571
FXUS02 KWBC 201902
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
202 PM EST SAT DEC 20 2025
VALID 12Z TUE DEC 23 2025 - 12Z SAT DEC 27 2025
...MAJOR SERIES OF ATMOSPHERIC RIVERS TO FUEL ROUNDS OF HEAVY
RAINFALL TO IMPACT THE WEST COAST, BUT ESPECIALLY CALIFORNIA
THROUGH THE HOLIDAYS...
..OVERVIEW
AN AMPLIFIED UPPER RIDGE WILL SHIFT FROM THE INTERIOR WEST/ROCKIES
THROUGH THE EAST-CENTRAL U.S. NEXT WEEK AND DEVELOP A SOUTH-
CENTRAL U.S. UPPER HIGH. THIS WILL OCCUR IN RESPONSE TO A SLOW BUT
STEADY EASTWARD TRANSLATION OF A DEEPENED UPPER TROUGH FROM THE
EASTERN PACIFIC INTO THE WEST COAST. IN THIS IMPACTFUL PATTERN,
ONSHORE FLOW WILL CONTINUE TO DIRECT LEADING ATMOSPHERIC RIVERS
(ARS) TO FUEL ROUNDS OF HEAVY PRECIPITATION AFFECTING CALIFORNIA IN
PARTICULAR, BUT WITH WIDESPREAD ACTIVITY FROM THE WEST COAST TO
THE INTERMOUNTAIN WEST/ROCKIES. TEMPERATURE-WISE, UNSEASONABLY TO
POTENTIALLY RECORD-BREAKING WARM TEMPERATURES ARE FORECAST TO
SPREAD NEXT WEEK FROM THE WEST/SOUTHWEST THROUGH THE SOUTH-CENTRAL
ROCKIES/PLAINS TO THE EAST-CENTRAL U.S., THOUGH THE NORTHERN TIER
STATES OUT THROUGH NORTHERN NEW ENGLAND CAN EXPECT COOLER PERIODS
WITH SOME SNOW CHANCES AS SHORTWAVES/FRONTAL SYSTEMS WORK THROUGH.
..GUIDANCE/PREDICTABILITY ASSESSMENT
LATEST GUIDANCE CONTINUES TO SHOW GOOD AGREEMENT ON THE LARGE
SCALE PATTERN THROUGH MUCH OF THE PERIOD, BUT SOME UNCERTAINTY ON
THE DETAILS. THE MOST UNCERTAIN PART OF THE FORECAST, AND ALSO THE
MOST IMPACTFUL, IS WITH UPPER TROUGHING THAT DEEPENS OFF THE
PACIFIC COAST AS IT MOVES TOWARDS CALIFORNIA AND POSSIBLY ONSHORE
BY LATE WEEK. 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 COUPLE OF ARS DURING THE
PERIOD AND HEAVY RAINFALL SHIFTING SOUTHWARD ALONG THE CALIFORNIA
COAST, BUT THERE IS SOME DISTRIBUTION/EXACT AMOUNTS UNCERTAINTY.
IMPACTFUL TIMING DIFFERENCES EMERGE BY FRIDAY, WITH SOME RECENT
MODEL RUNS SHOWING CLOSED LOW DEVELOPMENT NEAR CALIFORNIA AND
ISSUES WITH HOW QUICKLY THE ENERGY MOVES INLAND. THE ECMWF WAS A
BIT FASTER THAN CONSENSUS, BUT THE GFS REMAINS SLOW. THE AI
GUIDANCE IS VARIED TOO, SO PREFER THE ENSEMBLE MEANS AT THIS POINT
WHICH ARE CLOSER TO CONSENSUS AND IN THE MIDDLE OF THE SPREAD
(THOUGH LIKELY TOO WEAK).
THE WPC FORECAST FOR TODAY USED A GENERAL BLEND OF THE
DETERMINISTIC MODELS DAYS 3 AND 4, GRADUALLY INCREASING WEIGHTING
OF THE ENSEMBLE MEANS FROM DAY 5 AND ONWARD. THIS HELPS TO MITIGATE
SOME OF THE DETAIL DIFFERENCES IN THE MODELS AND ALSO TO MAINTAIN
CONTINUITY WITH THE PREVIOUS SHIFT.
