657
FXUS02 KWNH 180724
PREEPD
PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
224 AM EST THU DEC 18 2025
VALID 12Z SUN DEC 21 2025 - 12Z THU DEC 25 2025
...ADDITIONAL ROUNDS OF HEAVY PRECIPITATION LIKELY FOR THE WEST
COAST...
..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 IS QUITE GOOD WITH THE OVERALL PATTERN. MINOR
DIFFERENCES IN TROUGH AND FRONTAL TIMING ACROSS THE CENTRAL AND
EASTERN U.S. ARE WELL WITHIN TYPICAL SPREAD, AS IS THE UPPER HIGH
AND RIDGE PLACEMENT. THE MORE UNCERTAIN PART OF THE PATTERN IS WITH
TROUGHING IN THE EASTERN PACIFIC THAT DEEPENS THROUGH THE WEEK.
WHILE THERE IS GENERAL AGREEMENT FOR THIS TO OCCUR, MULTIPLE
NORTHERN STREAM AND SOUTHERN STREAM VORT MAXES MAKE THE DETAILS
UNCERTAIN, WHICH COULD BE IMPACTFUL TO SENSIBLE WEATHER ASPECTS
LIKE PRECIPITATION AMOUNTS IN CALIFORNIA. INTERESTINGLY THE 00Z
ECMWF AND AI-GFS ARE BOTH A BIT NORTHWEST OF THE GFS AND EC-AIFS
WITH THE EMBEDDED UPPER LOW BY NEXT THURSDAY. IN ANY CASE, THERE DO
NOT SEEM TO BE TRUE MODEL OUTLIERS AT THIS POINT. THE WPC FORECAST
BEGAN WITH A MULTI-MODEL DETERMINISTIC BLEND EARLY IN THE PERIOD,
AND ADDED SOME ENSEMBLE MEAN GUIDANCE TO THE BLEND FOR THE LATTER
PART OF THE PERIOD TO TEMPER INDIVIDUAL MODEL DIFFERENCES.
..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. THUS WILL START THE DAY 5/MONDAY ERO WITH A
MARGINAL RISK OF EXCESSIVE RAINFALL, AWAITING BETTER CLUSTERING IN
THE GUIDANCE TO SEE IF THIS MAY RAISE TO SLIGHT RISK LEVELS, GIVEN
PRECIPITATION IS OVER MUCH THE SAME REGION AS SUNDAY. 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. 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.
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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