Subj : Flood potential WA/OR To : All From : Mike Powell Date : Mon Dec 15 2025 08:33:41 AWUS01 KWNH 150936 FFGMPD ORZ000-WAZ000-152135- Mesoscale Precipitation Discussion 1263 NWS Weather Prediction Center College Park MD 435 AM EST Mon Dec 15 2025 Areas affected...Western WA and Northwest OR Concerning...Heavy rainfall...Flash flooding possible Valid 150935Z - 152135Z SUMMARY...A new atmospheric river will be arriving this morning across the Pacific Northwest. New rounds of heavy rain will be associated with this across especially western WA and northwest OR, and this will gradually pose a renewed threat of flooding. DISCUSSION...The latest GOES-W WV suite along with OSPO/CIRA ALPW data shows a new trans-Pacific atmospheric river approaching the Pacific Northwest. The source region of the long-fetch moisture transport is situated well southwest of Hawaii along 160W to 170W, with a northeast advection of it up along and ahead of a well-defined frontal zone that extends from Hawaii to offshore of the West Coast. Satellite imagery and surface data also shows additional northern stream energy and a reinforcing cold front approaching from areas near and south of the Gulf of AK. Over the next 6 to 12 hours, strong deep layer southwest flow with embedded shortwave energy will approach and begin crossing the Pacific Northwest which will drive the aforementioned offshore fronts gradually inland across the coastal ranges. Strong warm air advection and enhanced low to mid-level flow will drive increasing rainfall rates across the orographically favored coastal ranges from northwest OR up into the Olympic Peninsula and eastward into the foothills and higher terrain of the Cascades. Microwave-based CMORPH2 data shows heavy rainfall rates associated with the offshore shortwave energy focused along 130W, with some rates on the order of 0.75" to 1"/hour. This is associated with broken areas of convection as validated by GOES-W GLM data indicating intermittent lightning activity. Some of these heavier rates should arrive gradually throughout the morning, and the 00Z HREF guidance shows high probabilities (>60%) of rainfall rates exceeding a 0.50"/hour across the Olympic Peninsula and the foothills of the Cascades, with some low-end probabilities (approaching 30%) of seeing these rates max out near 1"/hour in the central and southern WA Cascades. These high rates will coincide with the arrival of high IVT magnitudes that will be on the order of 800 to 1000 kg/m/s into the coastal ranges, with an inland penetration of the core IVT plume into the Cascades. Some modest instability with MUCAPE values up around 250+ J/kg along and just ahead of the frontal boundaries will also be a contributor to these elevated rates. Expect as much as 3 to 5 inches of new rainfall over the next 12 hours, with the heaviest totals over the Olympic Peninsula and the windward slopes of the WA Cascades. Lesser amounts will be noted over northwest OR with as much as 1 to 3 inches here involving the orographically favored terrain. The region is extremely sensitive given the residual impacts from the recent high-impact multi-day atmospheric river. Elevated/high streamflows and saturated ground will support the additional rains going into runoff that will be capable of driving renewed areas of areal flooding. Some localized burn scar flash flooding, debris flows and landslide activity also cannot be ruled out today. Orrison ATTN...WFO...OTX...PDT...PQR...SEW... ATTN...RFC...PTR...NWC... LAT...LON 49262217 49252090 48782034 47882038 46802093 45402127 44392175 44302231 44662313 44682399 45742427 47102424 48002466 48222357 47692293 47822252 48592230 $$ --- SBBSecho 3.28-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .