Friday, December 28, 2018

Another Windstorm Tomorrow

This is almost getting routine.   Another wind event will occur tomorrow (Saturday), but with a bit of a different meteorological twist:  no major cyclone, but a warm front passage, followed by a strong cold front.

Today, I will show you some advanced meteorological technology for wind prediction, something developed at the UW with funding from Seattle City Light and the Northwest Modeling Consortium.   
This figure shows the peak wind gust forecast over the city of Seattle predicted by the National Weather Service (green line), a super-high resolution numerical model prediction  run at the UW(red line), and a collection of high-resolution forecasts (gray lines and blue line).    Showing many forecasts (or an ensemble), each slightly different in their initialization, provides a way of estimating the uncertainly in the prediction. All these predictions were started at 4 PM Thursday.
These forecasts are the gusts (short-term wind maxima), based on a calibration from historical periods.  Virtually all the models are going for a significant wind event on Saturday, with maximum gusts over Seattle getting to 40-50 mph (most places in Seattle will experience less) between 10 AM and 4 PM.    Note how the wind continuously ramp up starting this afternoon....this is associated with a warm front passage.

The above graphic is available to the public on the Seattle City Light Windwatch page (https://atmos.washington.edu/SCL/).  WindWatch was created by my group (mainly research meteorologist Jeff Baars) with support from Seattle City Light.

The latest high-resolution (1.3 km grid spacing) run is in (below).   At 1 PM Saturday, it is windy around central Puget Sound (gusts of 35-40 knots), and if you look carefully you will notice the windshift on the NW coast associated with the cold front.

 Three hours later, the cold front has reached Hoquiam, on the central WA coast, and and strong westerly winds have pushed into the Strait of Juan de Fuca.


Considering all the stress put on trees during the past two windstorms, I would expect for less damage this time, but I am sure there will be some power outages.

To give you a better view of what is going on, here is a plot of sea level pressure (solid lines), low level temperature (colors--yellow is warm, blue is cold) and surface winds.  The situation at 4 AM this morning, shows cold air over the interior and the warm front offshore (transition from green to yellow, and significant wind shift).

 By 10 AM Saturday, warmer air had spread over Washington and Oregon, leaving us in the warm sector.    Note the large north-south pressure difference--that is what will produce strong winds.


The winds should weaken Saturday evening, with cooler and partly cloudy conditions on Sunday.
New Year's Eve should be dry!





10 comments:

Rabbits' Guy said...

The BOW-WA Predictor stone is wet but with no list. But I best double tie the string on it in preparation for wind gusts.

WindChimes said...

This windy and wet December is reminding me of the record-setting winter of 2006-2007, although fortunately we’ve thus far avoided the disruptive lowland snow events (several occurring at evening rush hour) that occurred then. I distinctly remember how the windy rainstorms just kept coming during the 1st half of that season, punctuated by the grand finale of the Dec 14-15 windstorm. Interesting that the CPC’s 6-10 day outlooks recently have noted analog patterns to certain periods in December 2006 as well.

Chris H. said...

For backcountry skiers, wind in the mountains, especially during a snowstorm, means snow will be transported into wind slabs.

Wind slabs form on the lee side of terrain features such as ridge tops.

As wind blows across mountain faces,snow slabs will also form in a process know as cross loading. Slabs will form in gullies, the leeward side of terrain features and snow slopes far away from ridge tops.

Wind slabs have the potential and oftentimes are an injurious and deadly trap for skiers as these unstable snow slabs are primed to Avalanche through the release of their stored energy and break up.

Skiers tend to be snow slab triggers as they transfer enough energy to that slab formation to tip the balance of instability.

I remember one time stepping out from behind the protection of a large Rock onto an old slab as our group was making its way up into steeper Terrain.

Two members of our party had already crossed that section and as I stepped out I felt the snow moving but no crack propagation.

A friend, who was still behind the rock saw the whole thing and he said that the snow moved like a wave.

Luckily that old slab did not have enough stored energy to initiate a crack propagation and release as an avalanche. However the weak snow below the slab, that can typically form and persist around rock formations, eventually collapsed as our group placed more and more stress on the snowpack.

It's fortunate for us that sometimes the mountains give us warnings not proceed. That instability warning became our turnaround point instead of proceeding upwards to the increasingly steep terrain above us.

For snow Mountain Travelers of all abilities, now is the time to pull out your avalanche study books and brush up on wind slab formation.

You do have some books to study right?

Chris H
Heli-free North Cascades


Chris H. said...

Part 2.

In the example that I gave above, I wanted to clarify that as I stepped out onto that old slab the weak layer below collapsed and no crack propagated through the slab itself.

That weak layer collapse would be considered propagation through the weak layer and the wave that was observed was the resulting deformation of the slab.

Below is a link to some of the snow science that models that behavior.


Remember that these are just models and snow is a very complex medium and not completely understood.

Fortunately there's still some mystery to snow and that's why people say avoiding avalanches is an art. However I believe that much of what we consider art is luck combined with the practice of best safety practice protocols.

I also believe that an understanding of the science behind Avalanche formation will increase your odds of safe travel in Avalanche Terrain. For example 90% of the Avalanches that occur happen during or within 24 hours of the last snow storm. Simply avoiding those times will increase your odds.

It takes time for strengthening bonds to form between snow layers and snow itself. That time is variable between factors such as different snow grain types, snowpack depth, rate of snow deposit,and temperature.

Wind transported snow or snow that is deposited from rock walls above snow deposition zones rapidly increases the depth of that snow and places more strain on any existing weak layer.

During the last heavy snowfall amounts we saw a lot of large climax avalanches originating in wind deposited snow and or snow deposition zones below rock walls (snow loading).

We also have in place a weak faceted snow grain type layer at the ground/snow interface. There's also another persistent weak layer about a foot above the ground. Eastside North Cascades.

Other regions also have their own set of problems to consider with the addition of some rain crusts in the mix.





https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.igsoc.org/journal/58/209/j11J009.pdf&ved=2ahUKEwjQlo73wsXfAhUtJTQIHcvHDsQQFjAAegQIAxAB&usg=AOvVaw2gHsaQXujGXK_NQHIgCABw

TW B said...

Another day another wind storm? Just saying. I would give the nod to the UW Hi Res forecast. Very blustery in Issaquah through the night (25-35 mph).

John McBride said...

Maximum strength gusts appear to have been reached in the Wedgwood neighborhood of NE Seattle about 11:00 AM - 12:00 PM. I was out walking early this morning, from 8:00 - 9:00, and the wind was strong, averaging 10 - 15 mph, but probably no gust in excess of 25 - 30 mph. I don't have a wind gauge so my observations are anecdotal, but I've been alive 70 years and have an idea of relative wind speed. As events go I'd judge this one a typical Autumn, early Winter frontal passage event. But the science is interesting and may help prevent damage from more serious storms.

John said...

Recurring windstorms this December have also been a problem in Montana,too.A rapidly intensifying low in the Alberta Rockies today is causing sustained 35-50 mph winds with gusts to 70- 90 mph along the Front Range in Montana.

MAC in Bellingham said...

Did not really fully materialize as far as I can tell. There were a few mid-30's gusts mid-morning at the Bellingham airport, but that is very tame by our standards. Looks like Seattle and south did not get much with the biggest gust at Paine Field.

The original NWS forecast did not have a wind advisory for Whatcom County, but they updated it later to include us.

It appears to me that the NWS forecast was better than the UW or ensemble.

Stephen Murdock said...

Very dark/gloomy day 1.5MI SW of KBLI. I measured (Davis VP2) a maximum irradiance value of just 18W/m2 at 3:07PM. This is the lowest daily maximum irradiance value I've measured in over 3 years of records. There were just over 7HR of measurable (>/= 1W/m2) irradiance. For reference, the penultimate lowest such value I've measured in the past 25 days is 25W/m2 on 12/11 (a particularly low value). The maximum such value I've measured over the same period is 329W/m2 on 12/26 and the mean maximum daily value over the same period is 170W/m2.

Chris H. said...

In my area today Dec 30th, nwac is reporting two wind slab related skier trigger avalanches.