9pm Update: One More Snow Chance

November 21, 2010

Well tomorrow is the “big day” for us weather weenies.  I just finished perusing the 00z model runs…here are my thoughts:

1.  South wind kept us from getting a 1-2″ surprise in the Metro area today.  Models handled our little snow burst well, except the timing was off…it showed up 2-3 hours early.  That light onshore flow was expected.  Now it’s disappearing as a 2nd low is spinning down the coastline.  It’s near Astoria right now; models show it spinning overhead and into eastern Oregon by morning.

2. Behind that low, a colder atmosphere is pulled down from the north overnight.  That plus some clearing should make for an icy, but dry start Monday.

3. Between daybreak and around 4pm tomorrow, a stationary front (the leading edge of the arctic air) sits between Astoria-Portland-Condon.  A 3rd surface low develops along the front and moves right overhead midday or afternoon.  I’m depending on that to give us a possible 1-2″ of snowfall…a bit more moisture than models show (but that happened today too).   We start cool again in the morning tomorrow, plus our model (RPM) and WRF-GFS show the boundary right over us, instead of up north of Kelso.  That should mean no south wind and we shouldn’t have a problem getting snow to stick just about anywhere from Portland north.

4.  The arctic front then suddenly makes it’s move behind the low after 4pm, sagging south to Eugene by 10pm tomorrow night.  Most likely at least a trace of snow would accompany that front even down into Salem and Eugene.  As a result, we should freeze up quickly after dark tomorrow.

5.  If you have plans to be out and about after the noon hour tomorrow, make sure you have a plan to get un-stuck if we suddenly get a surprise 2″ dump of snow and traffic grinds to a halt!  That seems unlikely, but definitely possible.

Now this is the scenario that I’m basing my forecast on.  The NAM moves the low by a bit farther to the north, which puts us into a similar situation that we saw today (minus the eastern Clark County cold spot).  If so, we’d only get a trace or so as the arctic front finally slides south.

So here’s my forecast:

1-2″ snow Portland north to Longview midday-evening tomorrow.

Trace from Woodburn south into the Valley.

I lowered Tuesday’s high temp forecast based on at least 1″ of snow cover in the metro area.  If we don’t get snow on the ground, I’d guess 33-35 degree highs.

We can talk about that sudden appearance of moisture for Wednesday AFTER we get through tomorrow.  Actually, I suppose you can talk about whatever you want on here, but I’m busy focusing on tonight and tomorrow.

It’s still hard to believe it’s such a cold atmosphere plunging into our area tomorrow night.  -13 to -15 degrees C. at 850mb?  Imagine if it was January…then it would be quite a bit colder at the surface.  I’d probably go for highs in the low-mid 20s.

Chief Meteorologist Mark Nelsen

2 pm Snow Update: Snow Level Forecasts

November 21, 2010

This afternoon is a prime example of why you shouldn’t forecast snow levels in 200′, 300′, or 500′ increments.

At 2pm, we have a rain/snow mix and nothing on the ground here in Beaverton at about the 220′ elevation.  Yet in NE Vancouver I can see snow on everything except the roadway…AT THE SAME ELEVATION.

Precipitation intensity, leftover cold pockets from overnight cooling, and influence of milder southerly wind all plays a larger part of the snow/rain story than just a few hundred feet of elevation.

That’s why you’ll never see a forecast for snow to 300′ (or anything similar) on my forecasts.

It sure looks as if at least everyone has SEEN some snow this afternoon, if you didn’t get a dusting at your home, you have probably seen it in the air.  The best spot seems to be in central/eastern Clark county, and areas up around 1,000′ elsewhere.  A mild southerly wind is keeping temps in the mid-upper 30’s south of the Columbia River, but a leftover pocket of cooler temps (and no south wind) around/east of I-205 up in Clark County is giving you folks your first snow of the season.

The back edge of the heavy precipitation line is crossing the West Hills right now, so the low elevation snowfall will stop in the next hour, but then more showers between now at 8pm may give us an even better chance for sticking snow as sunset approaches!

Chief Meteorologist Mark Nelsen

Weather Page Fixed

November 21, 2010

That was REALLY annoying.  My main weather page with all the links I use (and I know a lot of you folks use) had been down since 7pm last night.  I felt meteorologically naked.  BUT, it turns out my provider (www.cascadeaccess.com) was somehow making any ftp’d files into zero bytes.  I use a program that inserts current conditions every 15 minutes, then sends the file from my computer to the server.  Don’t know what was wrong, but it’s fixed now.  Page is working.  In the future if it happens try markswxlinks2.html and that is a “static” version that doesn’t change.  Well, at least it will be there about 10 minutes from now.

Looks as if precip is going to move in a bit faster this afternoon.  Marginally cold enough for snow to stick in the lowlands right now, and with the new surface low (sliding along the arctic boundary) slightly to our north, we seem to get a light southerly breeze this afternoon…kiss of death.  But maybe not.  Let’s just hope for briefly heavy precip. intensity and a quick dusting for most of us.  Either way it’ll freeze up overnight, so watch out for ice once that happens.

Still looks good for tomorrow with moisture picking up midday and a colder atmosphere.  I’ll be surprised if everyone doesn’t at least get a dusting out of this from Salem north.