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windfind
Joined: 18 Mar 1997
Posts: 1901
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 Posted: Sun Apr 26, 2015 11:02 am Post subject: Blog: when NW and WSW winds fight. |
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Hi Gang,
Yesterday most sites in the Bay Area had good winds but, as forecast, Pt. Isabel and Berkeley had weak winds. This was frustrating for locals since the day before, in almost the exact same pattern, those sites had epic wind.
Here is the story of behind the weak winds at Berkeley and Pt. Isabel:
http://blog.weatherflow.com/why-did-i-forecast-strong-winds-everywhere-except-pt-isabel-and-berkeley/
Mike Godsey
iwindsurf.com
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gybearoo
Joined: 02 Apr 2005
Posts: 7
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| Posted: Sun Apr 26, 2015 10:40 pm Post subject: |
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| Thanks for the nice explanation, Mike.
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windward1
Joined: 18 Jun 2000
Posts: 1400
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| Posted: Sun Apr 26, 2015 11:08 pm Post subject: |
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Hi Windfind,
I appreciate your explanation and I believe I understand it and the chart, however, it seems counter-intuitive. It would seem the two forces joining at that low angle would enhance the wind rather than diminish it. I could see that if they were headed at an angle where they were coming more at each other that the energy would be absorbed in the collision and no wind would be the result.
Does it have to do with one being over top the other? The one from Tomales being over the one from the Golden Gate, thus the Golden Gate wind gets rolled and dispersed?
Regards,
Windward1
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windfind
Joined: 18 Mar 1997
Posts: 1901
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| Posted: Mon Apr 27, 2015 9:16 am Post subject: |
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| Quote: | Does it have to do with one being over top the other? The one from Tomales being over the one from the Golden Gate, thus the Golden Gate wind gets rolled and dispersed?
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Hi Windward,
Good reasoning!
I first figured out the NW vs. WSW wind battle by taking the ferry to Angel Island. Looking like a fool I hiked up Mt. Livermore with several helium filled silver mylar balloons on a strong NW day. At the top I found extremely gusty shifty winds from the NW to WSW. Since at that moment science outweighed political correctness I released the balloons and watched their pathway with binoculars. The balloons traveled erratically at first but once away from the lee of Angel Island they were picked up by the WSW flow from the Golden Gate. But after about a mile my binoculars showed them hitting NW flow and their course showed that they were in a extremely turbulent flow. From this I deducted that when there was strong NW wind just aloft it pushed the NW surface winds through the vague gaps between Tomales and the bay. This wind then pushed over the bay to deflect the WSW wind heading towards the Pt. Isabel/Berkeley zone.
The pathway from Tomales is not a straight shot near sea level gap like the other gaps in the coast range. Most days the NW wind is so diffuse that it does not impact Pt. Isabel or Berkeley. So to get the wind killing effect you need both surface NW wind and strong NW winds aloft.
If the NW wind aloft are mild then thinks get complicated since the WSW surface wind may overpower the NW wind effect. And then there are the impossible to forecast days when the two winds are more equal.
Since the WSW wind is COOL dense air it hugs the surface. But the NW wind is relatively warm since it has come over the land mass of Marin. So it is unstable and does not stick to the surface. This NW wind has a lot of vertical motion after coming over the coast range touching the surface at times and other times lifting from the surface. So if you are kiting or windsurfing in the Berkeley to Pt. Isabel area you may have solid WSW to W wind suddenly die as the NW wind swoops near the surface and deflects the WSW wind. That is why Berkeley and sometimes Pt. Isabel sometimes die so fast.
You can visualize all of this in the graphic below.
Mike Godsey
iwindsurf.com
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