067
FXUS02 KWBC 140804
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
304 AM EST SUN DEC 14 2025
VALID 12Z WED DEC 17 2025 - 12Z SUN DEC 21 2025
...ANOTHER ATMOSPHERIC RIVER WILL IMPACT THE PACIFIC NORTHWEST
PARTICULARLY THURSDAY...
..OVERVIEW
AS THE MEDIUM RANGE PERIOD BEGINS MIDWEEK, QUASI-ZONAL FLOW WILL
BE IN PLACE ACROSS THE LOWER 48 WITH QUICKLY MOVING EMBEDDED
SYSTEMS, INCLUDING A COUPLE OF SHALLOW SHORTWAVES OVER THE GREAT
LAKES AND THE NORTHWEST. THE NORTHWEST SHORTWAVE IS FORECAST TO
BROADEN/DEEPEN AS IT MOVES EAST INTO THE NORTH-CENTRAL U.S.
THURSDAY AND INTO THE EAST BY FRIDAY ACCOMPANIED BY A SURFACE LOW
PRESSURE SYSTEM. UPSTREAM, STRONG WESTERLY FLOW WILL FORCE ANOTHER
ATMOSPHERIC RIVER (AR) INTO THE NORTHWEST/NORTHERN CALIFORNIA BY
THURSDAY. ADDITIONAL FLOODING IS POSSIBLE ESPECIALLY IN AREAS THAT
HAVE SEEN EXCESSIVE RAINFALL RECENTLY, AND HEAVY SNOW IS LIKELY IN
THE HIGHER TERRAIN. TEMPERATURE-WISE, THE FLAT FLOW WILL ALLOW FOR
ABOVE AVERAGE TEMPERATURES FOR MOST OF THE CONUS, THOUGH THE
NORTHERN TIER COULD SEE COLDER PERIODS WITH SHORTWAVES MOVING
THROUGH.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE QUASI-ZONAL PATTERN MIDWEEK INTO NEXT WEEKEND WITH EMBEDDED
SHORTWAVES IN FAST FLOW LEADS TO SOMEWHAT LOWER CONFIDENCE IN THE
DETAILS OF THE PATTERN THAT COULD HAVE SENSIBLE WEATHER IMPACTS,
THOUGH THE BROAD PATTERN REMAINS REASONABLY AGREEABLE. AS THE
AFOREMENTIONED SHORTWAVE TRACKS QUICKLY ACROSS THE NORTHERN TIER,
MOST DETERMINISTIC AND AI MODELS SHOW REASONABLE CONSENSUS--ASIDE
FROM THE 12Z UKMET, WHICH WAS CONSIDERED AN OUTLIER FOR ITS DEEPER
AND SLOWER SOLUTION WITH THE SHORTWAVE. AT THE SURFACE GFS RUNS
HAVE BEEN A BIT SLOW WITH THE LOW PRESSURE SYSTEM NEAR THE CANADIAN
BORDER. THUS FOR THE EARLY PART OF THE PERIOD, USED A MULTI-MODEL
BLEND FAVORING THE ECMWF.
THERE IS GOOD MODEL AGREEMENT IN THE ATMOSPHERIC RIVER COMING INTO
THE PACIFIC NORTHWEST ON THURSDAY, BUT THE EXACT ORIENTATION WILL
MAKE A DIFFERENCE IN HOW MUCH OVERLAP OF HEAVY RAIN THERE IS WITH
THE WETTEST ANTECEDENT CONDITIONS OF THE RECENT STRONG AR. 00Z EC-
AIFS AND AIGFS ARE SHOWING A NORTHERN TREND REACHING SOUTHERN
WASHINGTON AND NORTHERN OREGON FOR MORE OVERLAP, WHILE THE 00Z
ECMWF AND GFS ALSO TRENDED A LITTLE NORTH AT LEAST OUT OF NORTHERN
CALIFORNIA, SO WILL CONTINUE TO MONITOR EVEN THESE RELATIVELY SMALL
MODEL DIFFERENCES THAT COULD BE IMPACTFUL.
