Snow Outlook

November 20, 2010

I happened to be scheduled to work this weekend…how convenient in this case.

So is it going to snow in the city of Portland?  I would be surprised if the ground doesn’t at least get coated white either tomorrow evening or Monday afternoon/evening.  Will we get 4″ or more?  That seems very unlikely, but 2″ sure is within the realm of possibilites.

It’s getting cold out there tonight with clearing skies and drier dewpoints working down from the north and east.  Tonight should be the coldest so far this season (finally a frost at PDX!) for most of us.

I’m amazed at how much information we have nowadays and how detailed it can be.  When I started in this business around 1991, a cold upper-level trough and associated arctic airmass would plunge south, you could generally just say we’ll see somewhere between a trace and 2″ of snow within 12 hours of the frontal passage, and then it would get cold.  But look at today’s (and tonight’s) mesoscale modeling.  Our RPM, the WRF-GFS, and to a lesser extent the NAM-MM5 from the UW, all show two separate precipitation espisodes the next 48 hours.

The first is a burst of showers around sunset tomorrow as a weak surface low slides overhead and east towards Pendleton.  Larger and coarser modeling wouldn’t even be able to see this in the past.  The 2nd is a slightly stronger low or elongated low that slides by Monday afternoon. 

I feel a bit better about snow compared to last night at this time for these reasons:

1. Tomorrow’s burst of showers around sunset is just that…around sunset and beyond…good timing with a marginally cold-enough atmosphere for lowland snow.  Dark = cooler = good.

2. Wind is calm or light northwest both tomorrow and Monday afternoon…that’s always good to allow snow to fall a bit lower.  No south wind to mess things up.

3. Models (the last two mesoscale runs) have been steadily moving the Monday afternoon precipitation a bit farther north.

So here’s my forecast I’ll use at 10pm:

Portland: 

-Trace to 1″ possible tomorrow evening around sunset and just beyond.  Best bet for that 1″ would be on the hills and eastside (Gresham, Camas, Sandy, Battleground, etc…).  If the precipitation is heavy enough, it could be everywhere.  If we only get light showers; forget about the lowest elevations.

-1-2″ possible Monday afternoon/evening, even better chance south into Salem and Eugene. This is the one that could really make a mess.  If indeed snow DOES start to fall at mid-afternoon, roads are wet/slushy and then begin freezing up???  Well…I just keep thinking of December 29th, 2009.

Forget about the 300′, 500′, 750′ elevation distinctions…especially with showery precipitation!  That drives me nuts.  One place could get sticking snow to sea level (under a heavier shower), and another spot at 1,000′ might just get a mix under a light shower for no accumulation.

Chief Meteorologist Mark Nelsen