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Pressure gradient: More images

 
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windfind



Joined: 18 Mar 1997
Posts: 1775

PostPosted: Sun May 15, 2022 10:47 am    Post subject: Pressure gradient: More images Reply with quote

Hi Gang,

Edit: there are a couple of questions in the posts below that I attempted to answer.

The graphic below is for a blog I will do later next week. But it is more timely today as our pressure gradient pattern changes a bit.

The key concept is that when the NW ocean wind reaches the coast it flows through gaps that are the easiest pathway towards the maximum pressure gradient in the Central Valley.

Since the Central Valley has a huge N-S range so the pressure gradient is sometimes unevenly distributed. So if the max pressure gradient is towards Bakersfield the wind curves through the Bay hitting certain sites.
If the max pressure gradient is towards Sacramento to Redding then the NW curves to hit other sites.

Of course, if the ocean winds are more SW, like when we have an eddy to a marine surge after a heatwave, the wind flow is very different than in these graphics.

Hopefully, the graphic is somewhat self-explanatory but if not wait for the blog.

Mike Godsey



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Last edited by windfind on Sat May 21, 2022 7:54 pm; edited 3 times in total
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jpf18



Joined: 13 Aug 2000
Posts: 338
Location: San Francisco

PostPosted: Sun May 15, 2022 1:34 pm    Post subject: Reply with quote

Crissy windsurfer here. I was wondering - based on that, is it possible to forecast how the wind's filling into the Bay? I've taken "Wind starts at the beach / parking lot / Palace of Fine Arts" as code for "wind quickly drops off mid bay", making for saileable but short, subpar runs in inside-current and holey conditions. We've had quite a bit of that this and last season. I have yet to hit the North tower this season! That said, with the wind forecast to start at the beach today, and considering even distribution of gradients to the Central Valley: Do we expect the wind to fill in end-to-end from East Beach to near Yellow Bluff?
It would be great to take the guesswork out of the forecast a little.
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windfind



Joined: 18 Mar 1997
Posts: 1775

PostPosted: Sun May 15, 2022 3:46 pm    Post subject: Reply with quote

jpf18 wrote:
Crissy windsurfer here. I was wondering - based on that, is it possible to forecast how the wind's filling into the Bay? I've taken "Wind starts at the beach / parking lot / Palace of Fine Arts" as code for "wind quickly drops off mid bay", making for saileable but short, subpar runs in inside-current and holey conditions. We've had quite a bit of that this and last season. I have yet to hit the North tower this season! That said, with the wind forecast to start at the beach today, and considering even distribution of gradients to the Central Valley: Do we expect the wind to fill in end-to-end from East Beach to near Yellow Bluff?


Hi jpf18,

Yes, you should be able to get near the north tower today but remember the forecast said that the Sacramento pressure gradient was the lesser of the pressure gradients.

Yes, if there are NW ocean winds and the pressure gradient is mostly towards Bakersfield there will probably be yellow caps in the restrooms. But with that pressure gradient, the Marin Headlands throw a wind shadow over the north tower area. To give the forecast all that detail for every site would make the forecast very long and the list of actual readers very short. So I am trying to get more of that type of detail in the notes below each site's table numbers. But I am handicapped by laziness & time so often those notes are short.

To get wind near Yellow Bluff (≈1 mile NNE of the north tower just past Horseshoe Cove) you need a strong pressure gradient towards Redding & Sacramento + a marine surge or an eddy. Unfortunately, that often means very light wind near Anita Rock and lots of fog. Two old Crissy tricks to use judiciously on that type of day is to risk your life launching near the North tower. Or launch from Crissy on a floaty board and slog out towards Harding Rock where you will encounter strong wind then tack back and forth until you get to the north tower to Yellow Bluff area.

This is the sort of local stuff I have to use in forecasting for the Rolex Cup/Boat series and the AC.

Mike
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jpf18



Joined: 13 Aug 2000
Posts: 338
Location: San Francisco

PostPosted: Mon May 16, 2022 10:57 am    Post subject: Reply with quote

windfind wrote:
Hi jpf18,

Thank you for the detailed explanation. Sunday field report: The wind direction and ebb current were set up for a trip to the North tower, but no dice. I attempted a couple of recon runs going long, but again, mid Bay was a massive hole, as in: Not enough to waterstart at times. Less pronounced so a little further downwind. I didn't see any takers working upwind, everybody was milking an approx. half mile long slot off the beach. There was some nice little swell to ride that helped with gliding through the windbare spots. Good thing it was ebbing strong. All in all a nice session similar to what we've seen for quite a while now.
The forecast was spot on for the wind to start early and let up towards 5pm. The folks who launched at the "usual time" got the short end of the stick.
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fxop



Joined: 13 Jun 1998
Posts: 190

PostPosted: Mon May 16, 2022 12:34 pm    Post subject: Reply with quote

Why is Palo Alto typically more northerly than other mid-peninsula sites?

fxop
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windfind



Joined: 18 Mar 1997
Posts: 1775

PostPosted: Mon May 16, 2022 12:34 pm    Post subject: Reply with quote

Thanks for the report. It is only by reports like this that we can improve our forecasts. Otherwise improvements only happen as computers get faster and machine learning improves upon traditional models.

Speaking of which, watch for our AIB virtual sensors to appear just offshore at some sites soon. These are model forecasts enhanced by machine learning for sites where the wind inside can be very different that the winds outside. 3rd. Ave., Crissy and Pt. Isabel will be first sites.

AIBs are operational in the La Ventana area where we did preliminary testing this winter. See image below.

Mike Godsey



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windfind



Joined: 18 Mar 1997
Posts: 1775

PostPosted: Tue May 17, 2022 8:26 am    Post subject: Reply with quote

Quote:
Why is Palo Alto typically more northerly than other mid-peninsula sites?

fxop


Hi fxop,

I love low-hanging fruit type questions I can actually answer. See the graphic below where I have simplified the wind flow with white arrows

(Unfortunately, the graphic below is for a more W not NW day so the depicted Palo Alto winds are not as NNW as usual)

The reason Palo Alto usually has NNW wind is the long NW to NNW oriented valley from the flats of San Jose over Morgan Hill to Dunnville. And this valley connects to the narrow valley heading towards the Pacheco pass over San Luis to the Central Valley.

When the ocean winds are NW and the pressure gradient is towards Bakersfield wind pours through the San Bruno Gap and curves as NW to WNW wind past Coyote and 3rd. Ave. as it is sucked towards the Altamont Pass area to the Central Valley. But part of that wind curves towards that long valley past San Jose. That curve makes the wind more NNW.

It is because the marine air is dense that it takes these circuitous routes through gaps in the coast range.

It often gets more complex because of the SW facing Highway 92 gap that messes up all this flow unpredictably as 3rd. Ave. folks know all too well.

Mike



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fxop



Joined: 13 Jun 1998
Posts: 190

PostPosted: Sat May 21, 2022 12:49 pm    Post subject: Reply with quote

Now PA makes sense! Thanks Mike!
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windfind



Joined: 18 Mar 1997
Posts: 1775

PostPosted: Sat May 21, 2022 7:59 pm    Post subject: Reply with quote

Today was a better day for visualizing the causes of NNW winds at Palo Alto. See the animation below.


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