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windfind
Joined: 18 Mar 1997
Posts: 1901
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 Posted: Tue Jun 07, 2016 10:16 am Post subject: UPDATED: Visual anatomy of a big Sherman dawn. |
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Hi Gang!
Below is some imagery I will turn into a blog later. Right now I am preparing to hit the water!
But the imagery below should be enough for you to construct the probable blog text.
Mike
iwindsurf.com/ikitesurf.com
Weatheflow.com
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Last edited by windfind on Wed Jun 08, 2016 3:19 pm; edited 1 time in total |
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estherjibe
Joined: 18 Apr 2011
Posts: 15
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| Posted: Tue Jun 07, 2016 10:41 am Post subject: Sherman wind |
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| Aren't these features caused by the somewhat cut-off low out near 135 deg.?
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windfind
Joined: 18 Mar 1997
Posts: 1901
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| Posted: Tue Jun 07, 2016 12:27 pm Post subject: Re: Sherman wind |
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| estherjibe wrote: | | Aren't these features caused by the somewhat cut-off low out near 135 deg.? |
Hi Esterjibe,
Good observation! Looking at the series of images below you can see that there is an upper trough west of us at 135 deg. at ≈ 18,000 ft.which is causing strong SW flow way aloft out in the pacific.
This upper trough never quite turns into a Cut-Off Low over the next 3 days and gets sucked up into the upper flow.
As you look at the imagery in the area of the upper trough to California going from ≈ 18,000 ft. down to 1000 ft. notice how the SW flow from the upper trough gets weaker and weaker as you near the surface and almost disappear near the Bay Area.
Also note how as we near the surface the southerly winds of Mendocino Eddy get stronger.
So to answer your question the near Cut-Off Low works with the eddy to the north to make a contribution to the weak surface SW flow at the surface and aloft which helps the Sherman Island winds today.
But without the strong pressure gradient to Redding, Sacramento, Bakersfield and the Great Basin as well as the deep marine layer and strong inversion Sherman Island would probably only be in the mid teens.
Mike
iwindsurf.com/ikitesurf.com
Weatheflow.com
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windfind
Joined: 18 Mar 1997
Posts: 1901
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| Posted: Wed Jun 08, 2016 11:11 am Post subject: |
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Hi Gang,
Here a video of the Cape Mendocino eddy/low pressure that helped stir up the all day Sherman Island winds yesterday.
Mike Godsey
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U2U2U2
Joined: 06 Jul 2001
Posts: 5467
Location: Shipsterns Bluff, Tasmania. Colorado
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windward1
Joined: 18 Jun 2000
Posts: 1400
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| Posted: Wed Jun 08, 2016 10:13 pm Post subject: |
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| Is that an eddy or a localized low pressure area?
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windfind
Joined: 18 Mar 1997
Posts: 1901
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| Posted: Thu Jun 09, 2016 12:09 pm Post subject: |
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| windward1 wrote: | | Is that an eddy or a localized low pressure area? |
Hi Windward1,
If you look at the 3rd. post above you can see there is a sort of counter-clockwise flow at 850 and 975 mb in the area where you see the spinning satellite imagery.
So strictly speaking the spinning mass is not a surface eddy like our typical Golden Gate Eddy. But since there is not a distinct low pressure above it is hard to really call the spinning mass a typical low pressure storm. So to save long winded geek talk I just called it an eddy.
Mike
iwindsurf.com/ikitesurf.com
Weatheflow.com
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windfind
Joined: 18 Mar 1997
Posts: 1901
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| Posted: Thu Jun 09, 2016 12:35 pm Post subject: |
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| U2U2U2 wrote: | I'm not a meteorologist .
Please explain why/ how wind at 18,000 feet effects the ground wind..? |
Hi U2U2U2,
I was hoping no one would ask that question since a simplified answer would take pages and complex graphics. Which I will do one day in a blog. But oversimplified...
1. Winds at the 500mb level which averages at 18,000 ft. play a critical role in moving the North Pacific High around this part of the pacific making the NW ocean wind build or fade. So tomorrow they move the NPH closer to the Bay so I expect much stronger NW surface wind.
2. The upper troughs these 500 mb winds form also can make the NPH push a ridge into the Pacific Northwest which makes our ocean surface winds more NNW and creates NNW to NNE winds just over the Bay Area which in turn makes the pressure gradient focus west of Treasure Island so Sherman Island dies and the East Bay is weak as you will probably see Saturday.
3. Upper ridges formed by the 500mb winds cause heat waves when they are over the Central Valley causing stronger Bay winds. But if the upper ridges are too strong the Central Valley thermal low expands over the Bay killing first the East Bay winds then possibly all the wind.
4. Strong 500mb winds are typically accompanied by strong wind closer to the surface. When these winds at say 1000 feet hit the coast range they climb up those mountains where they pick up momentum from the even stronger winds above the mountains. So when they descend down the coast range they are much stronger and gustier than they were over the ocean surface. You will experience this tomorrow!
5. As the winds at ≈ 18,000 ft. flow over us there are areas where this wind flow converges. This acts to cause descending air that induces higher pressure towards the surface. Tomorrow for instance this happens over the North Pacific High so it grows and expands towards the Bay Area hence I am forecasting much stronger NW wind Friday.
6. By the same token winds at ≈ 18,000 ft. sometimes diverge so they cover a wider area. This tends to cause wind below to rise which deepens low pressure at the surface. I expect this to happen over the N. Great Basin tomorrow which should increase the SFO-Winnemucca pressure gradient which adds a few knots to our strong winds tomorrow.
7. through 100... to be continued.
Mike Godsey
iwindsurf.com/ikitesurf.com
Weatheflow.com
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