607
FOUS30 KWBC 180832
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
332 AM EST THU DEC 18 2025
DAY 1
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 THE
NORTHWEST CORNER OF CALIFORNIA...
A WELL DEFINE PLUME OF DEEP MOISTURE WILL CONTINUE TO APPROACH THE
COAST AND THEN PROGRESS INLAND BRINGING WIDESPREAD COVERAGE OF
MODERATE OR GREATER RAINFALL TO PORTIONS OF WESTERN OREGON AND A
SMALL PART OF ADJACENT WASHINGTON AND THE NORTHWEST CORNER OF
CALIFORNIA. AFTER SEVERAL RUNS WHERE THE GUIDANCE HAS SHOWN A
SUBTLE NORTHWARD SHIFT OF THE MOISTURE PLUME AND ASSOCIATED
RAINFALL...THERE WAS SUBTLE SHIFT SOUTHWARD IN THE LATEST SUITE OF
GUIDANCE NECESSITATING A SMALL SHIFT IN THE NORTH/SOUTH POSITION OF
THE SLIGHT RISK ALONG/NEAR THE OREGON CASCADES. OVERALL...THE RISK
WITHIN THE SLIGHT RISK AREA REMAINED ALONG THE WEST ASPECT OF THE
CASCADES WHERE THE MOISTURE PLUME ENCOUNTERS THE CASCADES AND IS
FORCED TO LIFT OROGRAPHICALLY. GUIDANCE SHOWED WIDESPREAD 3 INCH TO
6 INCH PRECIPITATION AMOUNTS WITHIN 24HRS WITH ISOLATED HIGHER
TOTALS DEPICTED BY THE CONVECTIVE ALLOWING MODELS IN AND NEAR THE
TERRAIN. BOTH THE HREF AND RRFS CONTINUED TO SHOW HIGH
PROBABILITIES OF 0.50"+/HR RAINFALL RATES ESPECIALLY AFTER 21Z
TODAY WHICH PERSISTS THROUGH 12Z FRIDAY. 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.
BANN
DAY 2
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...
..PACIFIC NORTHWEST
THE ATMOSPHERIC RIVER THAT CAME ON-SHORE IN THE DAY 1 PERIOD WILL
BE ON-GOING ACROSS OREGON...ALTHOUGH THE EVENT SHOULD BE WINDING
DOWN IN TERMS OF RAINFALL RATES AND ADDITIONAL AMOUNTS FAIRLY EARLY
IN THE OUTLOOK PERIOD. 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 700 UNITS ARE FORECAST TO COVER
MUCH OF OREGON INITIALLY...THEN WEAKEN AS THE AXIS OF THE MOISTURE
PLUME AND ASSOCIATED PRECIPITATION SETTLES INTO NORTHERN
CALIFORNIA. MAINTAINED A LARGE AMOUNT OF CONTINUITY IN TERMS OF
PLACEMENT OF THE SLIGHT RISK AREA CLIPPING NORTHWEST CALIFORNIA AND
A PORTION OF ADJACENT SOUTHWEST OREGON.
SAW LITTLE REASON TO MAKE ANY SIGNIFICANT CHANGES TO THE EXPANDED
COVERAGE OF THE SLIGHT RISK AREA MADE YESTERDAY TO COVER MORE INTO
THE TRINITY MOUNTAINS GIVEN LATEST QPF.
..SOUTHERN NEW ENGLAND
A TROUGH CROSSING THE EASTERN UNITED STATES WILL DRAW INCREASING
AMOUNTS OF MOISTURE NORTHWARD INTO PARTS OF NEW ENGLAND...HELPING
SET THE STAGE FOR LOCALLY HEAVY RAINFALL WHEN FLOW ALOFT BECOMES
INCREASINGLY DIFLUENT AND A SURFACE WARM FRONT APPROACHES FROM THE
SOUTH EARLY ON FRIDAY. WPC DETERMINISTIC QPF IN THE 1.0 TO 1.4 INCH
RANGE IS FORECAST ALONG THE TRACK OF THE LOW. OFFSETTING THE
POTENTIAL OF EXCESSIVE RAINFALL BY LIMITING RAINFALL RATES/AMOUNT IS
THE MEAGER INSTABILITY EXPECTED TO BE IN PLACE.
BANN
DAY 3
VALID 12Z SAT DEC 20 2025 - 12Z SUN DEC 21 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN PORTIONS OF
NORTHERN CALIFORNIA|...
RENEWED RAINFALL IS EXPECTED ACROSS NORTHERN CALIFORNIA AS YET
ANOTHER PUSH OF MOISTURE ARRIVES ON THE HEELS OF THE AR WHICH
DEPARTED DURING THE DAY ON FRIDAY, THE AXIS OF THE STRONGEST ON-
SHORE WINDS AND THE GREATEST ON-SHORE MOISTURE TRANSPORT LOOKS TO
BE SOUTH OF THE AXIS OF THE PREVIOUS EVENT, BUT THERE IS LIKELY TO
BE AT LEAST SOME OVERLAP BETWEEN THE TWO RAINFALL FOOTPRINTS.
PRECIPITABLE WATER VALUES INCREASE TO BETWEEN 1.2 AND 1.4 INCHES
ALONG THE NORTHERN CALIFORNIA COAST NORTH OF SAN FRANCISCO SOUTH
OF THE OREGON/CALIFORNIA BORDER BY EARLY SATURDAY EVENING. GIVEN
SUCH HIGH SNOW LEVELS...THERE IS NOT A LOT OF CONCERN ABOUT
PRECIPITATION TYPE UNTIL THE MOISTURE LADEN FLOW REACHES THE
NORTHERN END OF THE SIERRA NEVADA RANGE (SAVE FOR A FEW MOUNTAIN
TOPS ALONG THE TRAJECTORY INLAND FROM THE COAST). WITH THE AREAL
AVERAGE AMOUNT OF THE RAINFALL FORECAST BEING IN THE 1 TO 2 INCH
RANGE...WILL KEEP THE MARGINAL RISK INHERITED FROM YESTERDAYS DAY
4 ERO AND PLAN TO ADJUST AS NEEDED WHEN PLACEMENT AND AMOUNTS
BECOME MORE LOCKED IN,
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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