147
FOUS30 KWBC 180025
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
725 PM EST WED DEC 17 2025
DAY 1
VALID 01Z THU DEC 18 2025 - 12Z THU DEC 18 2025
THE PROBABILITY OF RAINFALL EXCEEDING FLASH FLOOD GUIDANCE IS LESS
THAN 5 PERCENT.
HURLEY
DAY 2
VALID 12Z THU DEC 18 2025 - 12Z FRI DEC 19 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL MAINLY OVER WESTERN
OREGON EXTENDING INTO A SMALL PORTION OF WASHINGTON AND
CALIFORNIA...
2000 UTC UPDATE: HAVE ADDED A 25% AREA -- WHICH IS AN ENHANCED OR
'HIGHER- END' SLIGHT RISK -- ACROSS EASTERN PORTIONS OF THE
WILLAMETTE VALLEY INTO THE WESTERN SLOPES OF THE CASCADES, WHILE
ALSO CLIPPING THE FAR EASTERN REACHES OF PORTLAND METRO ALONG WITH
THE WESTERN COLUMBIA GORGE INTO FAR SOUTHERN WA. THE 12Z GUIDANCE
CAME IN EVEN WETTER, OWING TO THE STRONG AR WHICH GIVEN BETTER
SAMPLING, THE INTENSITY OF WHICH IS COMING INTO BETTER VIEW NOW
WITHIN 24-36 HOURS OF THE MAIN EVENT. CW3E GLOBAL ENSEMBLE GUIDANCE
(GEFS AND ECENS) SHOW PEAK IVT VALUES ~1000 KG/M-S, WHILE THE
WESTERN REGION WRF MEAN PEAKS CLOSER TO 1200. GUIDANCE QPFS SHOW
WIDESPREAD 4-8" AMOUNTS WITHIN 24HRS, WITH HIGHER TOTALS DEPICTED
FROM THE CAM (PARTICULARLY IN THE 'ENHANCED' SLIGHT RISK AREA). 12Z
HREF PROBABILITIES OF QPF EXCEEDING 5" WITHIN THE DAY 2 PERIOD ARE
90% OR HIGHER WITHING THE ENHANCED SLIGHT RISK AREA, WHILE WITHIN
THE SAME AREA, PROBABILITIES OF EXCEEDING 8" PEAK BETWEEN 60-80+
PERCENT. BOTH THE HREF AND RRFS SHOW HIGH PROBABILITIES OF
0.50"+/HR RAINFALL RATES BETWEEN 16Z WED THROUGH 12Z THU (PROBS
OVER 50%; IN MANY AREAS OVER 70-80%), WITH QUITE A BIT OF OVERLAP
WITHIN THE SAME AREAS THROUGH ~03Z THU.
THEREFORE, THE POTENTIAL FOR SMALL STREAM AND/OR RIVER FLOODING
CONTINUES TO INCREASE WITH THIS EVENT (PER COLLABORATION WITH THE
NATIONAL WATER CENTER). HOWEVER THE POTENTIAL FOR FLASH FLOODING
(MORE SHORT-FUSED INUNDATION), ESPECIALLY OVER BURN SCARS, IS ALSO
INCREASING WITHIN THE ENHANCED OR 'HIGHER-END' SLIGHT RISK AREA.
NEXT SHIFT WILL EVALUATE THE POTENTIAL NEED FOR A MODERATE RISK.
HURLEY
PREVIOUS DISCUSSION BELOW...