..WEATHER/HAZARDS HIGHLIGHTS
THE BIGGEST STORY DURING THE PERIOD WILL FOCUS OUT WEST WITH
AMPLIFYING UPPER TROUGHING AND SYSTEM GENESIS WORKING TOWARD THE
WEST COAST AND PERSISTENT LEAD ONSHORE FLOW DIRECTING PACIFIC
MOISTURE INTO THE WEST COAST THROUGH NEXT WEEK. HIGH MOISTURE
ANOMALIES ARE EXPECTED WITH ATMOSPHERIC RIVERS IMPACTING
NORTHERN/CENTRAL CALIFORNIA EARLY IN THE WEEK AND EVENTUALLY
CENTRAL/SOUTHERN CALIFORNIA THROUGH CHRISTMAS EVE/CHRISTMAS.
ATMOSPHERIC RIVERS WILL FUEL A CONTINUED WET PERIOD FOR ESPECIALLY
CALIFORNIA DURING THE UPCOMING BUSY HOLIDAY WEEK. THE GUIDANCE
SIGNAL IS GROWING IN SUPPORT OF A RENEWED DEEP MOISTURE FEED EARLY-
MID NEXT WEEK FOR NORTHERN-CENTRAL CALIFORNIA, WITH HIGH RAIN RATES
(1+ INCH PER 3 HOURS) AND AREAL AVERAGED RAINFALL OF 3"+ TOWARD
COASTAL AREAS AND 5"+ TOWARDS THE SIERRA NEVADA. A MARGINAL RISK
WITH EMBEDDED SLIGHT RISK IS STILL HIGHLIGHTED ACROSS THIS REGION
FOR THE DAY 4/TUESDAY EXCESSIVE RAINFALL OUTLOOK, AND UPGRADES ARE
NOT OUT OF THE QUESTION AS PRECIPITATION PROVES SLOW TO SHIFT FROM
AREAS WITH ALREADY WET CONDITIONS DUE TO SHORT RANGE RAINFALL. BY
ABOUT MIDWEEK AND INTO MID-LATER NEXT WEEK, SOME CONNECTION TO
TROPICAL PACIFIC MOISTURE IS LIKELY TO STREAM IN AND INCREASINGLY
IMPACT SOUTHERN CALIFORNIA AS A SURFACE LOW APPROACHES THE COAST.
MARGINAL AND SLIGHT RISK EROS ARE IN PLACE FOR WEDNESDAY AS WELL,
STRETCHING INTO SOUTHERN CALIFORNIA AND PARTS OF THE SOUTHERN GREAT
BASIN. SIGNIFICANT RAINFALL IS EXPECTED, PARTICULARLY FOR THE
TRANSVERSE RANGES OF SOUTHERN CALIFORNIA. HIGHER ELEVATIONS OF THE
SIERRA SHOULD SEE AN EXTENDED HEAVY SNOW EVENT. TO THE NORTH, THE
PACIFIC NORTHWEST CAN ALSO EXPECT ORGANIZED PRECIPITATION INTO
EARLY NEXT WEEK, BUT WITH LESSER AMOUNTS AND INTENSITIES.
ADDITIONALLY, MOISTURE IS FORECAST TO SPILL OVER THE CASCADES AND
INTO THE INTERMOUNTAIN WEST AND NORTHERN/CENTRAL ROCKIES NEXT WEEK
TO PRODUCE HEAVY SNOW AT HIGHER ELEVATIONS.
SHALLOW SHORTWAVES MOVING THROUGH THE NORTHERN TIER AND ASSOCIATED
SURFACE SYSTEM REFLECTION FOCUS WILL BRING LIGHT PRECIPITATION
ACROSS THE NORTH-CENTRAL AND NORTHEASTERN U.S. THROUGH THE EARLY
PART OF NEXT WEEK, INCLUDING SOME LIGHT-MODERATE SNOWS. FARTHER
SOUTH, SOME WEAK TO MODERATE GULF MOISTURE MAY STREAM INTO TRAILING
FRONTAL SYSTEMS ACROSS THE SOUTH AND EAST-CENTRAL U.S. AND LEAD TO
LIGHT 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,
WITH WIDESPREAD TEMPERATURE ANOMALIES UPWARDS TO 20-30 DEGREES
ABOVE NORMAL. HIGHS INTO THE 80S ACROSS PARTS OF THE SOUTHWEST TO
TEXAS COULD SET DAILY RECORDS. UNSEASONABLE WARMTH IS LIKELY TO
SPREAD EAST ACROSS THE MID-LATITUDES TO THE MISSISSIPPI, OHIO, AND
TENNESSEE VALLEYS AND THE EAST OVER THE CHRISTMAS HOLIDAY PERIOD.
SANTORELLI/SCHICHTEL
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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