052
FXUS02 KWBC 211858
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
158 PM EST SUN DEC 21 2025
VALID 12Z WED DEC 24 2025 - 12Z SUN DEC 28 2025
...MAJOR SERIES OF ATMOSPHERIC RIVERS WITH HIGHLY ANOMALOUS
MOISTURE 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. THIS 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 POSITION
FROM THE EASTERN PACIFIC INTO THE WEST COAST. IN THIS IMPACTFUL
PATTERN, ONSHORE FLOW WILL CONTINUE TO DIRECT LEADING ATMOSPHERIC
RIVERS (ARS) AND BRING HEAVY TO SIGNIFICANT PRECIPITATION TO
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 THIS WEEK FROM THE WEST/SOUTHWEST THROUGH THE
SOUTH-CENTRAL ROCKIES/PLAINS TO THE EAST-CENTRAL U.S.. THE NORTHERN
TIER CAN SEE COLDER PERIODS WITH SNOW CHANCES AS MORE NORTHERN
STREAM SHORTWAVES AND ASSOCIATED FRONTS PASS THROUGH.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE LATEST GUIDANCE CONTINUES TO SHOW GOOD AGREEMENT THE FIRST
HALF OF THE PERIOD WITH A DEEPENING TROUGH OFF THE WEST COAST.
UNCERTAINTY INCREASES GREATLY BY NEXT WEEKEND REGARDING TIMING OF
THIS TROUGH/CLOSED LOW AS IT SHIFTS INTO THE SOUTHWEST, FINALLY
BRINGING AN END TO THE RELENTLESS ATMOSPHERIC RIVERS FOR CALIFORNIA
DURING THE PERIOD. THE GFS REMAINS THE SLOWEST TO PROGRESS INLAND
WITH THE ECMWF AND CMC FASTER. THE 12Z EC TRENDED FASTER (WITH A
DEEPER TROUGH OVER THE MIDWEST LATE PERIOD) WHILE THE 12Z CMC WENT
SLOWER (WITH A LESS AMPLIFIED TROUGH DOWNSTREAM). UNCERTAINTY IS
HIGH ON THE EXACT PATTERN EVOLUTION FOR NEXT WEEKEND ACROSS THE
CONUS, BUT THE ENSEMBLE MEANS AND AI GUIDANCE DOES OFFER SOME HELP
WITH A MORE CENTRAL LOCATION FOR THE WESTERN TROUGH. THE WPC
FORECAST TRENDED INCREASINGLY HEAVIER TOWARDS THE ENSEMBLE MEANS,
WITH IT ENCOMPASSING 70 PERCENT OF THE BLEND BY DAY 7. THIS HELPS
TO MAINTAIN CONTINUITY WITH THE PREVIOUS WPC FORECAST AS WELL.
..WEATHER/HAZARDS HIGHLIGHTS
A PREMIER STORY DURING THE PERIOD WILL FOCUS OUT WEST WITH
AMPLIFYING UPPER TROUGHING AND SYSTEM GENESIS WORKING TOWARD THE
WEST COAST AND LEAD DEEP ONSHORE FLOW DIRECTING PACIFIC MOISTURE
INTO THE WEST COAST THROUGH THE WEEK. HIGH MOISTURE ANOMALIES WITH
ATMOSPHERIC RIVERS IMPACTING NORTHERN/CENTRAL CALIFORNIA EARLY WEEK
WILL SHIFT TO CENTRAL TO SOUTHERN CALIFORNIA CHRISTMAS
EVE/CHRISTMAS BEFORE AFFECTING PARTS OF THE SOUTHWEST. BY MIDWEEK
INTO LATER WEEK AND LINGERING INTO NEXT WEEKEND, SOME CONNECTION TO
TROPICAL PACIFIC MOISTURE IS LIKELY TO STREAM IN AND INCREASINGLY
IMPACT SOUTHERN CALIFORNIA AS SURFACE LOW/FRONTAL SYSTEM APPROACHES
THE COAST. MARGINAL AND EMBEDDED SLIGHT RISK ERO THREAT CONTINUE
TO BE DEPICTED FOR WEDNESDAY/THURSDAY, STRETCHING FROM NORTH-
CENTRAL CALIFORNIA TO SOUTHERN CALIFORNIA AND THE SOUTHERN GREAT
BASIN. FOR DAY 4/WEDNESDAY, AFTER COORDINATION WITH AFFECTED WFOS
LOX AND SGX, WENT AHEAD AND UPGRADED PORTIONS OF THE SANTA BARBARA-
LOS ANGELES- TRANSVERSE RANGES TO A MODERATE RISK GIVEN OVERLAP
WITH DAY 3/TUESDAY HEAVY RAINFALL AND THE BUSY HOLIDAY PERIOD.
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 EARLY WEEK, BUT
WITH LESSER AMOUNTS. ADDITIONALLY, MOISTURE IS FORECAST TO SPILL
OVER THE CASCADES TO THE INTERMOUNTAIN WEST AND NORTHERN/CENTRAL
ROCKIES NEXT WEEK TO PRODUCE HEAVY SNOW AT HIGHER ELEVATIONS. AS
THE UPPER LOW SHIFTS INTO THE SOUTHWEST NEXT WEEKEND, RAINFALL
SHOULD WANE FROM NORTH TO SOUTH ACROSS CALIFORNIA.
ELSEWHERE, SHALLOW SHORTWAVES MOVING THROUGH THE NORTHERN TIER AND
ASSOCIATED SURFACE SYSTEM REFLECTION FOCUS WILL BRING LIGHT
PRECIPITATION FROM THE NORTH-CENTRAL TO THE OHIO VALLEY AND
NORTHEAST THIS WEEK.
WARMER THAN AVERAGE TEMPERATURES ARE FORECAST ACROSS THE INTERIOR
WEST AND CENTRAL U.S. THIS WEEK. THE HIGHEST ANOMALIES WILL LIKELY
BE ACROSS THE INTERIOR WEST INTO THE PLAINS, WITH WIDESPREAD
TEMPERATURE ANOMALIES UPWARDS TO 20-30 DEGREES ABOVE NORMAL, WITH
WIDESPREAD DAILY HIGH TEMPERATURE RECORDS POSSIBLE. MORNING LOWS
WILL BE JUST AS ANOMALOUS WITH EVEN MORE WIDESPREAD WARM MORNING
LOW RECORDS FROM THE SOUTHWEST INTO THE CENTRAL U.S. INTO THE
WEEKEND. 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 AND THE WEEKEND, BUT
WITH MODERATING ANOMALIES.
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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