BEHIND THIS AR, SHALLOW TROUGHING SHOWS SOME TIMING DIFFERENCES
THAT ARE HARD TO PICK OUT WITH THE FAST OVERALL FLOW. THE 12Z CMC
BECAME OUT OF PHASE BY LATE WEEK, BUT THE 00Z CMC APPEARS TO ALIGN
BETTER WITH THE OTHER GUIDANCE. OVERALL, INCREASED THE PROPORTION
OF ENSEMBLE MEANS IN THE MODEL BLEND TO ABOUT HALF DAY 6 AND MORE
DAY 7 GIVEN THE INCREASING SPREAD.
..WEATHER/HAZARDS HIGHLIGHTS
THE PACIFIC NORTHWEST CAN EXPECT A RELATIVE BREAK IN THE HEAVY
PRECIPITATION ON WEDNESDAY. HOWEVER, SOME LINGERING RAIN IS
FORECAST IN THE PACIFIC NORTHWEST, AND THE NEXT AR MAY START TO
AFFECT THE REGION OVERNIGHT WEDNESDAY. THIS COULD LEAD TO ISOLATED
FLOODING CONCERNS GIVEN THE SENSITIVE ANTECEDENT CONDITIONS, SO A
LOW END MARGINAL RISK IS IN PLACE FOR THE DAY 4/WEDNESDAY-WEDNESDAY
NIGHT ERO FOR NORTHWEST OREGON INTO WESTERN WASHINGTON. THE NEXT
AR IS FORECAST TO IMPACT THE NORTHWEST ON THURSDAY. AS DISCUSSED
ABOVE, THE EXACT ORIENTATION AND NORTH-SOUTH AXIS OF THE AR WILL
MAKE A DIFFERENCE IN THE IMPACTS, BECAUSE OF THE POTENTIAL FOR
OVERLAP WITH THE WET ANTECEDENT CONDITIONS WITH A NORTHERN AR, OR
LESS OVERLAP IF THE AR IS FARTHER SOUTH. BUT THE MAGNITUDE OF THE
IVT AND GLOBAL MODELS SHOWING EASILY 5-7" OF QPF GIVES SUPPORT FOR
A SLIGHT RISK IN THE NEW DAY 5/THURSDAY ERO FOR WESTERN OREGON INTO
SOUTHWESTERN WASHINGTON FOR THE FLOODING POTENTIAL. MEANWHILE,
HEAVY SNOW IS LIKELY INTO THE CASCADES AND INTO THE NORTHERN
ROCKIES WITH THE AMPLE MOISTURE COMBINING WITH SUFFICIENTLY COLD
TEMPERATURES. THE IVT PLUME IS LIKELY TO DRIFT GRADUALLY SOUTH
TOWARD NORTHERN CALIFORNIA ON FRIDAY AND THEN LESSEN SATURDAY.
AS IMPULSES MOVE OUT OF THE NORTHWEST ACROSS THE NORTHERN TIER,
RAIN AND SNOW COULD SPREAD ACROSS THE NORTHERN PLAINS 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 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. LIGHT TO
MODERATE PRECIPITATION MAY GET RENEWED ACROSS THE OHIO VALLEY BY
SATURDAY, THOUGH THIS IS UNCERTAIN.
WARMER THAN AVERAGE TEMPERATURES ARE FORECAST ACROSS THE WESTERN
AND CENTRAL U.S. THROUGH MUCH OF THE PERIOD. THE HIGHEST ANOMALIES
WILL LIKELY BE ACROSS THE HIGH PLAINS MOST DAYS, AS HIGHS IN THE
60S EXPAND INTO WESTERN NEBRASKA AND SOUTH DAKOTA ON WEDNESDAY AND
AGAIN ON FRIDAY (AFTER A COLD FRONT BRINGS BRIEFLY COOLER
TEMPERATURES THURSDAY. BY SATURDAY TEMPERATURES IN MUCH OF TEXAS
COULD REACH THE UPPER 70S TO LOW 80S. THE EASTERN U.S. CAN EXPECT
MORE TRANSIENT ABOVE AVERAGE TEMPERATURES AROUND THURSDAY AND
PERHAPS AGAIN INTO NEXT WEEKEND, BUT WITH PERIODS OF NEAR NORMAL
TEMPERATURES IN BETWEEN. MEANWHILE THE NORTHERN TIER IS FORECAST TO
SEE SHOTS OF COLD AIR WITH SHORTWAVES AND COLD SURFACE HIGHS
BEHIND COLD FRONTS.
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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