316
FXUS02 KWNH 190802
PREEPD
PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 AM EST FRI DEC 19 2025
VALID 12Z MON DEC 22 2025 - 12Z FRI DEC 26 2025
...A SERIES OF ATMOSPHERIC RIVERS WITH ADDITIONAL ROUNDS OF HEAVY
RAINFALL TO IMPACT THE WEST COAST THROUGH THE PERIOD...
..OVERVIEW
PREDICTABILITY IS ABOVE AVERAGE THAT UPPER RIDGING WILL WORK OVER
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 UPPER TROUGHING. IN THIS
IMPACTFUL PATTERN, ONSHORE FLOW WILL CONTINUE TO DIRECT
ATMOSPHERIC RIVERS (ARS) TOWARDS THE WEST COAST, CAUSING HEAVY
PRECIPITATION TO CALIFORNIA IN PARTICULAR, WITH PRECIPITATION
PUSHING INTO THE INTERMOUNTAIN WEST/ROCKIES AS WELL. TEMPERATURE-
WISE, UNSEASONABLY TO POTENTIALLY SOME RECORD-BREAKING WARM
TEMPERATURES ARE FORECAST ACROSS THE SOUTHWEST AND PORTIONS OF THE
PLAINS, THOUGH THE NORTHERN TIER CAN EXPECT MODESTLY COLDER
PERIODS WITH SHORTWAVES AND FRONTAL SYSTEMS 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 THERE IS SOME DISTRIBUTION/EXACT AMOUNTS
UNCERTIANTY.
DESPITE THIS AND IN AN EFFORT TO MITIGATE SMALLER SCALE DIFFERENCES
AS CONSISTENT WITH INDIVIDUAL FEATURE UNCERTAINTIES, THE WPC
FORECAST DID UTILIZE A FULLY DETERMINISTIC GFS/ECMWF/CANADIAN/UKMET
MODEL COMPOSITE FOR DAYS 3-5 BEFORE SWITCHING TO A BLEND OF STILL
COMPATIBLE ENSEMBLE MEANS TO HELP FURTHER SMOOTH OUT DETAIL
DIFFERENCES LATE PERIOD ALONG WITH USAGE OF SOME WPC CONTINUITY.
..WEATHER/HAZARDS HIGHLIGHTS
AMPLIFYING UPPER TROUGHING AND SYSTEM GENESIS WORKING TOWARD THE
WEST COAST AND PERSISTENT LEAD ONSHORE FLOW WILL DIRECT PACIFIC
MOISTURE INTO THE WEST COAST WELL INTO NEXT WEEK, WITH THE HIGHEST
MOISTURE ANOMALIES EXPECTED WITH ATMOSPHERIC RIVERS IMPACTING
NORTHERN/CENTRAL CALIFORNIA EARLY IN THE WEEK AND EVENTUALLY
CENTRAL/SOUTHERN CALIFORNIA THROUGH CHRISTMAS EVE/CHRISTMAS.
AN AR IS FORECAST TO TAKE AIM AT NORTHERN CALIFORNIA DURING THE
SHORT RANGE PERIOD INTO SUNDAY, BRINGING HEAVY PRECIPITATION WITH
SOME HIGH RAIN RATES (1+ INCH PER 3 HRS), AND AREAS OF RAIN AMOUNTS
3"+ TOWARD COASTAL AREAS AND 5"+ TOWARD THE SIERRA NEVADA. THIS
LEADS INTO A CONTINUED WET PERIOD NEXT WEEK. THE GUIDANCE SIGNAL IS
GROWING IN SUPPORT OF A RENEWED DEEP MOISTURE FEED DAY 4/MONDAY
AND DAY 5/TUESDAY FOR NORTHERN-CENTRAL CALIFORNIA. HAVE INTRODUCED
WPC EXCESSIVE RAINFALL OUTLOOK MARGINAL RISK THREAT AREAS WITH
EMBEDDED SLIGHT RISK THREAT AREAS FOR BOTH DAYS AS PRECIPITATION
MAY PROVE SLOW TO SHIFT FROM AREAS ALREADY WITH WET CONDITIONS.
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. 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 BY NEXT MIDWEEK, SOME CONNECTION TO TROPICAL
PACIFIC MOISTURE IS LIKELY TO STREAM IN AND MAY INCREASINGLY IMPACT
SOUTHERN CALIFORNIA TO MONITOR WITH SYSTEM TRANSLATION. HIGHER
ELEVATIONS OF THE SIERRA SHOULD SEE A PROTRACTED HEAVY SNOW EVENT.
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 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
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 UPWARDS 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 OVER THE CHRISTMAS HOLIDAY PERIOD.
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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