518
FXUS02 KWBC 130800
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 AM EST SAT DEC 13 2025
VALID 12Z TUE DEC 16 2025 - 12Z SAT DEC 20 2025
...MULTIPLE ATMOSPHERIC RIVERS WILL IMPACT THE PACIFIC NORTHWEST NEXT
WEEK...
..OVERVIEW
NEXT WEEK, THE PATTERN ALOFT WILL MAINLY BE TRANSITIONING TO
QUASI-ZONAL FLOW OVER THE LOWER 48, THOUGH WITH MULTIPLE SHORTWAVES
CAUSING POTENTIALLY IMPACTFUL WEATHER. ROUNDS OF ENERGY WILL
DIRECT ATMOSPHERIC RIVERS INTO THE PACIFIC NORTHWEST, CAUSING
ADDITIONAL CONCERNS FOR HEAVY RAIN AND FLOODING, ALONG WITH
MOUNTAIN SNOW. SHORTWAVES AND FRONTAL SYSTEMS CONTINUING EAST COULD
LEAD TO AT LEAST MODEST PRECIPITATION CHANCES ACROSS THE NORTHERN
TIER AS WELL AS TO THE MIDWEST/MISSISSIPPI VALLEY TOWARDS MIDWEEK
AND TOWARD THE EASTERN U.S. THURSDAY. TEMPERATURE-WISE, A WARMING
TREND IS LIKELY ACROSS MOST OF THE CONUS GIVEN THE FLATTENING
UPPER-LEVEL PATTERN.
..GUIDANCE/PREDICTABILITY ASSESSMENT
SHORTWAVES WILL DOMINATE THE POTENTIAL FOR IMPACTFUL WEATHER AS
THE MEDIUM RANGE PERIOD BEGINS TUESDAY. GUIDANCE IS IN REASONABLE
AGREEMENT WITH A NORTHERN TIER SHORTWAVE MOVING FROM THE NORTHERN
HIGH PLAINS INTO THE GREAT LAKES TUESDAY-WEDNESDAY, ASIDE FROM THE
12Z CMC BEING A LITTLE FASTER/SHALLOWER, AND A WEAK SOUTHERN STREAM
TROUGH POSITIVELY TILTED ACROSS THE SOUTHERN PLAINS/ROCKIES IS
ALSO REASONABLY AGREEABLE. THE WPC FORECAST USED A MULTI-MODEL
BLEND FAVORING THE 12Z ECMWF FOR THE EARLY PERIOD.
THERE ARE INCREASING DETAIL DIFFERENCES IN THE MODELS AFTER
MIDWEEK THAT COULD HAVE SENSIBLE WEATHER IMPACTS. MODELS DO SHOW
ANOTHER SHORTWAVE MOVING ACROSS THE U.S./CANADA BORDER ALONG WITH A
SURFACE LOW, ATOP THE NORTHWEST WEDNESDAY AND DEEPENING/BROADENING
A BIT MORE INTO A NOTABLE TROUGH MOVING ACROSS THE MIDWEST AND
THEN THE EAST LATER NEXT WEEK. NON-NCEP MODELS AND EVEN THE AI-GFS
HAVE SHOWN THIS PATTERN FOR A COUPLE OF DAYS NOW, WHILE GFS RUNS
HAVE BEEN SHALLOWER. HOWEVER, THE 18Z AND NEW 00Z GFS RUNS ARE
GRADUALLY TRENDING TOWARD THE BETTER CONSENSUS FOR THE TIMING AND
DEPTH OF THIS TROUGH, WHILE IT IS POSSIBLE THE ECMWF MAY BE TOO
AMPLIFIED COMPARED TO THE MIDDLE GROUND EC-AIFS. THUS A BLENDED
SOLUTION SEEMS MORE REASONABLE NOW. ADDITIONAL ENERGIES MOVING
ACROSS THE NORTHEAST PACIFIC AND ACROSS THE NORTHERN U.S./SOUTHERN
CANADA ALSO SHOW SPREAD, AND THE NEW 00Z CMC BECOMES OUT OF PHASE
WITH THE PATTERN BY NEXT SATURDAY IN THE NORTH-CENTRAL U.S. DUE TO
THESE DIFFERENCES. THE WPC FORECAST TRENDED TOWARD USING MORE
ENSEMBLE MEANS IN THE FORECAST AS THE PERIOD PROGRESSED, WITH THE
MEANS COMPRISING HALF THE BLEND DAYS 6 AND 7 TO TEMPER INDIVIDUAL
MODEL DIFFERENCES.
