Winter and Spring in The Forecast

March 4, 2012

Today was a wonderful late winter/early spring day with temps in the mid-upper 50s around the metro area; a bit warmer than average.  Tomorrow and Tuesday morning winter takes a run at us again, then it’s on to our warmest weather so far this season for later in the week.  The Spring “roller-coaster” is beginning…

For tomorrow we’ve got a very sharp cold front on the coastline at daybreak, heading through the Portland area maybe 10am or so, then east of the Cascades through the afternoon.  When I say “sharp”, I mean snow levels around 6,000′ ahead of it and as low as 1,000′ right behind it in the heaviest precipitation.  This is one of those unusual “anafronts”, which leads to much of the precipitation on the cold side of the front.  These can sometimes lead to unusually low snow levels behind the cold front due to steady, heavy precipitation for a few hours (or maybe just an hour or two tomorrow).  I remember several of these have brought sticking snow to Boring, Sandy, and Corbett when we had expected the snow level to be well above 2,000′.  One was maybe Halloween 1994 or 1995, or maybe it was 1996, but it was definitely Halloween night…  Memory is fading…  That will happen midday to early afternoon tomorrow, not the best timing in early March, but something to keep an eye on.  If you live anywhere east of Gresham, watch for “chunky rain” after 11am or so!

Then a very cold upper-level trough sweeps through the Pacific Northwest tomorrow night and early Thursday morning.  850mb temps look to be around -9 deg C.  That’s cold enough to get sticking snow to sea level at night; however almost all of the precipitation is gone by that time.  Models have been trying to produce a cluster of showers that come south in the late night hours tomorrow night.  The MM5-NAM and WRF-GFS have this idea.  Our RPM (at least at 00z) does not.  Regardless, no models are showing any sort of heavy precipitation, so I don’t see how anyone could get more than an inch, but anyone could see a dusting to sea level tomorrow night and the Tuesday morning commute could see slippery spots.  The good news is that we quickly dry out.  It’ll be melting quickly after sunrise anyway and then it’s on to a mostly sunny Tuesday afternoon with highs in the mid 40s.


Beyond Tuesday, nice upper level ridging pops up later Wednesday through Friday.  Take a look at that 500mb chart for Thursday afternoon:

Looks nice doesn’t it?  We NEVER saw this kind of ridging through last March, so this is a big improvement, hopefully a sign of a more reasonable spring to come?  We’ll see.  850mb temps rise up to around +6 to +10.  It’s time to pull out the March Magic Chart!  The warm season must be approaching since I only use these March-September.  It says somewhere into the low-mid 60s with offshore flow.  That would be why we have a 62 & 64 in the forecast.  Hopefully that’s not a warm bias; the 00z GFS is flattening the ridge more rapidly Friday, so 64 might be high.

00z GFS ensemble chart here: 

You can see near or above average temps from Thursday through Sunday, then a bit cooler as a large, cold upper level trough sits in the eastern Pacific.  If you take all 16 days, it appears about “average” to me.

Chief Meteorologist Mark Nelsen