Challenging Snow Forecasts

February 15, 2011

Quite a tough forecast the next 7 days!  First we have to figure out how low the snow level goes the next 2-3 days and how much falls where, then it’s on to the possibility of more significant snowfall early next week.

You’ve seen my thoughts in the previous post.  Lots of onshore flow and brisk south/southwest wind continues at the lowest elevations through Thursday night.  My gut forecast instinct is that means nothing other than a dusting possible in isolated low elevation spots.  I think our RPM model (and probably the UW NAM model again…it’s a real pain) are too agressive with overnight cooling both tomorrow night and Thursday night.  They both show temps down around freezing even at the lowest elevations both nights.  Sure, that’ll happen in isolated spots (Forest Grove?) if the south wind dies down, but I find that really hard to believe for most of us in the Valleys.  That’s where those models both combine the overnight temps and continuing showers into several inches of snowfall.  So I’m discounting that in my forecast (the previous post).

Drying commences on Friday (especially the GFS, it takes all the moisture south), and that continues through Saturday night.

It seems to me the GFS is trending a bit more towards the ECMWF on the Sunday-Monday system, swinging far more energy offshore before bringing it inland on Monday.  At least that’s the GFS trend the last 3 model runs.  But as we’ve seen in the recent past, (November and again in early January), those small details on the global models have huge effects down here near sea level in the Pacific Northwest where we live in such a borderline snow climate.    The ECMWF (Sun-Mon) is another borderline snow situation, in fact I don’t see any sticking snow in the Valleys Sunday or Sunday evening on that model.  The GFS (as of 00z) is slightly colder and looks a bit better for SOME snow Sunday night or Monday morning, but this could easily just be a “dusting” event again.  Still too far away to know.  I see both models are drier after that time.  Regardless, as of now I don’t see a great setup for a major snowstorm (3″+).  But even that could still change.

Chief Meteorologist Mark Nelsen


Snowy Weather Update

February 15, 2011
The big question the next few days will be “is it going to snow at my house?”.  Here are the thoughts I sent to the newsroom a few minutes ago…
 
First, things we know WON’T happen in the next 5 days:
-High winds
-Flooding rains
-Freezing Rain
-A Cold Blast or Freeze, or “Arctic Chill”
 
Lots of cold showers streaming in off the Pacific through Friday, no single “storm”, although the showers pile up against the Cascades, which makes it more of a continous snowfall (storm) up there.
 
The showers get a bit colder and the snow level (the lowest elevation that snow sticks) lowers tomorrow through Friday.  At this point it appears the the lowest that we’ll see an inch or more of sticking snow is ABOUT 1,000′.  Each morning there will probably be some sticking snow below that elevation in spots; basically if a heavy shower passes over, it allows the snow level to briefly dip a bit more.
 
In the hills (up near 1,000′ and above): Much better chance for some snow on the ground the next 3 mornings, but even that will be random. 
 
So will we see a widespread sticking snow (1-2″+) here in the lowest parts of the Metro area? 
Unlikely, just not quite cold enough with relatively mild air coming in off the Pacific through Friday.
 
Will everyone see SOME snow in the air, or a dusting on cars/grass?
Very likely, especially Thursday and Friday mornings when it’ll be slightly colder.  At times it could get real exciting with fat snowflakes falling, but then melting.  Snow will be a “talker” the next 4 days.
 
Will it affect the morning or evening commutes?
No issues with main roads/highways in the lowest elevations, but expect snowy roads at times in the hills (top of West Hills, Sandy, etc…), probably only during the AM commutes.  This time of year, temperatures generally jump into the 40s in the afternoons because it’s getting so late in the winter season.  One sunbreak and it’s 44 degrees.
 
This will be the best snow “event” we’ve seen this winter in the Cascades and hills around town.
 
 
As for Sunday and Monday, details on that are really up in the air, some models show a good combination of moisture and cold for a widespread snowfall even down at the lowest elevations; others do not.
 
Chief Meteorologist Mark Nelsen