THE NEXT IN A SERIES OF ATMOSPHERIC RIVERS WILL ARRIVE ALONG THE
WEST COAST OF NORTH AMERICA EARLY ON THURSDAY. THE THREAT FOR
SIGNIFICANT RAINFALL REMAINS ELEVATED ACROSS THE WESTERN HALF OF
OREGON WITH A FOCAL POINT FOR MAXIMUM PRECIPITATION CENTERED OVER
THE SOUTHERN CASCADES AND THE ADJACENT VALLEYS ON EITHER
SIDE...ALTHOUGH A SUBTLE NORTHWARD SHIFT WAS NOTED IN THE 17/00Z
GUIDANCE. THE KEY IN THIS THREAT IS THE PROJECTED "SPILLOVER" OF
HEAVIER PRECIP TO THE LEEWARD SIDE OF THE CASCADES DUE TO A
PRONOUNCED 700MB JET FROM THE PACIFIC LEADING TO SIGNIFICANT
PACIFIC MOISTURE BEING DRAWN INLAND. THIS IS THE AREA WHERE ARI
FORECASTS ARE HOVERING BETWEEN THE 25-100 YEAR EXCEEDANCE
INTERVALS, MATCHING THE OUTPUT FROM THE PAST SEVERAL MODEL RUNS AND
DETERMINISTIC QPF FROM WPC. AS A RESULT, DAY 2 EXCESSIVE RAINFALL
OUTLOOK WAS EXPANDED THE SLIGHT RISK NORTHWARD ACROSS PORTIONS OF
THE WASHINGTON CASCADES IN RESPONSE TO THE SUBTLE NORTHWARD SHIFT
NOTED EARLIER...WITH LITTLE REASON TO EXPAND ANY FARTHER EASTWARD.
BANN
DAY 3
VALID 12Z FRI DEC 19 2025 - 12Z SAT DEC 20 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL MAINLY OVER
PORTIONS OF NORTHWEST CALIFORNIA...
THE ATMOSPHERIC RIVER THAT CAME ON-SHORE ON THURSDAY WILL BE ON-
GOING ACROSS MUCH OF OREGON AS THE DAY 3 PERIOD BEGINS AT 12Z ON
FRIDAY. A WELL-DEFINED PLUME OF DEEP LAYER MOISTURE WILL CONTINUE
TO BE DRAWN FROM THE EASTERN NORTH PACIFIC OCEAN EASTWARD INTO
PARTS OF THE NORTHERN GREAT BASIN BY FAST WESTERLY FLOW ALOFT. IVT
VALUES AT OR ABOVE 600 UNITS IS FORECAST TO COVER MUCH OF OREGON
INITIALLY...THEN WEAKEN AS THE AXIS OF THE MOISTURE PLUME AND
ASSOCIATED PRECIPITATION SETTLES INTO NORTHERN CALIFORNIA. THE
SYSTEM LOOKS TO HAVE TRENDED A BIT MORE PROGRESSIVE ON FRIDAY THAN
SUGGESTED BY EARLIER RUNS...SO SOME OF THE QPF VALUES OVER OREGON
AND NORTHWEST CALIFORNIA HAVE COME DOWN A BIT. EVEN SO...OPTED TO
MAINTAIN A LARGE AMOUNT OF CONTINUITY FROM THE INITIAL OVERNIGHT
ISSUANCE WITH THE SLIGHT RISK AREA OVER NORTHWEST CALIFORNIA AND A
PORTION OF ADJACENT SOUTHWEST OREGON GIVEN THE POSSIBILITY OF
FURTHER WOBBLES IN THE PLACEMENT OF THE AXIS OR VARIATIONS IN MODEL
QPF. THE ONLY NOTABLE CHANGE FOR THE 20Z UPDATE WAS TO EXPAND THE
SLIGHT RISK EASTWARD AND MORE INTO THE TRINITY MOUNTAINS WHERE QPF
VALUES AND SNOW LEVELS ARE HIGH. AREA AVERAGE PRECIPITATION
AMOUNTS ARE CURRENTLY IN THE RANGE OF 1-2 INCHES WITH LOCALIZED
HIGHER AMOUNTS MAXIMA IN THE TERRAIN, SOME OF WHICH MAY FALL AS
SNOW.
SANTORELLI/BANN
DAY 1 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/94EPOINTS.TXT
DAY 2 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/98EPOINTS.TXT
DAY 3 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/99EPOINTS.TXT
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