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   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   
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