Snow Outlook Tonight-Thursday Morning

March 20, 2012

A big taste of winter FOR SOME AREAS the next 2 days; quite a challenging forecast!  Here are the nuts and bolts…

Steady precipitation (rain or snow) from now through late tomorrow night all areas over and west of the Cascades is an easy call, but high uncertainty on details; models are disagreeing a bit still.  Maybe it’s best to tell you what we DON”T expect the next day or so.

What we don’t expect through Thursday morning:
–          Strong or gusty wind.  It’ll be nearly calm from this evening through Thursday AM.
–          “Stormy” weather except way up in the mountains.
–          Flooding in the north/central Willamette Valley
–          A snowstorm in the Portland metro area.

What we’re confident with:
–          A ton of snow in the foothills near 1,500’ and above, Coast Range, and Cascades
–          Lots of rain, but we’ll probably avoid flooding due to snow in hills and quick cutoff of precipitation tomorrow night.
–          Afternoons just wet at the lower elevations, even if we get A.M. snow

The snow level gradually lowers late tonight to somewhere around 1,000’ in the Northern Willamette Valley, and possibly all the way down to the Valley Floor from Salem to Eugene.  Then it’ll rise during the “daytime heating” all day tomorrow through evening.  Possibly dipping down into the hills briefly again tomorrow night. 

FORECAST

Northern Willamette Valley (Portland), North Coast, and Columbia Gorge:
Zero-Dusting possible briefly tomorrow morning and maybe again late tomorrow night.
2-4” once you get up to 1,000’
Likely no commute issues tomorrow morning or evening.

Central/South Valley (Salem-Albany-Eugene):
1-3” lowest spots late tonight or tomorrow morning, possibly again tomorrow night.
5-7” once you get up to 1,000’…S. Salem Hills and Santiam Canyon might get nailed.
Could be messy morning commute in parts of Salem.

Near/above 1,500 (includes Coast Range, and Cascade Foothills):
-10-15” late tonight through Thursday morning.

When I have time later this evening I’ll blog about all the meteorological details…

9:00 PM UPDATE

Looks like sticking snow level is SLOWLY dropping as expected this evening, maybe approaching 1,000′ in the Cascade Foothills and somewhere just above 1,500′ over Portland based on KPTV temps.

I’ve had a few emails wondering why the push for snow to our south  to lower elevations vs. up here in Portland. 

1. The dividing line, a stationary front, will linger somewhere around Roseburg to Bend through tomorrow night.  All areas to the north of that line COULD see snow if precipitation rates are heavy enough to drive the snow level all the way down to the surface.

2.  Models (all except the NAM) keep the heaviest precipitation from Salem to Roseburg.

3.  So the best chance for the lowest elevations to see snow is somewhere south of Portland to somewhere north of Roseburg.  Assuming models are handling things correctly.

The 00z RPM and WRF-GFS are similar to earlier runs, although both were SLIGHTLY farther south with the heavier snow.  Check out the 24 hour snowfall off the WRF-GFS from 5am tomorrow through 5am Thursday.  I really like the 11″+ at Junction City!

And the Salem forecast sounding still goes with a snow profile for 7am tomorrow:

The Portland sounding is only 1-2 degrees warmer in the morning, thus the forecast of 2-4″ in the West Hills and a trace in the lowest elevations.  A 2-3 degree error in temperature forecast will make the difference between no snow in Portland and 2″!

The NAM continues to push precipitation much farther south, in fact the UW-MM5-NAM has Portland and areas to the north mainly dry tomorrow!

Our RPM has shifted the better chance for sticking snow in the Salem and Portland areas from late tonight to tomorrow night instead.  It also totally cuts off precipitation after midnight tomorrow night, then a light southwest wind kicks in.  That tells me Thursday morning’s commute will probably be fine with several hours of temperatures above freezing even if we get snow around 10pm-midnight tomorrow night.

I’m staying with my forecast above…we’ll see how it goes.

Theoretically, I’m driving my small car (with snow tires removed) tomorrow at 9am to Reno, Nevada for the start of a 10 day vacation…hmmm.  We’ll see.

Chief Meteorologist Mark Nelsen