Thursday, January 21, 2016

Atmospheric River Approaches

During the past several weeks, mountain snow lovers have enjoyed a bonanza.
More and more snow, with no rain periods.   Deep soft snow.  Little ice.

But good times are over.  A warm, wet atmospheric river is upon us.  And Cascade Concrete is about to return.

The latest WRF model forecast for the 24-h total precipitation through 4 PM Thursday, shows moderate to heavy precipitation over our region, with over 5 inches in some portions of the Olympics and north Cascades


This precipitation is the result of a relatively narrow plume of moisture coming out of the subtopics....known as an atmospheric river. Below is a forecast of the vertically integrated water vapor for 4 AM--the total amount of water in the vertical.  The atmospheric river (red and white colors) is obvious.

The infrared image at 9 PM Wednesday night shows substantial clouds with the atmospheric river, with moisture streaming from the SW to the NE.


The atmospheric river is associated with strong winds and quite warm temperatures.  Here is the forecast 850 hPa (about 5000 ft) for 4 AM Thursday.  The orange colors shows the warm air.   The snow will rise to around 7000 ft today....well above the passes. Initially, cool easterly flow will keep snow falling in the passes, but sometime today the strong, warm flow will win and rain will fall on the snow.


 If you mix lots of snow with warm temperatures and heavy rains, what do you get?

Avalanche threat.  Particularly in the north Cascades, where the rain will be very heavy.  My colleagues at the NW Avalanche Center have serious warnings out (see below).  Be very careful in the backcountry for the next few days.

More rain on Friday...sorry.





12 comments:

lhsouthern said...

What about flooding?

Unknown said...

Still showing snow on snoqualmie, Stevens, Blewitt and White Pass this morning at 7:00 a.m. hopefully the temperatures will remain colder than what was forecasted

blackandsmart said...

I have been reading a lot about a drier than normal winter forecast for the PNW due to an exceptionally strong El Nino. Is my perception that Dec and, after a brief respite, Jan are very wet indeed in the Puget Sound region? I check West Coast radar frequently, and while N Cal is getting its fair share, S Cal is looking much drier than what had been anticipated. Am I wrong, or did El Nino not live up to expectations?

jno62 said...

This drier than normal pattern is awesome. Makes for great cycling. Love El Nino.

The Drennans said...

Maybe some people enjoy the back country in any condition. But really, how many folks really enjoy a slushy slog in feet of wet snow, with wind and pouring cold rain? and a big avalanche threat to boot?

The Drennans said...

Surely jno62 is jest. Drier than normal? lol.

brotherzaneosf said...

Here on Bainbridge Island.....still waiting for that "drier than normal El Nino" that has been discussed. I am beginning to have my doubts.

measuredmass said...

The rains have arrived just in time. Our snowpack was beginning to become unpleasant, with the first persistent weaknesses becoming troublesome. There's no substitute for a big warm rain to return our snowpack to its frequently-docile nature.

soaringbrain said...

I have given up all hope of seeing snow this year. (In Seattle) :(

Matt Thompson said...

I think the pattern is going to change in mid-February, but don't quote me on that. Yes all agencies and especially NOAA have been wrong for November, December and January. But, I have been having a back and forth with some meteorologists online and the wet pattern is supposed to change out in mid-February with above normal temps coming back. One meteorologist I emailed with has stated that this rain and snow is a direct result of the strong El Nino, and that it has weakened slower than expected. It keeps getting weaker, and the weather should moderate. The recent uptick, as I understand, is because a couple weeks ago, El Nino got slightly warmer again, due to strong trade winds or something. And the winds have died down so the waters are starting to slowly cool again. I know I am sick of the snow and cold, but it looks like some warmer temps, rain and melting snow are in store for Central Washington, which will be nice. I hope it doesn't flood.

BTW, from what I understand a strong El Nino can go either way. Some years in the past were way wetter than normal and some way drier, it all depends on many different factors. And it is way too early to say La Nina for the next winter. Given how the El Nino region is slowly cooling off, a reasonable forecast is ENSO Neutral.

richard583 said...

Main colder air mass having begun to move and spread more southward [again], post its general regression over and during the past two weeks, approximately. (?)

Joseph Ratliff said...

What Matt points out in his last paragraph (if true) would indicate why the media and certain public figures need to be more responsible in their evaluation of potential climate (and resulting weather) conditions.