..WEATHER/HAZARDS HIGHLIGHTS
INTO TUESDAY, ANOTHER ATMOSPHERIC RIVER IS LIKELY TO BE TAKING AIM
AT THE NORTHWEST. THE CORE OF THE IVT WILL BE ORIENTED TOWARD
NORTHERN CALIFORNIA FOR THE LONGEST DURATION, WHICH HAS NOT SEEN AS
MUCH RAIN IN RECENT DAYS AS WASHINGTON/OREGON AND IS THUS NOT AS
SENSITIVE TO HEAVY RAIN. MEANWHILE A COUPLE ROUNDS OF MORE
TRANSIENT IVT ARE EXPECTED FARTHER NORTH INTO THE PACIFIC
NORTHWEST. THIS REGION HAS MUCH MORE SENSITIVE ANTECEDENT
CONDITIONS, BUT A COLDER REGIME COMPARED TO THE RECENT EVENT WILL
MEAN MORE OF THE PRECIPITATION FALLS AS SNOW ACROSS MUCH OF THE
CASCADES. ADDITIONALLY, THIS MOISTURE PLUME SHOULD BE ORIENTED MORE
SOUTHWESTERLY RATHER THAN COMPLETELY ORTHOGONAL TO TERRAIN. A
MARGINAL RISK IS IN PLACE FOR DAY 4/TUESDAY ERO STRETCHING ACROSS
NORTHERN CALIFORNIA INTO THE PACIFIC NORTHWEST. CONTINUE TO MONITOR
IF A SLIGHT RISK IS NEEDED, BUT GIVEN LIMITING FACTORS FOR
FLOODING IN BOTH REGIONS, WILL CONTINUE TO HOLD OFF. WEDNESDAY
SHOULD SEE A RELATIVE BREAK IN QPF OVERALL. HOWEVER, SOME LINGERING
RAIN IS FORECAST IN THE PACIFIC NORTHWEST AND COULD LEAD TO
ISOLATED FLOODING CONCERNS GIVEN THE SENSITIVITY. THUS A LOW END
MARGINAL RISK IS IN PLACE FOR DAY 5/WEDNESDAY FOR NORTHWEST OREGON
INTO WESTERN WASHINGTON. YET ANOTHER AR SEEMS LIKELY TO IMPACT THE
NORTHWEST BY THURSDAY, WHICH COULD BE STRONG, SO ADDITIONAL
FLOODING WILL BE A CONCERN. THE IVT PLUME IS LIKELY TO DRIFT
GRADUALLY SOUTH TOWARD NORTHERN CALIFORNIA ON FRIDAY.
AS IMPULSES MOVE OUT OF THE NORTHWEST ACROSS THE NORTHERN TIER,
RAIN AND SNOW COULD SPREAD ACROSS THE NORTHERN PLAINS LATE TUESDAY
INTO WEDNESDAY AND INTO THE MIDWEST AND GREAT LAKES REGION
WEDNESDAY-THURSDAY. FARTHER SOUTH, SOME GULF MOISTURE RETURN IS
POSSIBLE FOR LIGHT TO MODERATE RAIN ACROSS EASTERN TEXAS INTO THE
LOWER AND MIDDLE MISSISSIPPI VALLEY FOR TUESDAY AND WEDNESDAY. BY
THURSDAY THESE SYSTEMS ARE LIKELY TO INTERACT AND SPREAD BROADENING
PRECIPITATION CHANCES ACROSS THE EASTERN THIRD OF THE U.S. MOST OF
THIS PRECIPITATION IS LIKELY TO BE RAIN EXCEPT IN THE GREAT LAKES
TO INTERIOR NORTHEAST. MOST PRECIPITATION SHOULD BE PULLING AWAY ON
FRIDAY, BUT WITH SOME LAKE EFFECT PRECIPITATION POSSIBLE BEHIND
THE MAIN SYSTEM.
COLDER THAN AVERAGE TEMPERATURES ARE LIKELY TO PERSIST INTO
TUESDAY ACROSS EASTERN PORTIONS OF THE COUNTRY, BUT WITH LESSER
ANOMALIES THAN THE COLD SHORT RANGE PERIOD. LOW TEMPERATURES IN THE
20S ARE STILL EXPECTED FOR THE SOUTHEAST THOUGH, 10-20 DEGREES
BELOW AVERAGE. MEANWHILE WARMER THAN AVERAGE CONDITIONS ARE
FORECAST FOR MOST OF THE WEST INTO THE PLAINS. THESE WARMER THAN
AVERAGE TEMPERATURES SHOULD GRADUALLY EXPAND EASTWARD, REACHING THE
EAST ON THURSDAY FOR A WARMER THAN AVERAGE PERIOD, UNTIL ANOTHER
COLD FRONT PASSES FOR LATE WEEK. THE HIGHEST ANOMALIES WILL LIKELY
BE ACROSS THE HIGH PLAINS MOST DAYS, AS HIGHS IN THE 60S EXPAND
INTO WESTERN NEBRASKA AND SOUTH DAKOTA. BY SATURDAY TEMPERATURES IN
MUCH OF TEXAS COULD REACH 80F.
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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