744
FXUS02 KWBC 181925
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
225 PM EST THU DEC 18 2025
VALID 12Z SUN DEC 21 2025 - 12Z THU DEC 25 2025
...A SERIES OF ATMOSPHERIC RIVERS WITH ADDITIONAL ROUNDS OF HEAVY
RAINFALL TO IMPACT THE WEST COAST THROUGH THE PERIOD...
..OVERVIEW
A SHALLOW UPPER TROUGH IS LIKELY TO MOVE ACROSS THE EASTERN U.S.
SUNDAY WITHIN RELATIVELY ZONAL FLOW. UPSTREAM, LOW AMPLITUDE
RIDGING IS FORECAST TO MOVE THROUGH THE INTERIOR WEST AND CENTRAL
U.S. EARLY NEXT WEEK, STRENGTHENING INTO MIDWEEK WITH A SOUTH-
CENTRAL U.S. UPPER HIGH SETTING UP IN RESPONSE TO DEEPENING EASTERN
PACIFIC TROUGHING. ONSHORE FLOW WILL CONTINUE TO DIRECT
ATMOSPHERIC RIVERS (ARS) TOWARDS THE WEST COAST, CAUSING HEAVY
PRECIPITATION TO CALIFORNIA IN PARTICULAR, WITH SOME PRECIPITATION
PUSHING INTO THE INTERMOUNTAIN WEST/ROCKIES AS WELL. TEMPERATURE-
WISE, UNSEASONABLY TO POTENTIALLY RECORD-BREAKING WARM TEMPERATURES
ARE FORECAST ACROSS THE SOUTHWEST AND PORTIONS OF THE PLAINS,
THOUGH THE NORTHERN TIER CAN EXPECT COLDER PERIODS WITH SHORTWAVES
MOVING THROUGH.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL AGREEMENT ACROSS THE CONUS REMAINS IN GOOD AGREEMENT, AT
LEAST SYNOPTICALLY. THERE ARE SOME MINOR/LESS IMPACTFUL
DIFFERENCES IN TROUGH AND FRONTAL TIMING ACROSS THE CENTRAL TO
EASTERN U.S. EARLY WEEK. THE MOST UNCERTAIN AND IMPACTFUL PART OF
THE PATTERN IS WITH TROUGHING THAT DEEPENDS OFF THE EASTERN PACIFIC
AS IT MOVES TOWARDS CALIFORNIA. THE MODELS DO AGREE THAT THIS WILL
OCCUR, BUT THE DETAILS OF ENERGIES WITHIN THE TROUGH AND VORT MAXES
TOWARDS THE COAST WILL CREATE CONSIDERABLE IMPACTS TO SENSIBLE
WEATHER ASPECTS. THERE CONTINUES TO BE A SIGNAL FOR A SERIES OF ARS
AND HEAVY RAINFALL SHIFTING SOUTHWARD ALONG THE CALIFORNIA COAST
WITH TIME, BUT THE DISTRIBUTION AND EXACT AMOUNTS REMAIN VERY MUCH
IN FLUX.
DESPITE THIS, THE WPC FORECAST DID UTILIZE A FULLY DETERMINISTIC
MODEL BLEND (MORE WEIGHTED TOWARDS THE ECMWF) FOR DAYS 3-5. BLENDED
IN SOME OF THE ENSEMBLE MEANS TO HELP SMOOTH OUT THE DETAIL
DIFFERENCES LATE PERIOD, STILL MAINTAINING DETERMINISTIC MODELS AS
HALF THE BLEND. OVERALL, THIS MAINTAINED GOOD AGREEMENT WITH THE
PREVIOUS WPC FORECAST AS WELL.
