Dissection of a North wind event in the Bay Area.

Why are we expecting such strong North winds tomorrow yet so few sites receive this wind

by Mike Godsey, mike@iwindsurf.com

If you saw the gobbledegook forecast I posted yesterday, Oct 1, 2013, you have to wonder what all the stuff about strong ridges and the valley low expanding over
North Winds the coast means.  Every fall you see one or two forecasts like this promising strong AM north winds. And you may notice that the NWS stated this morning: “Have issued a Fire Weather Watch for the North and East Bay hills as well as the Santa Cruz Mountains starting at this time. Will also have to consider Wind Advisories as the event nears.” And for most of you wonder what is the big deal since in the past you see little hint of those big North winds. So let’s use imagery to bring some graphic clarity to the gobbledegook forecast and explain why few of you will sail or kite these winds.

For comparison let’s first look at what is behind Ben’s forecast for today Wed. Oct. 2 for NW winds to the upper teens+ or or so at Waddell, Crissy, Coyote & 3rd, and then compare it the expected North winds Thursday.

Image #1. In this image for today find the center of the North Pacific High in the middle left of the image.

Picture the high as a huge dome of high pressure air that
wanders around the east pacific often between Hawaii and the west coast in the summer. Then notice the isobars that form irregular circles around the high pressure. Each of these isobar lines represents areas that have equal pressure.

Lines nearer the center of the NPH represent higher pressure while lines further from the center represent areas of lower pressure. So there is a pressure gradient from the center of the high pressure to its edges.

Notice for the most part these isobars are far apart meaning the pressure gradient is weak. But note how the isobars are much closer together along the California coast. This means there is a stronger pressure gradient on our coast.

Now picture winds spiraling out from the center of the North Pacific High. At the center of the NPH  there is no pressure gradient hence no wind (as I once found out the hard way after using a college scholarship to buy a 24 foot sloop and naively set sail to Hawaii.) But anywhere the isobars are close together there is a strong pressure gradient and the winds spiraling from the high pressure strengthen.

This tightening of isobars often occurs in the summer near Hawaii creating the NE trade winds and along the west coast creating strong NW winds.

Image #2 shows the modeled winds from the NPH today. Notice the strong winds near the Northern California coast. Later today as the Central Valley heats up the SFO-SAC pressure gradient  will suck those NW wind to the coast and they will CURVE into bay as WNW to WSW winds.

Now take a quick look back at Image #1. Notice how the NPH dome of high pressure is pushing a ridge into far Northern California. Today all that will do is create weak NE winds aloft in the AM. But tomorrow……

Image #3 is hard to believe! Suddenly the shape of the NPH has changed dramatically in 24 hours!

Look at this image and  note: 1. the strong ridge of high pressure from the NPH covering much of Northern California north of Sonoma. 2. check out how the NPH’s isobars have moved westward away from the coast. 3. see how the Central Valley thermal low is has expanded way out into the pacific.

4. Most importantly note that all of this creates a HIGH pressure towards Redding and a LOW pressure to the south. So we have a N. to S. pressure gradient which creates the N to NNE winds.

Image #4 shows the impact of all of this on the wind patterns. Notice how the strong NNE to NE winds coming down the Central Valley and over the Sherman Island area. Also notice how the NW ocean winds are turned a bit NNE by this pattern and pushed a bit away from the coast. And with the Central Valley thermal low over the bay the pressure gradient is weak so the thermal wind to the valley is faint and little of the NW ocean wind curves into the bay.

But still…why does so little of this strong N. wind reach sites inside the bay and why am I recommending that the hard core head to the old sites like the Jungle towards Rio Vista rather than Sherman Island?

Normally the Bay Area has cold dense marine air moving in our summer thermal winds. This type of air is called “stable” air since it is heavier than the air above it. So when it hits topography it goes up a hill and then flows down the hill and reattaches to the surface. That is why the wind is usually remarkably steady in the summer in the Bay Area despite all of our hills. But the air in this North wind event is warm air that has a density about the same as the air just aloft. So when it hits a hill it rises up the hill and does not easily reattach to the surface.

Looking at a topographic map you will notice that Sherman Island has hills to the north hence unreliable North wind. This is also true for almost all of the sites in the Bay Area. But looking at the part of the river towards Rio Vista you will notice it is open to the north.

Of course I am leaving out a lot of factors in this brief dissection of N. winds. For example there is the venturi effect of the coast range and the impact of the winds aloft. But I suspect few of you have even read this far so it is time to stop.



Posted in San Francisco