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Video: Cut-Off Low kills Southern California coast winds

 
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windfind



Joined: 18 Mar 1997
Posts: 774

PostPosted: Sun Aug 18, 2013 7:28 pm    Post subject: Video: Cut-Off Low kills Southern California coast winds Reply with quote

Hi Gang,

Go to this video link to see how the Cut-Off Low over San Francisco totally killed the Southern California winds today:

http://blog.weatherflow.com/?p=14423

Mike Godsey



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cbknap



Joined: 03 Jun 1997
Posts: 300

PostPosted: Sun Aug 18, 2013 11:56 pm    Post subject: Reply with quote

I would politely suggest that the convective cloud buildup south of SB is what shut off SC wind Sunday. It was glaringly obvious on the NWS satellite 1km as early as 10 am Sunday...surprised Mike did not notice. Yes there were giant cumulo clouds towering over the slopes of the SG mts but that can actually supercharge coast winds. In this case the CCW rotation around the cutoff low that Mike called out built a dam that shut off thermal winds.
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windfind



Joined: 18 Mar 1997
Posts: 774

PostPosted: Mon Aug 19, 2013 10:19 am    Post subject: Reply with quote

cbknap wrote:
I would politely suggest that the convective cloud buildup south of SB is what shut off SC wind Sunday. It was glaringly obvious on the NWS satellite 1km as early as 10 am Sunday...surprised Mike did not notice.
[/quote]

Hi Chris,

Thanks for the feedback. I mentioned the high clouds forming towards SB in the 7AM forecast as a possible factor in weakening the wind. Watching those clouds through the morning it became clear that they were unlikely to impact the afternoon wind so I dropped that warning at 11:30 AM. But I decreased the forecast winds anyway due to the late death of the faint eddy. In the satellite animations I use (see blog using the link in my post) clouds in the SB area do not seem to be a factor.

The key to the Southern California winds in this pattern is the pressure gradient between the coast waters and inland valleys both near and distant from the coast. Plus some help from the North Pacific High's surface NW winds curving into the beaches as WSW wind in response to that pressure gradient. On stronger thermal wind day like Saturday you can watch the temps go up in those valleys paralleled by the pressure gradient increase and then later the wind velocity on the coast. Yesterday those inland valleys temps headed upwards and then plateaued early as the clouds you see in the video developed. So the expected pressure gradient or winds barely materialized.

cbknap wrote:
Yes there were giant cumulo clouds towering over the slopes of the SG mts but that can actually supercharge coast winds.


It is true that towering convective clouds can locally supercharge or kill winds depending on their location. And in a spring type clearing pattern they may help pull in NW ocean winds over the coast. But for summertime thermal winds the key is really hot temps inland. And yesterday the overall cooling impact of those clouds and the resulting drop in the pressure gradient results in a net decrease in coast wind.

cbknap wrote:
In this case the CCW rotation around the cutoff low that Mike called out built a dam that shut off thermal winds.


The great bulk of the counter-clockwise spinning wind of the Cut-Off Low are roughly at the 500mb level around 18,000 feet up. Unless they have a winter type surface low below them those upper winds don't directly impact the surface winds except on the highest hills. These upper level winds can make subtle shifts in the movement and thickness of the marine layer clouds and sometimes you can even see ripples in the top of the marine layer due to some of the upper winds impacting lower level winds. But there was no hint of that yesterday so a "dam" is pretty unlikely. Plus those upper level winds were SW over Cabrillo yesterday so they would not block the wind. But they could possibly helped the eddy.

Thanks for your interest and continuing support.

Mike Godsey
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coolmtnbiker048



Joined: 01 Jul 2005
Posts: 372
Location: California

PostPosted: Tue Aug 20, 2013 9:46 pm    Post subject: Reply with quote

What is happening to the wind in Southern Californial? Seems like it has been gradually been getting worse every summer for the past 5 years.
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windfind



Joined: 18 Mar 1997
Posts: 774

PostPosted: Wed Aug 21, 2013 9:35 am    Post subject: Reply with quote

coolmtnbiker048 wrote:
What is happening to the wind in Southern Californial? Seems like it has been gradually been getting worse every summer for the past 5 years.


Hi Coolmtnbiker048,

I really don't know. Just when I thought I was beginning to learn how to forecast Southern California winds things began change and become more subtle.

Having watched the North Pacific High daily for 25 years it is clear that the NPH is moving past Southern California faster each spring and early summer on the average. So often Southern California has less frequent and shorter clearing winds.

And the upper level high pressure ridges expanding from the east that underlie the surface pressure gradient that drives the summer time thermal winds seem to be getting larger. So more "heat bubble" issues.

I am not remotely a climatologist so the above are just impressions. For what it is worth the average wind for many of the S. F. Bay Area sites and the Gorge average wind seem weaker on the average than 10-20 years ago. The same is true at La Ventana in Baja.

And the California marine layer is definitely thinner on the average than it used to be. There are parts of the Bay Area that were foggy most summer days that are mostly clear now. And sites like Larkspur, that only get wind when the marine layer is really deep, see much less wind than 20 years ago.

Bottomline... I really don't know. I just get on the water every possible chance to enjoy what wind I can find.

Mike Godsey
iwindsurf.com
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spennie



Joined: 13 Oct 1995
Posts: 809
Location: Thousand Oaks, CA

PostPosted: Wed Aug 21, 2013 10:55 am    Post subject: Reply with quote

Mike, it seems to me that we're getting an unusually high number of cut-off lows as well. Do you agree?
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stevenbard



Joined: 11 Nov 1993
Posts: 4012

PostPosted: Wed Aug 21, 2013 12:35 pm    Post subject: Reply with quote

Back in the 80's if it was a building heat wave you could count on both Cabrillo and Secos to hit 20+ every day. (no sensor needed) In addition you could count on 3-4 days of sailing per week from July 1- Oct 1. This held well through the 90's. One other component was surf. Wow, we had much better surf in the 80's-90's than today. (in combo with wind)

Now it seems that we get some surf but no wind, or a lot of wind and no surf during the summer. The global pattern has changed. We must adapt. I saw that the AWT had a couple of weeks of wind and surf in Baja. One very large swell with a few days of wind. In the 90's the swells down there were larger, and it blew 24/7 for weeks on end. Today they mountain bike and SUP down there. It was too windy for those kind of activities in the 90's. Very Happy
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