Mesoscale Discussion 0060
   NWS Storm Prediction Center Norman OK
   0750 AM CST Sat Jan 13 2024

   Areas affected...Northwest OR...including parts of the Willamette
   Valley

   Concerning...Freezing rain 

   Valid 131350Z - 131845Z

   SUMMARY...Freezing rain is expected to increase in coverage and
   intensity this morning. Some mixing with sleet is also possible.

   DISCUSSION...An expansive anticyclone and related Arctic airmass is
   centered over the northern Rockies this morning, and cold
   temperatures have spread westward through the interior Northwest and
   into the Columbia Gorge and Willamette Valley. Meanwhile, a
   subtropical moisture plume has been drawn northward by a deep-layer
   cyclone over the central Pacific (centered near 27N/149W), and then
   eastward by a vigorous mid/upper-level shortwave trough and
   deepening surface cyclone (centered near 44N/131W) approaching the
   Pacific Northwest coast. This moisture, combined with ascent
   attendant to the approaching shortwave, will support increasing
   precipitation from southwest to northeast across much of OR today.
   This will lead to increasing winter-precipitation rates in areas
   where cold air is now entrenched.   

   In particular, expanding precipitation within the shallow cold
   airmass and a strengthening warm nose in the 900-800 mb layer will
   lead to an increasing freezing-rain threat this morning across parts
   of the Willamette Valley. Freezing rain has already been reported in
   Eugene, though areas farther north in the valley may initially mix
   with sleet or snow, given the initially deeper subfreezing layer
   noted in the 12Z SLE sounding. A transition to freezing rain is also
   expected across parts of the coastal ranges, and in areas along the
   immediate coast where adjacent terrain favors a greater intrusion of
   shallow cold air. 

   Liquid-equivalent rates of 0.05-0.1 inch/hour will be possible
   through the morning as precipitation gradually spreads northward.
   Ice accretion will be rather efficient where temperatures can hold
   in the mid/upper 20s F, as suggested by the observation of 0.05-inch
   flat-ice accretion in the 13Z KEUG METAR when an initial band of
   freezing rain moved through.

   ..Dean.. 01/13/2024

   ...Please see www.spc.noaa.gov for graphic product...

   ATTN...WFO...MFR...PQR...

   LAT...LON   44292275 43932291 43802326 43782350 43852370 44072374
               44442367 44552395 45122397 45522386 45452364 45242341
               45252276 44912266 44632271 44292275