211
FXUS02 KWBC 230703
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
203 AM EST MON DEC 23 2024
VALID 12Z THU DEC 26 2024 - 12Z MON DEC 30 2024
...UNSETTLED WEATHER FOR THE WEST COAST WITH PERIODS OF HEAVY RAIN
AND 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 AS A
RELENTLESS SERIES OF PACIFIC SYSTEMS SHOVES THE DOWNSTREAM RIDGE
ALONG. THIS WILL ALLOW PROGRESSIVE SYSTEMS TO MOVE FAIRLY QUICKLY
FOR THE MOST PART, WITH THE MAIN EXCEPTION BEING TO LOW PRESSURE
SYSTEM MOVING NORTH FROM TX INTO THE UPPER MIDWEST AHEAD OF A BROAD
BUT NEGATIVELY TILTED UPPER TROUGH. FOR THE MOST PART, THIS SHOULD
BE A MILD PATTERN FOR THE LOWER 48.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE GUIDANCE SHOWS SMALL TO MODEST DIFFERENCES EARLY ON, DRIVEN BY
THE 12Z ECMWF WANTING TO MOVE AN UPPER LOW AND UPPER TROUGH THROUGH
THE MEAN RIDGE. EVEN WITHIN THE ECMWF GUIDANCE, THERE'S A MIXED
SIGNAL FOR A PREFERENCE FOR THE CLOSED SYSTEM IT ADVERTISES ACROSS
THE NORTHEAST ON THURSDAY MORNING -- THE APEX OF THE RIDGE IS FAR
ENOUGH NORTH TO SUPPORT A PROGRESSIVE MID-LEVEL LOW AROUND THAT
LOCATION, BUT THE LOCATION AND INTENSITY OF THE MID-LEVEL POSITIVE
ANOMALY APPEARS TOO CLOSE BY. SINCE IT CAN'T BE RULED OUT
COMPLETELY, THE PREFERENCE USED FOR THE 500 HPA HEIGHTS,
FRONTS/PRESSURES/QPF/POPS LEANS TOWARDS A DETERMINISTIC BLEND OF
THE 12Z RUNS OF THE UKMET, ECMWF, CANADIAN AND THE 18Z GFS. LATER
ON, SOME 12Z ECMWF/NAEFS MEAN WAS USED TO ACCOUNT FOR STRENGTH AND
POSITION UNCERTAINTY. THE REMAINDER OF THE GRIDS STAY CLOSER TO
THE 01Z NBM.
..WEATHER/HAZARDS HIGHLIGHTS
MULTIPLE ROUNDS OF MODERATE TO HEAVY RAINFALL ARE EXPECTED TO
IMPACT THE WEST COAST LATER THIS WEEK AS MULTIPLE FRONTAL SYSTEMS
APPROACH THE COAST SOUTH OF LOWS HEADED FOR SOUTHWEST CANADA, WITH
THE SIGNAL FOR HEAVY RAINFALL BECOMING STRONGER BY THE DAY BETWEEN
THURSDAY AND NEXT SUNDAY AND EVENTUALLY CONCENTRATING NEAR THE
CALIFORNIA/OREGON BORDER. FLOODING WILL BE POSSIBLE, ESPECIALLY
WHERE BURN SCARS EXIST AND POSSIBLY NEAR THE STEEP TERRAIN OF THE
OLYMPICS AND 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 THURSDAY AND
FRIDAY.
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 WILL
FOCUS OVER THE NORTHWEST ON CHRISTMAS AND EXPAND THROUGH THE
INTERMOUNTAIN WEST/ROCKIES LATER THIS WEEK.
DOWNSTREAM, MULTIPLE FRONTAL SYSTEMS WILL BRING RAIN AND
THUNDERSTORM CHANCES TO THE PLAINS, MISSISSIPPI VALLEY, AND
SOUTHEAST. SOUTHERLY FLOW WILL BRING MOISTURE UP FROM THE GULF OF
MEXICO INTO THE ARK-LA-TEX REGION WHERE MULTIPLE ROUNDS OF RAIN ARE
EXPECTED. A MARINGAL RISK OF EXCESSIVE RAINFALL EXISTS FOR THAT
AREA ON THURSDAY/THURSDAY NIGHT.
WIDESPREAD ABOVE NORMAL/MILD TEMPERATURES BY LATE DECEMBER
STANDARDS ARE EXPECTED ACROSS THE CENTRAL U.S. FOR THE ENTIRE
FORECAST PERIOD, RANGING FROM 5 TO 20 DEGREES ABOVE AVERAGE.
RELATIVE MILDNESS SPREADS INTO THE EAST WITH TIME. THE GREATEST
POSITIVE ANOMALIES ARE LIKELY ACROSS PORTIONS OF THE UPPER MIDWEST
AND EXTENDING INTO THE OHIO VALLEY BY SATURDAY. READINGS CLOSER TO
CLIMATOLOGY ARE EXPECTED NEAR THE WEST COAST.
ROTH/DOLAN
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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