AWUS01 KWNH 240610 FFGMPD CAZ000-ORZ000-241800- Mesoscale Precipitation Discussion 0025 NWS Weather Prediction Center College Park MD 110 AM EST Tue Feb 24 2026 Areas affected...Southwest Oregon and Northwest California (Coastal Ranges and Klamath Mountains)...Northern Sierra Nevada Concerning...Heavy rainfall...Flash flooding possible Valid 240610Z - 241800Z SUMMARY...A strong atmospheric river advancing inland across portions of the West Coast will direct anomalous moisture into the coastal ranges of southwest OR and northwest CA over the next 6 to 12 hours. Heavy rainfall rates as high as 0.50" to 0.75"/hr are expected going into Tuesday morning. This coupled with moistening soils, rising streamflows, and high freezing levels with rain-on-snow, will lead to an increasing threat of runoff problems and a localized threat of flash flooding. DISCUSSION...Regional radar mosaics and GOES-W satellite imagery depict a well-defined atmospheric river arriving across portions of the Pacific Northwest and northern California. OSPO ALPW shows a deep southwestward extension of the moisture axis down into the subtropics to the east of Hawaii, with the atmospheric river focusing out ahead of a deeper layer trough and associated frontal system offshore of the West Coast. IVT magnitudes are forecast by the latest GFS and ECMWF solutions to increase rather dramatically over the next several hours across the immediate coastal ranges of southwest OR and northwest CA including the Klamath Mountains. Already IVT values of 500 to 750 kg/m/s are arriving across these areas, and the latest guidance indicates these numbers rising to 750 to 1000 kg/m/s by 12Z. This will be facilitated by enhanced low to mid-level southwest flow out ahead of a surface low pressure wave transiting the aforementioned offshore front which has a northeast extension currently into western OR. This low center will arrive across western OR between 12Z and 18Z on Tuesday which will help drive the front a bit farther south down the coast. The 850/700 mb moisture flux anomalies are forecast to rise to 3 to 5 standard deviations above normal, with the moisture having some deep subtropical origins. This moist kinematic profile is nearly orthogonal to the coastal terrain and the Klamath Mountains, maximizing forced orographic ascent, and thus favoring efficiently high rainfall rates. The latest HRRR and 00Z HREF guidance remains in excellent agreement with the HREF depicting high probabilities (>60%) of 0.50" to 0.75"/hour rainfall rates over the next 6 to 12 hours. Rainfall totals by 18Z/Tuesday of up to 3 to 6 inches are expected, with the southwest-facing slopes seeing the greatest totals. Given the increasingly moist soils, rising streamflows, and rain-on-snow considerations over the higher terrain (including the northern Sierra Nevada), these rains are expected to drive concerns for runoff problems. A localized flash flood threat will exist for the more sensitive locations which may include a couple of burn scar areas. An increased risk of rock and landslide activity will exist along with urban flooding impacts. Orrison ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...EKA...MFR...MTR...REV...STO... ATTN...RFC...PTR...RSA...NWC... LAT...LON 42852396 42642362 42332339 41982258 41702272 41562257 41512208 41262168 40842159 40442115 40112105 39732073 39472094 39512151 40022201 40422206 40442247 40252276 39872268 39412275 39052258 38592267 38472330 38942414 40612463 41932471 42752445