How can it be so windy on the coast but not in Seattle?
November weather is now in full swing, with the big 3 weather fall/winter weather events in Seattle -- lowland snow, windstorms and flooding rain -- all poised to make an appearance within the same week!
The lowland snow is done, and now come the other two. In fact, the pattern setting up is so stormy that the National Weather Service has rare double wind advisories issued for parts of the region to cover two separate, but fast moving storms.
A lesser Wind Advisory is in effect for the coast and Northwest Interior for Monday morning into midday hours for potential gusts to 40-45 mph, then a stronger High Wind Watch is in effect for Monday evening into Tuesday for potentially 60-65 mph gusts.
These first two storms are the opening salvo of what could be as many as five storms to blow through the Pacific Northwest this week and we could see more of these types of watches, warnings and advisories for the coast and Northwest Interior.
But Seattle and Puget Sound areas? Not so much. While it'll be breezy to blustery, winds are not expected to be as strong in any of these storms.
How is that? While all five of these forecasted incoming storms are strong, their tracks are just not as conducive to wind in the Seattle area as they are on the coast and northwest interior.
To get a good windstorm in the greater Seattle area, we want the center of a storm to make landfall along the northern tip of Washington or into southern or central Vancouver Island.
As a storm approaches the Pacific Coastline, southerly winds will rapidly increase as the air goes north to attempt to fill in the deep area of low pressure. Out on the coast, there is no topography to offer much resistance, and the winds can really roar unabated.
Then, as the storm is just about to make landfall and is due northwest of the Northwest Interior region, the way our geography aligns between the Strait of Juan de Fuca and Vancouver Island, this region will get a strong southeasterly wind -- and with the wide open waters of the eastern Strait and the way Admiralty Inlet is aligned, there is little land resistance here either.
But in both of these periods, the Olympic Mountains stand in the way between Seattle and the storm's center, and thus winds in Seattle are relatively calm while it can be raging on the coast and on Whidbey Island.
For those winds to reach Seattle, we need the storm to continue to track east/northeast to where it is still a strong storm when it crosses due north of Seattle. That's when we get the southerly surge of air.
But in seemingly all five of the cases this week, the storms -- while pretty intense -- are making landfall into the northern tip of Vancouver Island or even up farther north. And the storms are continuing north and fizzling to where by the time they get due north of Seattle, they're quite far away and the difference in pressure isn't all that great.
Here are some good illustrations, courtesy of the UW MM5 forecast model. In our case, the winds will flow perpendicular to the black bars on these maps (the isobars). The closer to the black lines are together, the stronger the difference in pressure and in turn, the stronger the winds will be.
This map shows 4 a.m. Monday:
Look at the number of lines drawn across the Washington coast -- and also look if you note the way the lines are pointing north of Seattle - draw another line at a right angle to those lines and it's aligned northwest-southeast -- perfect for southeast winds to roar in those areas.
But in Seattle, we want those isobars to be aligned east-west to get a good south wind. Here, we get some but it's more northwest-southeast here and not as packed together. This is still a windy period for Seattle, but not as windy as the other spots.
And here is 10 a.m Monday:
Note that by the time the storm moves inland, the lines are nowhere near as packed over Seattle.
How about Storm #2?
This map shows 10 p.m. Monday night:
It's got a strong 965 mb low, but way far away - even north of Vancouver Island. But lots of closely packed lines over the coast and northwest interior again, while Seattle is still protected by the mountains with a light southeast wind.
And here is 4 a.m Tuesday:
OK, actually that is a moderately decent pattern for gusty winds in Seattle with a 983 mb low due north. (Full disclosure -- I started writing this blog before the late maps came out and they weren't as impressed as they are now.) Still, this is probably gusts to 40-45 or so in Seattle while the watches are for 60-65 mph gusts in the coast/Northwest Interior areas.
The wind events later in the week follow a similar progression -- enough to make it windy on the coast and northwest interior, but nowhere near as strong in the Puget Sound region.
But for those of us who rely on traditional electricity-required methods of cooking a Thanksgiving turkey, we'll take the advantage.