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Updated 3rd. Ave battle lines today & tomorrow

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Joined: 18 Mar 1997
Posts: 1689

PostPosted: Fri Aug 31, 2018 2:26 pm    Post subject: Updated 3rd. Ave battle lines today & tomorrow Reply with quote

Below see new post in this thread showing how the Hwy. 92 gap impacted the 3rd. Ave. winds yesterday and probably today and for sure tomorrow.

Hi Gang,

The animation below is for a future blog about hows gaps in the coast range funnel marine layer and winds to various sites.

Since I used today's fog and winds as an example I thought I would post the imagery now.

The marine layer is a cool dense higher pressure air mass that hugs the surface of the ocean. When a pressure gradient sucks it inland it flows through low spots or gaps in the coast range. This creates a very local venturi effect accelerating and redirecting the wind.

Which gaps are producing the most wind depends upon where the lowest pressure is in the Central Valley. For example, if the pressure gradient is strongest to the northern Central Valley then the favored pathway is through the Golden Gate, Hwy. 92, Pacheco Pass and Muir gaps. And this, in turn, favors sites like Pt. Isabel, Larkspur and Sherman Island. While if the pressure gradient is strongest towards Bakersfield the San Bruno Gap and over Morgan Hill routes thru Pacheco Pass are favored so Coyote and 3rd. Ave. have good winds.

I have not shown all the gaps in this animation and it is very simplified but you can see many of the gaps as they are defined by the fog.

To use the animation concentrate on ONE gap and watch it through the entire animation otherwise it is really confusing.

Incidentally, the animation below is NOT a forecast! It just shows the location of different gaps and how fog moves towards or into those gaps. For example for Larkspur or San Luis to blow the fog would have to be much further inland than it was on this particular day.

Mike Godsey

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Last edited by windfind on Sat Sep 01, 2018 11:40 am; edited 5 times in total
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Joined: 27 Jun 1995
Posts: 987
Location: Santa Cruz Ca

PostPosted: Fri Aug 31, 2018 5:23 pm    Post subject: Reply with quote

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Joined: 18 Mar 1997
Posts: 1689

PostPosted: Sat Sep 01, 2018 11:39 am    Post subject: Reply with quote

Hi Gang,

Here are some of the e-mails I received from customers about the winds INSIDE at 3rd. Ave. & Coyote yesterday:

"Reading your report this morning, yesterday was indeed a super weird day. We had NW winds inside at Coyote (with some fluttering SW gusts), with up and down (sometime strong) W to SW winds halfway towards the channel. I have sailed here for 15+ years. Is it me, or is this frequent eddy thing a new phenomenon? Every time I feel like we have a good tide set up for some fun, our friend eddy shows up and ruins the party. Smile"

"10m kite twin tip board. 4:50pm to 6:20pm.
Struggle the whole time. Up and down wind. Lots of down wind low teens even times when hard to keep the kite flying. And many up times soild mid 20s even a little over powered. I hung in there but it was really a constant struggle. Also had a significant westerly component! I was not alone I discovered after taking to other kiters about the conditions. Thanks for listening!"

"4:00 wind was NW, filled in to the shore
4:30 wind had turned W, light at the shore. I had to work hard to keep the kite up getting through the golf course wind shadow at the lower beach.
4:30-5:00 wind was light and gusty
5:30 wind got strong and weak in streaks
We kept crossing from strong to weak to strong."

So what the hell is going on! For many years 3rd. was one of the most reliable sites in the Bay Area but in recent years and especially this year the winds have often been weird especially inside. It has taken me a very long time to figure out what is going on and it is still a challenge to forecast the issues in advance.

First, look at the fog animation in the first post above so you can see how the Hwy. 92 gap funnels the marine layer clouds towards 3rd. This funneling effect is especially pronounced when there is an eddy along the coast.

Then check out the 2nd. animation below. (for larger imagery with the text click on this blog link:

Find the 3rd. Ave. launch sites area. Then notice the solid WNW winds coming through the San Bruno Gap towards the channel.

Now notice the W to WSW eddy winds near Half Moon Bay that are funneling through the Hwy. 92 gap area.

Notice how the WNW winds and the WSW winds converge and the battle near the launch sites. Yesterday this battle occurred near the launch area hence the reports above.

But on some days this battle line may be a mile inland which would mean great winds inside and outside at 3rd.

At this time I am pretty good forecasting which days are likely to have such a battle and mention this in the text, bullets and table text. But I still cannot forecast exactly where the battleline will be.

Worse, as K. above discusses, the WNW vs. WSW battle line can move around during the afternoon. So if you arrive at the right time you are praising the forecast, the wrong time you are cursing! And Coyote, often on the edge of the battle is even harder to forecast during eddy conditions. While Palo Alto and Natural Bridges and San Luis benefit from the eddy.

For example yesterday I incorrectly thought the battle would be a bit inland of 3rd. and forecast good winds inside but gave lots of warnings about eddy issues. And tomorrow I am pretty certain the eddy will win.

Keep sending or posting reports. I love criticism but reports are more likely to lead to improved forecasts.

Mike Godsey

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