Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 08 2025 16:53:22 AWUS01 KWNH 081653 FFGMPD ORZ000-WAZ000-090400- Mesoscale Precipitation Discussion 1258 NWS Weather Prediction Center College Park MD 1152 AM EST Mon Dec 08 2025 Areas affected...Western Washington...Far Northwest Oregon... Concerning...Heavy rainfall...Flash flooding possible Valid 081700Z - 090400Z SUMMARY...Very strong atmospheric river with persistent .25-.5"/hr rates falling on increasingly saturated soils will result in high run-off, swelling rivers and poor drainage/urban flooding but at minimum further set the stage for expected additional rainfall into the mid-week. DISCUSSION...Regional RADAR mosaic is showing the leading warm/moisture advection surge from the atmospheric river is expanding across the Pacific Northwest and into the Washington Cascades at this time. CIRA LPW animation depicts a long fetch core to the atmospheric river with a long tap back to the tropics near the Dateline; though the strength of the polar stream with 150-170kt fairly zonal upper-level jet has resulted in very strong deep layer flow which has resulted in a fairly long/gradual isentropic incline to deplete any potential of vertical instability, the strength of moisture flux anomalies are pushing the maximum climatological percentiles for this time of year. This is generally confirmed by CIRA LPW percentiles in the 95-99th range through all layers but most noted in the 700-500mb at this time. The total PWat values are at or above 1.25" and have come ashore with the subtle but fast moving warm front denoted with wind shift from south to southwest and increase to 40kts at surface, 50-60kts near boundary layer and over 70kts at 700mb, this has lead to IVT values very near 1000 kg/m/s analyzed just offshore while remaining above 700 kg/m/s into the Cascades currently, but is expected to rise as the effective warm sector moisture intersects the terrain fairly orthogonally as well. As a result, broad ..25-.33"/hr rates have been seen near the coastal areas so far and will expand into W WA over the next few hours. The nose of the upper-jet remains north for a bit and height-falls on the western side of the core will not start to dip southward until well after 00z as the cold front reaches the mouth of the Juan de Fuca Strait around 00-01z. The strong flux, upslope flow will allow for rates of .5"/hr regularly through the mid to upper slopes of the Cascade Range as the WAA has freezing heights above all but the highest peaks. As such upper slopes will see 3-5" totals by 03z with foothills likely to be 2-4" and lower valleys less than 2"...with isolated totals to 6-7"+ along the core of the AR in the southern WA Cascade Range. Coastal Ranges including the Olympics may see 4-6". Overall, FFG values are high enough even to encompass all but those highest extreme values being greater than 3.5/3hrs and over 5"/6hrs. However, NASA SPoRT LIS 0-40cm soil saturation values are generally above 70-80% across which is generally a bit above normal for the Coastal Range, but near 75-85th percentiles for the Cascade Range; however, full saturation should occur through the late afternoon, and as the main surge occurs toward 00z, much of the rainfall should convert to run-off. This should swell the rivers fairly quickly with compounding issues resulting in riverine flooding (please refer to discussion and graphical products from NW River Forecast Center and National Water Center for further details). Given this is the first surge of a prolonged AR event, the potential for flash flooding is likely limited to the highest slopes/reaches of the watersheds or some localized urban flooding due to poor drainage. With the saturation of the soils through depth, there will be a slowly increasing potential for mudslides especially near/downslope any recent burn scars. Gallina ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!41Gpl_K0C4oZ8oksLbVkshXJ6rbtTPELhF_yqA-pb03G5RNZm1ZMxwDtMJ02CLLoM3h0= Gh03LT2xq3MSxojGryzr5JQ$ for graphic product... ATTN...WFO...OTX...PDT...PQR...SEW... ATTN...RFC...PTR...NWC... LAT...LON 49112107 48812060 48292030 47312040 46882082=20 45752140 45492147 45512177 45902229 46102264=20 46062298 45822333 45732387 46362422 47182433=20 47832456 48222487 48462487 48292398 47902348=20 47362331 46992302 46942246 47252212 47702194=20 48252209 48572212 48962190 49092160=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) .