Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Dec 09 2025 21:12:11 AWUS01 KWNH 092112 FFGMPD ORZ000-WAZ000-100910- Mesoscale Precipitation Discussion 1259 NWS Weather Prediction Center College Park MD 411 PM EST Tue Dec 09 2025 Areas affected...Northern Oregon and Western Washington Concerning...Heavy rainfall...Flash flooding possible Valid 092110Z - 100910Z Summary...The ongoing atmospheric river event over the Pacific Northwest will continue into tonight over portions of western Oregon and Washington. The axis of heaviest rainfall is expected to shift back north into Washington and also increase in intensity, which will bring an increasing flood and landslide risk to these areas. Discussion...The expectation is that the atmospheric river impacting northern OR will shift northward back into western WA this evening. After a temporary drop in Integrated Water Vapor Transport (IVT) today, values are forecast to increase again towards 03z resulting in an uptick in rainfall coverage and intensity into the overnight hours. With no forecast instability, rainfall rates will be driven mostly by the magnitude of moisture transport into the terrain. This will put a cap on the upper bound of rainfall rates, although this will be compensated by IVT values above the climatological 99th percentile and layered precipitable water (PW) generally above the 95th percentile at all atmospheric levels. This deep moisture and strong moisture transport will support hourly rainfall in the 0.25"-0.40" range, potentially approaching 0.50" in the highest mountain peaks. These rates should increase in coverage after 03z (per the 12z HREF probabilities), moving from northern OR into western WA overnight. Additional rainfall through 09z Wednesday is generally expected to be in the 1-3" range. Under typical conditions, rainfall rates and totals of this magnitude would not pose a significant hazard to this region. However, the past 24 hours have already brought 4-10" of rain to these locations, leading to saturated ground and elevated streams and rivers. Although parts of WA saw a brief lull today allowing for some recovery, hydrologic conditions remain sensitive. Consequently, when heavier rain returns this evening into tonight, additional flood impacts are expected. The primary impacts are expected to be flooding of rivers and streams, other low-lying flood-prone areas, and poor drainage and urban areas. Given the saturated ground, the risk of landslides and debris flows in areas of steep terrain will also increase tonight as rainfall intensity ramps up. Chenard ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!43p-OkfUop706I6cKLYBBIwzA6nB0hPZtCxkYgIrTsd-VTMoHaWWkfw9qP6uMmlQkjOt= HZdFGfUJBx9ZZGkLPPdfEFs$ for graphic product... ATTN...WFO...OTX...PDT...PQR...SEW... ATTN...RFC...PTR...NWC... LAT...LON 47942123 47642074 46792112 46432130 45912145=20 45412151 45322145 44952164 44572184 44412215=20 44522255 44712277 45042250 45422224 45882270=20 45822308 45252331 44842354 44972399 45782413=20 46532407 47232417 47682412 47842384 47782333=20 47392322 47042294 46852251 47122208 47482197=20 47762182=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .