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   WAZ000-160900-   
      
   Mesoscale Precipitation Discussion 1264   
   NWS Weather Prediction Center College Park MD   
   456 PM EST Mon Dec 15 2025   
      
   Areas affected...western Washington   
      
   Concerning...Heavy rainfall...Flash flooding possible   
      
   Valid 152153Z - 160900Z   
      
   SUMMARY...A subtle increase in rainfall intensity is expected over   
   western WA through about 03Z with hourly rainfall peaking near 0.5   
   inches. 12 hour accumulations of 1 to 2 inches will be possible   
   through 09Z. While these higher rainfall intensities should remain   
   isolated in nature across the Olympics and Cascades, the region   
   remains saturated and quite sensitive due to last week's heavy   
   rainfall event.   
      
   DISCUSSION...The latest GOES West water vapor imagery showed the   
   approach of a mid to upper-level vorticity max about 250 miles   
   west of the mouth of the Columbia River, tracking toward the ENE.   
   Total precipitable water values have come down since 12Z this   
   morning as seen in recent blended TPW imagery with 0.6 to 0.8   
   inches observed from the Washington coast to the Cascades at 21Z.   
   850-700 mb mean layer winds were observed to be 50-60 kt via KLGX   
   VAD wind data, forecast to decrease into the 30-45 kt range by 03Z   
   via recent RAP forecasts. Hourly rainfall within the upslope   
   regions of the Olympics and Cascades has peaked in the 0.2 to 0.3   
   inch range over the past 2 hours with 2-4 inches (locally higher)   
   observed over the past 24 hours in the Olympics and Cascades.   
      
   The arrival of the offshore vorticity max has been preceded by   
   cooling clouds tops on infrared satellite imagery and rainfall   
   rates are likely to increase again over the next few hours over   
   the Olympics and Cascades, at least locally, despite continued   
   lowering of IVT values. This will be due to increased lift ahead   
   of the vorticity max and the left-exit region of a powerful jet   
   centered near 250 mb, centered ~1000 miles west of the WA/OR   
   coastline. Recent GOES West DMVs measured a few points with 170 kt   
   (10-15 kt higher than RAP forecasts). This jet max is forecast to   
   continue strengthening as it translates eastward with left-exit   
   ascent moving across western Washington tonight.   
      
   Strengthening forcing for ascent coupled with weak elevated   
   instability and mean WSW flow could support a couple of isolated   
   hourly rainfall values near 0.5 inches and brief training of   
   stronger echoes. The greatest probabilities for higher rainfall   
   rates will occur with higher elevations/orographic lift which will   
   be experiencing a transition to snow as freezing levels fall in   
   the wake of a cold front which will limit contributions to   
   additional flooding for the higher elevations above 5000 ft.   
   However, where p-type will remain liquid, limited potential for a   
   few hours of hourly rainfall in the 0.25 to 0.50 inch range   
   through 06Z (locally higher) and additional rainfall totals of 1   
   to 2 inches through 09Z within the windward slopes could result in   
   additional flood concerns for the region.   
      
   Due to the highly sensitive ground conditions, due largely in part   
   to last weeks atmospheric river event, any additional rainfall   
   will have the potential to renew or exacerbate flood concerns. The   
   potential will also exist for isolated landslides/debris flows   
   across the more sensitive regions of the Olympics and Cascades.   
      
   Otto   
      
   ...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=   
   DZ3fjg!_ihbPIbF-hZ7NCZRjxSSeMKqV1C7qq5CnKKiK7KlcTB2BViBGIpgmm-d77a52eEHi0Iv=   
   TvKo-4Ujjt6a7458NaeaiW8$  for graphic product...   
      
   ATTN...WFO...PDT...PQR...SEW...   
      
   ATTN...RFC...PTR...NWC...   
      
   LAT...LON   48992144 48882113 48732105 48482127 48352141=20   
               48162144 47962132 47792125 47612136 47432143=20   
               47192137 47092151 47042160 47032170 47012180=20   
               46962186 46892188 46832185 46782179 46762164=20   
               46732161 46542156 46422167 46342213 46392236=20   
               46582254 46762263 46912265 47082312 47122349=20   
               47292387 47592402 47922419 48092409 48102367=20   
               47972318 48052273 48532226 48942185=20   
      
   =3D =3D =3D   
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