Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 247 PM EST Sat Dec 6 2025 Day 1 Valid 16Z Sat Dec 06 2025 - 12Z Sun Dec 07 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Cook/Wegman Day 2 Valid 12Z Sun Dec 07 2025 - 12Z Mon Dec 08 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Orrison Day 3 Valid 12Z Mon Dec 08 2025 - 12Z Tue Dec 09 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE PACIFIC NORTHWEST... The 06Z/12Z model guidance suggests a slightly more suppressed axis of greater IVT magnitudes impacting the Pacific Northwest on the front end of the upcoming atmospheric event for the region. The NAM/GFS/ECMWF solutions though still support very strong warm air advection with enhanced IVT magnitudes reaching 800 to 1000+ kg/m/s across the western WA and northwest OR coastal ranges with the first wave that arrives in the 12Z/08 to 00Z/09 time frame. As strong surface low pressure and associated shortwave/jet energy crashes into British Columbia, the southern flank of the attendant cold front will slow as it is crosses WA state and this should allow for a relatively longer persistence of greater IVT values across southern WA and northern OR going into the 00Z to 12Z/09 time period. The deeper layer moisture fetch will have a trans-Pacific origin that extends down to just west of Hawaii, and satellite data would suggest a likelihood for tropical and subtropical moisture feeds into this atmospheric river event which will drive very efficient and high rainfall rates that may reach well into the 0.50" to 1"/hour range near the periods of max IVT and deeper layer ascent. The coastal ranges in particular across southwest WA and northwest OR along with the west-facing slopes of the Cascades farther inland should see enhanced rainfall totals that reach as high as 4 to 6+ inches which will heavily aided by deep layer orthogonal flow relative to the terrain. Given the subtle southward shift in the axis of greater IVT magnitudes and overall higher rainfall rates/totals for the northern parts of the Olympic Peninsula and the northern WA Cascades, the Slight Risk for this area was trimmed just a tad, with very little to no change elsewhere. This is consistent with the overall 06Z/12Z guidance QPF footprint. Regardless, the wet antecedent conditions and heavy rainfall totals are expected to drive concerns for locally enhanced areal and riverine flooding, with urban flooding also likely to become a concern. A threat for landslides will exist, especially toward the end of the period and thereafter as additional upstream atmospheric river activity/moisture transport arrives with more heavy rainfall. Orrison Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt