Friday, January 23, 2009

The Fraser Gap Wind


One of the more interesting local weather features of our region is the gap flow through the Fraser River valley...and this phenomenon will be quite evident this weekend. In fact, it has already started.
Take a look at the topographic map I have attached. The most substantial weakness in the Cascades north of the Columbia Gorge is the Fraser River Valley, located just north of Bellingham. When cold, dense air fills the interior basin of BC, a big pressure difference occurs down the Fraser..and as a result cool, dry air pushes along its axis to the SE. Bellingham often gets strong NE winds and cool, dry air as a result.
The surface chart for tomorrow at 4 PM shows cold air (blue and purple colors) and high pressure over the interior of BC. This will enable a strong Fraser River flow.
In fact, the Fraser Gap flow started already this afternoon. The plot of wind speed (attached) showed a decided acceleration at Bellingham around 20 UTC (noon PST). At the same time, the relative humidity plummeted as the continental air from the interior hit that location (see plot). Another plot of temp and temp point shows a drop in humidty (dewpoint declined), but the temperature rose quickly. Why? The reason is that strong winds from the Fraser mixed up the air, destroying the inversion and scouring out the cold air that was in place. Later on, as the primo cold air from the interior pushes through the Fraser, Bellingham will be a relative cold spot.
The winds exited the Fraser will be quite strong, especially Sunday morning. Take a look at the computer simulation for 4 AM on Sunday...sustained NE winds of 35 knots are being predicted around the northern portion of the San Juan Islands...people there should be prepared for a good blow.
On Sunday, dry cold air will spread across the region...and it is clear that lowland snow will not occur. Only the mountains will get a light dusting late Saturday and early Sunday. Expect sun the second half of Sunday from Seattle northward and Monday should be sunny too. But the period will be cool, particularly Monday morning, when temps will drop in the 20s and even upper teens in the colder interior locations.




7 comments:

JewelyaZ said...

We'll be in Portland so we'll miss whatever happens in Bellevue, but my weather station will record it for me. :-)

Does the same thing happen down the Columbia Gorge to a lesser extent when the air is cold in Eastern Washington? It seems like it should... but I am pretty unfamiliar with the weather to the south of us.

Cliff Mass Weather Blog said...

Yes..there will be easterly flow through the Gorge...cliff

mb in Port Angeles said...

Great topo map. Where can I find it online? (Hope it's not Google Earth. Google earth gives my poor old PC the pip.)

As always, 10,000 thank-yous, Cliff, for sharing your knowledge with us.

Anonymous said...

Cliff, is the inversion gone?? As I sit here on the banks of Hood Canal, Kitsap county, I wonder why there is after this the third day of 'good' quality air in Bremerton I can't fire up my space age catalytic wood furnace and HEAT my house rather than huddle in one room around an inadequate space heater!

andycottle said...
This comment has been removed by the author.
andycottle said...

From what is seen in the GFS models as of late, it looks like this is going to one dry trough with no precip for us at all. At least up here in Western Wa. Still the best chance for any frozen precip to fall looks to be NW OR where perhaps a dusting to just flurries may fall in that area. Also, it does`nt really look like 925mb temps go above freezing until about Wed or so. So certainly some chiller days coming up!

EDIT: Re-adding my post as I did`nt say say were precip may fall when I first posted.

mb in Port Angeles said...

yaay. found it. It's just Google Maps terrain view. That my PC can handle...