Wednesday, February 14, 2018

La Nina Snows Over the Pacific Northwest Followed by Very Cold Air

The atmosphere over the eastern Pacific is thoroughly locked into a La Nina-type pattern, with the snowpack being refreshed for NW skiers and those concerned about the water supply next summer.

The snow water equivalent (the amount of water in the snowpack if melted) is in decent shape (see below), being above normal for the northern and far eastern portions of Washington State, but slipping to about 3/4 normal for the southwestern Cascades.

There was some improvement last night, when 8 inches to a foot of new snow  over the Cascades due to an upper level trough moving southward along the eastern flank of a big ridge over the northeastern Pacific (see below).

As I have noted in earlier blogs, a ridge in the eastern with cool northerly/northwesterly flow moving over the Northwest is typical of mature La Nina events. That sets up the cool temperatures, while the upper level trough moving southward provides the precipitation.

But to get lowland snow one needs to get an upper trough of just the right amplitude and position, which doesn't happen often.  Too far inland, and the cool air and precipitation are too far east.  Too far offshore, we are cold and dry.

Another, more vigorous, trough will approach on Saturday (see below), resulting in another snowy period in the mountains.

The 48h total snowfall ending 4 PM Saturday is enough to make a skier smile, with over two feet at high elevations in the Cascades and a relatively low snow level on Saturday (Snoqualmie Pass the eastern Cascade slopes will get plenty).

Another trough comes through on Sunday morning (see below), with most of the action slipping southward into Oregon, which really needs the snow.

The 48h snowfall total ending

But what really got my attempt was the very cold air predicted to move into our region early next week.  The coldest air in a very long time.  Here is the UW WRF model surface (2-m) air temperature forecasts for 4 AM Monday and Tuesday.

WOW.   East of the Cascades many locations will get below 0F, some locations will be way below.  Twenties near the water and teens elsewhere in western WA.

You might think about protecting your exposed water pipes.


MacD said...

-7C in Vancouver forecast. Is this February?

jeff said...

So you're saying The Groundhog was right yet again?

John said...

Finally,the ridges and troughs are in the proper position for a La Nina winter.Better late then never.I just hope the models verify;they have been very poor and inconsistant this winter,especially the ECMWF.
Daily record snowfall over here in Spokane today, 7-11" at most locations.First appreciable snow in over a month.

John said...

Pretty good chance that Sea-Tac will set at least some new record lows next week, for the first time in seven years.The records for the 20th through the 22nd are 25,25,
and 30, respectively.

Unknown said...

Right now it does not appear that we will be breaking any cold record, but it will be close.

JW said...


In an earlier blog post, you mentioned the end of February as being the end of meteorological winter. Now it looks like La NiƱa is finally kicking in. Will its effects be short lived since we are so close to Spring?

John Marshall said...

Don't care one way or the other if we get record cold, but for those of us on the NE Olympic Peninsula, this is exciting.

We always get a nice upslope near Sequim when we have a Fraser River Outflow. If any moisture is still around when the Arctic air flows across the Strait (as is currently predicted), then we're going to get some serious snow and ice on our 1000' hilltop that's only a bit more than a mile from the water. A nice steep rise for the icy winds to push moisture from over the Strait up while losing a few more degrees to precipitate heavily. We sometimes get a foot or more of hilltop snow out of these setups.

This one is forecast perfectly given that once the snow falls, it is going to freeze hard for 3 or 4 days with only sunshine melting in midday to create ice after dark. Basically, that's perfect to turn our very steep north and east facing access roads into high-speed sledding runs. Even vehicles with chains won't be able to climb them.

I'll make sure to buy milk and supplies on Saturday to last the week, and pile the firewood high. Being snowed/iced in for the better part of a week is one of the special pleasures of living on our hilltop. (But please, only once per winter.)

If it all comes to pass, it'll be a nice ending for an otherwise dull, somewhat dry winter at my house. I've already been cutting my grass and was thinking about preparing for planting. But one more hardy blast of seriously snowy, cold weather will leave some strong Winter memories and make Spring all the more welcome when it does come.