March 2017 is a March 2012 Redux So Far

March 5, 2017

9:30pm Sunday…

This month sure seems similar to March 2012.  That year we saw a series of close calls with low elevation snow through the 3rd week of the month.  Officially we only received 0.8″ in Portland very close to sea level that March.  But up around 1,000′ there were 3 periods in which significant snow fell.  That month I had 21″ at 1,000′ at home in the far eastern metro area spread through the month (unrelated to cold Gorge wind).  There was also a record-setting late season wet snowfall in the southern Willamette Valley on the 21st.  Remember this viewer pic?  Not quite the spring break they imagined…

springbreak

Now so far we’ve had less snowfall than 5 years ago, but the pattern wants to continue quite chilly.   In fact this is the exact same weather pattern we saw the first half of December, January, & February.  But this time we are heading into spring and we don’t have a cold east wind to deal with that could bring a widespread snow storm to the metro area…whew!

For the folks who just want to know if they’ll have a snow day tomorrow?  The short answer is NO in the lowlands, but a couple of properly placed heavy snow showers 4-9am hitting a few hills could easily give us a bunch of 2 hour delays…we’ll see.  Regardless, but noon all areas below 1,000′ should be around 40 degrees or above…it can’t last long.

marksnow_upcominginmetroarea

Check out the 500 millibar height map for midweek, that’s flow in the upper atmosphere up around 18,000′.  gfs_namer_084_500_vort_ht  Our old friend the Alaskan Upper High is back.  Flow is coming down from the north on the backside of that high then converging with westerly flow and moving right into the Pacific Northwest.  This general setup gave us the big snowstorm in January and led to numerous close calls and ice storms through the winter.  As I mentioned, at least this time of year there is no cold air east of the Cascades (50+ the last few days at The Dalles) as the increasing sun angle kills low-level cold airmasses quickly.

All of us should see snow in the air the rest of tonight and tomorrow morning, but the limiting factor once again is the persistent onshore flow of mild marine air off the Pacific Ocean at the lowest elevations.  The Eugene to Olympia pressure gradient is pretty strong this evening, over 7 millibars.  That should equate to gusts 20-30 mph.  This means a mixing southerly breeze continues through tomorrow morning and most of the time that’ll keep the snow level up in the hills.  But any heavy batch of snow showers can easily dump snow right down to sea level since it’s so cold overhead.  To summarize, my gut feeling is most of the metro area will not be white at 7am tomorrow.  Once we get past 9-10am temperatures will definitely be too warm for sticking snow below 1,000′.  And for all you “window-licking wishcasters” out there…stop reading those snow accumulation maps!  Most of the time the mesoscale models and to a lesser extent the global models have been far too aggressive with low elevation snow in these onshore flow setups…I think we’ve seen it for every event this cold season.  It’s not that the model itself is bad, it’s just how the developers have chosen to represent snow in the algorithms, it may not be optimized for this setup.  Look at 850/925mb temps, precipitation intensity, and amount of mixing wind to start.  A lower dewpoint helps too.

Beyond tomorrow it looks wet, wet, wet…yuck.  Hard to find a dry day to work outside this week!  So it goes with a La Nina March…

Chief Meteorologist Mark Nelsen