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   Mesoscale Precipitation Discussion 1265   
   NWS Weather Prediction Center College Park MD   
   859 AM EST Thu Dec 18 2025   
      
   Areas affected...Northwest Oregon...Far Southwest Washington...   
      
   Concerning...Heavy rainfall...Flash flooding possible   
      
   Valid 181400Z - 190200Z   
      
   SUMMARY...Extreme surge of moisture flux into terrain as warm   
   front passes will result in mid to upper slopes of coastal and   
   Cascade Ranges to receive up to 1"/hr rates.  Localized 5-7"   
   totals are possible and may result in rapid rises and possible   
   flooding in/along streams/rivers.   
      
   DISCUSSION...GOES-W WV suite depicts a strong well   
   anti-cyclonically curved ridge continue to dominate the eastern   
   north Pacific with strong core of 150-180kt westerly 250 jet   
   crossing the Juan de Fuca Strait attm.  At the trailing edge   
   increased divergence and subtle sheared shortwave is streaking   
   through just west of 130W.  This will focus surface to boundary   
   layer cyclogenesis with a low 990mb low expected to lift northward   
   toward the Strait by 21z.   The energy through the atmospheric   
   column is highly dynamic along and behind a surging warm front.=20   
   This warm front has yet to reach the Pacific Coast, but is   
   expected around 16-17z from south to north along the Oregon coast.   
    Weaker southerly flow will quickly shift with 925mb flow swinging   
   WSW to align through nearly the entire column to 500mb.  Winds   
   will increase to over 60kts with occasional speeds up to 70kt.=20=20   
      
   CIRA LPW denotes the WAA aloft as just recently reached the coast   
   though the 850-700mb layer with core of 99th and Maximum record   
   values in this an 700-500mb layers (displaced further east over   
   central OR attm due to the gentle slope of the deeper AR as a   
   whole).  As the warm front passes the total PWats will be over   
   1.25" and fluxed on that strength of wind (50-80kts from   
   925-700mb) will result in IVT values over 1000 kg/m/s with higher   
   resolution guidance suggesting localized peaks nearing 1200 kg/m/s   
   placing the surge toward the extreme range.  While orientation to   
   the terrain is not fully orthogonal, the magnitude of flux   
   convergence and orographic ascent will support .75"/hr rates in   
   the mid to upper slopes of the central to northwest Oregon Coastal   
   Range and perhaps an hour later into the upper slopes of the   
   Cascades north of Lane county.  These rates are expected to surge   
   and ebb through 00z, with an isolated 1"/hr rate possible.  As   
   such, rainfall totals will quickly total toward 5"+ in the terrain   
   with rain-shadowed areas perhaps not even reaching .5".  Early   
   arriving 12z Hi-Res CAMs suggest isolated totals over 7" by 03z   
   are not out of the realm of possibility.  These values are pushing   
   24hr ARI (Average Return Intervals) definitely into the 25 year   
   range, with some suggestions of exceedance of 50 to 100 year   
   values, in the upper slopes of the central Oregon Cascades.  So   
   the rainfall is highly atypical and therefore flooding is   
   possible.   
      
   FFG values in the region are static and are not likely to be   
   exceeded at 1 or 3hr periods but may push the 6hr+ time range.=20   
   Yet, NASA SPoRT LIS 0-40cm soil saturation remain above 70% given   
   the connection to last week's event that was focused further   
   north, and may result in above average run-off as upper profiles   
   will quickly saturate. As such, even the glancing blow across into   
   W WA could exacerbate ongoing flooding there and has been included   
   in the area of concern, though rates/totals would be reduced to   
   the core in central to northern Oregon.  So any flooding is not   
   necessarily going to be flashy in nature but notable surge in   
   river levels will be noted.   As such, please continue to monitor   
   River Forecasts from local forecast offices, Northwest River   
   Forecast Center, and products/discussion from the National Water   
   Center.   
      
   Gallina   
      
   ...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=   
   DZ3fjg!66DWPwXIdIRzihZ8KNWR2kFs3hqm9EhGdO2Y4CVe9gNTFathDiknnp_0mkOk5RDuyJAM=   
   -pULRKiW904eJ5HzsifTnVs$  for graphic product...   
      
   ATTN...WFO...PDT...PQR...SEW...   
      
   ATTN...RFC...PTR...NWC...   
      
   LAT...LON   46812171 46592138 46062138 44512157 43922193=20   
               43642242 43972308 44012371 44032419 44232423=20   
               44802415 45412406 46042402 46202370 46122328=20   
               45572281 45862238 46232230 46612213=20   
      
   =3D =3D =3D   
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