Close Call for Snow Tuesday

December 28, 2009

Looks like we’ll escape without snowfall in the lowest elevations of western Oregon and Washington tomorrow, but by just 5 degrees or so.  An occluded front is approaching the Pacific Northwest tonight.  We still have a cold and dry airmass pouring through the Gorge at the surface, while the airmass above isn’t all that warm.  A long cold, clear night for maximum radiational cooling away from the Gorge is on tap once again as well.  Assuming precipitation arrives as most models forecast…around noon-3pm…we should stay all liquid here in the lowlands.  Our RPM model (not exactly a stellar performer so far this winter season) shows precipitation arriving soon after 9am, indicating some snow up against the Coast Range with some cold air damming there.  Since it’s the only model showing an earlier arrival, I’ll ignore it.  But I like the idea of cool air allowing snow to relatively low elevations from Portland north and west.  In my 10pm weathercast I’ll mention snow to 1,000′ north and northwest of Portland.  I’m thinking Vernonia, Buxton, and the hills in northern Clark County, Cowlitz County, and Columbia County in Oregon.

If you’re traveling I-84 through the Gorge tomorrow, I’d get it done before 4pm.  After that expect a steady sticking snowfall for a few hours…maybe 3″ at river level for messy freeway travel. 

There will probably be an area of freezing rain/ice pellets in a narrow band in the western Gorge.  The cold air is  very shallow west of Multnomah Falls out to Troutdale.  So above about 500′ (Corbett, Washougal Hills) the precipitation will probably be in the form of rain but surface temps will evaporationally cool down to 30-31 degrees.  Once the wind goes southerly tomorrow night those areas will be exposed to that warming, ending the threat after just a few hours while deeper into the Gorge the cold air will remain untouched and just gradually modify the next two days.  Just explaining myself so you don’t think I’m wacko and calling for a large area of freezing rain.  It’ll be localized, but it involves my home so I get to talk about it.

The Thursday system looks juicy…a wet New Year’s Eve.

The 00z GFS then builds ridging/splittiness over us beginning Sunday.  ECMWF shows some sort of ridging as well into the first week of the New Year.

Chief Meteorologist Mark Nelsen