
SNOW OVER THE HIGHER/HIGHEST TERRAIN OF THE CASCADES AND ROCKY
MOUNTAINS, AND PORTIONS OF THE SIERRA NEVADA. HEAVY SNOW IS
EXPECTED AT ELEVATION THROUGH SUNDAY. TIGHT PRESSURE GRADIENTS MAY
RESULT IN SOME PERIODICALLY GUSTY WINDS FOR SOME LOCATIONS AS
WELL.
DOWNSTREAM, MULTIPLE FRONTAL SYSTEMS WILL BRING RAIN AND
THUNDERSTORM CHANCES TO THE PLAINS, MISSISSIPPI VALLEY, AND
SOUTHEAST, WHICH SPREADS INTO THE EAST LATE THIS WEEKEND INTO EARLY
NEXT WEEK.
A WIDESPREAD THAW IS EXPECTED ACROSS A SIGNIFICANT PORTION OF OHIO
VALLEY AND THE SOUTHERN MID-ATLANTIC STATES, WITH HIGHS RISING INTO
THE 50S TO LOWER 60S FOR DAYS ON END. LOCATIONS IN THE PLAINS AND
MIDWEST WILL SEE TEMPERATURES 20-25F ABOVE AVERAGE IN SPOTS FRIDAY
THROUGH MONDAY, WITH SIMILAR ANOMALIES ACROSS NEW ENGLAND ON NEW
YEAR'S EVE. LOW TEMPERATURE ANOMALIES IN THE NORTH WOODS OF
MINNESOTA ARE FORECAST TO BE IMPRESSIVE, 30-32F FRIDAY AND
SATURDAY, ALBEIT STILL BELOW THE FREEZING MARK SINCE IT'S LATE
DECEMBER. READINGS CLOSER TO CLIMATOLOGY ARE INITIALLY EXPECTED
NEAR THE WEST COAST, BUT WITH THE BROADENING BUT MODEST UPPER LEVEL
TROUGH DEVELOPING EARLY NEXT WEEK, THE AREA OF NEAR AVERAGE
READINGS WILL MOVE EAST FROM THE WEST COAST INTO THE HIGH PLAINS
INTO NEW YEAR'S EVE.
ROTH/SANTORELLI
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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