..WEATHER/HAZARDS HIGHLIGHTS
PERSISTENT ONSHORE FLOW WILL DIRECT PACIFIC MOISTURE INTO THE WEST
COAST INTO NEXT WEEK, WITH THE HIGHEST MOISTURE ANOMALIES EXPECTED
WITH ARS IMPACTING NORTHERN CALIFORNIA EARLY IN THE WEEK AND
EVENTUALLY SOUTHERN CALIFORNIA INTO MIDWEEK. AN AR OF MODERATE
STRENGTH IS FORECAST TO TAKE AIM AT NORTHERN CALIFORNIA AS THE
PERIOD BEGINS SUNDAY, BRINGING HEAVY PRECIPITATION WITH REASONABLY
HIGH RAIN RATES (1+ INCH PER 3 HRS), AND AREAS OF RAIN AMOUNTS 3"+
TOWARD COASTAL AREAS AND 5"+ TOWARD THE SIERRA NEVADA. A SLIGHT
RISK REMAINS IN EFFECT IN THE DAY 4/SUNDAY ERO FOR SCATTERED
FLOODING POTENTIAL, ESPECIALLY AFTER A FEW DAYS OF MODERATE TO
HEAVY RAIN IN THE SAME VICINITY. MODEL GUIDANCE VARIES ON HOW
STRONG THE INTEGRATED VAPOR TRANSPORT REMAINS INTO MONDAY IN
NORTHERN CALIFORNIA. AT LEAST SOME MODERATE ADDITIONAL
PRECIPITATION IS LIKELY, BUT THERE LOOKS TO BE A GENERAL DOWNWARD
TREND IN RAIN AMOUNTS SO PLAN TO HOLD AT JUST A MARGINAL RISK FOR
NOW ON THE DAY 5/MONDAY ERO. WILL WAIT FOR BETTER CLUSTERING IN
THE GUIDANCE TO SEE IF THIS MAY EVENTUALLY NECESSITATE A SLIGHT
RISK, CONSIDERING PRECIPITATION IS OVER MUCH THE SAME REGION AS
SUNDAY. TO THE NORTH, THE PACIFIC NORTHWEST CAN ALSO EXPECT SOME
PRECIPITATION SUNDAY AND MONDAY, BUT WITH LESSER AMOUNTS AND
INTENSITIES. ADDITIONALLY, MOISTURE IS FORECAST TO SPILL OVER INTO
THE INTERMOUNTAIN WEST AND NORTHERN/CENTRAL ROCKIES, WHICH COULD
PRODUCE HEAVY SNOW AT HIGHER ELEVATIONS WITH SUFFICIENTLY COLD
TEMPERATURES. BY TUESDAY THE PATTERN CHANGES A BIT TO A MORE
SOUTHWESTERLY FLOW ORIENTATION, BUT STILL AFFECTING CALIFORNIA WITH
MODERATE TO LOCALLY HEAVY PRECIPITATION AS A SURFACE LOW APPROACHES
THE CA COAST. PARTICULARLY INTO WEDNESDAY, SOME CONNECTION TO
TROPICAL PACIFIC MOISTURE IS LIKELY TO STREAM IN AND AFFECT
SOUTHERN CALIFORNIA AS WELL.
SHALLOW SHORTWAVES MOVING THROUGH THE NORTHERN TIER COULD BRING
LIGHT PRECIPITATION ACROSS THE NORTH-CENTRAL AND NORTHEASTERN U.S.
THROUGH THE EARLY PART OF NEXT WEEK, INCLUDING SOME LIGHT SNOW.
FARTHER SOUTH, SOME WEAK TO MODERATE GULF MOISTURE MAY STREAM INTO
FRONTAL SYSTEMS ACROSS THE SOUTH AND EAST-CENTRAL U.S. AND LEAD TO
PRECIPITATION (MOSTLY RAIN) CHANCES ACROSS THE LOWER MISSISSIPPI,
TENNESSEE, AND OHIO VALLEYS FOR EARLY TO MID NEXT WEEK.
WARMER THAN AVERAGE TEMPERATURES ARE FORECAST ACROSS THE INTERIOR
WEST AND CENTRAL U.S. THROUGH MUCH OF THE PERIOD. THE HIGHEST
ANOMALIES WILL LIKELY BE ACROSS THE INTERIOR WEST INTO THE PLAINS
MOST DAYS, WITH WIDESPREAD TEMPERATURE ANOMALIES OF 20-30 DEGREES
ABOVE NORMAL. HIGHS IN THE 70S AND 80S ACROSS THE SOUTHWEST TO
TEXAS COULD SET DAILY RECORDS. UNSEASONABLE WARMTH IS LIKELY TO
SPREAD EAST INTO THE MISSISSIPPI, OHIO, AND TENNESSEE VALLEYS BY
CHRISTMAS DAY. MEANWHILE, THE MIDWEST/GREAT LAKES IS FORECAST TO
SEE A SHOT OF COLDER THAN AVERAGE TEMPERATURES SUNDAY, SHIFTING
INTO THE EAST BY MONDAY UNDERNEATH UPPER TROUGHING. THESE AREAS
SHOULD GENERALLY MODERATE CLOSER TO AVERAGE AS THE WEEK PROGRESSES.
SANTORELLI/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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