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FAST changing Eddy today: Five facts & tricks.

 
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windfind



Joined: 18 Mar 1997
Posts: 1542

PostPosted: Wed Jul 17, 2019 10:17 am    Post subject: FAST changing Eddy today: Five facts & tricks. Reply with quote

Hi Gang,


Update 9:40 AM compare these 2 animations of today's eddy and notice how radially the eddy has changed in the last 2 hours.

From 7:30 AM: Predawn when I was working up the forecast no eddy was visible in the vague satellite imagery but the pattern suggested and eddy forming so I factored in the eddies southerly flow in the forecast.

Sure enough, with daylight you can clearly see the eddy and its southerly flow now in the animation below. And today one model sort of forecast the mere existence of an eddy.

To forecast how the eddy will impact Pt. Isabel or worse a site like 3rd. which is near the south-facing Hwy. 92 gap we need to know 5 things:

1. Will there be an eddy? No model can forecast that reliably. And often the eddy develops AFTER a forecast and unless we notice it and put out a Special Update the forecast will be wrong.

2. How strong the eddy become? No model can forecast that yet so if we guess wrong then the forecast will be wrong.

3. What direction will the eddy move? Does it stay north of Bodega or does it move west of Half Moon Bay. No model can forecast that yet so if we guess wrong then the forecast will be wrong

4. Will the eddy persist all day or will it die abruptly: If the eddy lasts all day then Pt. Isabel rocks and 3rd. is unreliable inside. If totally dies the 3rd. rocks and Pt. Isabel is weak. No model can forecast that yet so if we guess wrong then the forecast will be wrong.

5. How small or big will the eddy be? The models sometimes get this right. If we forecast a huge elongated eddy running down the entire coast you can be pretty sure the forecast will be accurate ie. 3rd. sucks, Pt. Isabel blows.

Do you see a pattern here? Models can not forecast eddies. So on some days I follow the eddy all morning and try to fill in 1-5 myself. Other days I don't.

Frequent eddies are a very new feature to Bay Area weather. We used to get maybe or two per season. Now they are commonplace as the Pacific continues to change.

Until the models improve how do you deal with eddy changing after the 11:30 AM forecast?

First, know how the eddy impacts your site:

A. If the eddy endures or becomes stronger:
Crissy more likely to be fogged in and to be very light inside.
Pt. Isabel, Larkspur and sometimes Alameda more likely to have strong wind starting near shore.
3rd. way more likely to have WSW and/or unreliable wind near shore

B. If the eddy dies or becomes weaker:
Crissy has less fog and winds closer to Anita Rock
3rd. more likely to have good wind at launch site
Pt. Isabel and especially Larkspur have weaker wind.

What to look for after 11:30 forecast to see if eddy is dying or continuing.

1. Look at satellite imagery for a counter-clockwise spinning mass of clouds
2. Are the clouds holding in Muir Gap and Hwy. 92 gap if so eddy is enduring.
3. Are the Ft. Funston and Half Moon Bay SW winds fading or holding.

There are a lot more "nowcast" tricks you can use in the early and mid afternoon, especially for 3rd., that the forecaster can not see when they do the 11:30 AM update. If there is enough interest I could do an blog on the topic.

Mike Godsey
iwindsurf.com/ikitesurf.com
Weatheflow.com



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Last edited by windfind on Wed Jul 17, 2019 11:42 am; edited 1 time in total
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derekd



Joined: 12 Mar 1994
Posts: 68

PostPosted: Wed Jul 17, 2019 11:01 am    Post subject: Reply with quote

Thank you for this information.
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kirk2



Joined: 26 Aug 2014
Posts: 5
Location: Los Altos, CA

PostPosted: Thu Jul 18, 2019 9:21 am    Post subject: Reply with quote

Excellent information.
Thanks!

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