Snow Thoughts For The Weekend

January 11, 2012

For those of you just checking in and not wanting much detail, here’s what I have for you:

  1. No dramatic changes in thinking since last night at this time
  2. Timing of the arrival of colder air (and rain) has moved up.  Precipitation arrives Saturday afternoon as rain.
  3. By Sunday morning, and continuing through Monday midday or so, anything that falls out of the sky, even at the lowest elevations, will be in the form of snow.  Everyone will at least see snow in the air Sunday and/or Monday.
  4. Will there be “sleddable snow” in Portland?  I’d give it a 40% chance right in the city; just slightly lower confidence than last night.  My reasoning is below.
  5. Up on the higher hills (near and above 1,000′)?  Near 100% chance of 1″ or more Sunday-Monday.
  6. Traffic issues in the metro area?  Probably only on the hills Sunday morning and again Monday morning.  I doubt this will impact main roads/highways, but that could change.

Okay, now on to the details:

Gusty east wind continues this evening, with peak gusts earlier today in the 30-40 mph in many exposed spots around the metro area.  Vista House wind picked up this evening with a peak gust of 80 mph; safe to say it gusted to 100 mph “on the railing”.  Overnight the wind should retreat mainly to areas east of I-205 and near/south of the Columbia River.

Did you notice the extremely dry air today?  In spots (Clark County and western Washington County) where the wind has dropped off, we’ve seen 9pm temps already down in the mid 20s!  Lots of upper teens coming up later tonight as the wind keeps dying down away from the Gorge.

More of the same, but less wind, the next 2 days.

Obvious change in the past 24 hours is the timing has been moved forward.  The main cold front arrives late Saturday afternoon now.  Much colder atmosphere behind the front drops the snow level very low overnight Saturday night.  Expect a snowy Coast Range well before daybreak Sunday.

From Sunday morning to Monday afternoon then we have the classic (annoying) setup of cold showers streaming onshore from the west and northwest.  At the same time, a southerly breeze is blowing up the western valleys.  850 mb. temps drop to around -7 to -9 C.  That’s right at the threshold for snow sticking to the Valley floor.  So IF we get a good cluster of showers, there’s no reason anyone couldn’t see some accumulation of snow.  But moisture seems to be lacking during this whole period.  It’s a “showers and sunbreaks” sort of pattern.  That’s the main reason I’m not too excited that we’ll see significant lowland snow.  Note the 24 hour precipitation forecast from the WRF-GFS from Sunday morning to Monday morning.  It often appears to be shifted slightly west of reality due to the contouring.  So assume that whole dry area, rain-shadowed by the Coast Range, is running down the middle of the valley.  This model only shows a trace to .15″ over the driest part of the valley:

Then note the corresponding snowfall forecast:
So technically the WRF-GFS shows a dusting or less for most of the lowest elevations, much better chance of an inch or two once you get up in the higher hills.

Beyond that, the snow level gradually rises Monday evening into Tuesday.  You can see this nicely in the 00z GFS 850 mb. ensemble chart that just came out a couple minutes ago:

Great agreement (best we’ve seen in a few days) on the pattern through Tuesday for sure, then a few members have a shot of cooler air on Wednesday, maybe those are still trying to send a surface low right overhead on Wednesday, pulling in the cooler air.  Regardless, the trend in the 00z GFS is definitely milder the rest of next week, but lots of rain and tons of mountain snow.

I should point out that earlier GFS runs and their ensembles today had the whole pattern shifted a bit farther south, for a more marginal snow situation Tuesday-Wednesday.  It’ll be interesting to see if the 00z ECMWF shifts things a bit farther north as well.

To sum it up, it’s quite possible we get nothing sticking officially here in Portland Sunday-Monday, but if we get lucky some cluster of showers will form and drop a quick couple of inches.

Chief Meteorologist Mark Nelsen