Posted: Tue Aug 28, 2018 2:29 pm Post subject: Unending eddy ends...briefly
20 years ago eddies were so rare that I never bothered even looking for them when forecasting. Maybe once a summer an unforecast eddy would spin up destroying my forecast for Larkspur, Pt. Isabel and Sherman Island as those sites ripped way above the forecast while the coast, Coyote and 3rd. Ave. forecasts would totally fail as they unexpectedly received unreliable or weak winds. But with forecast fiascos so common eddies were not worth worrying about.
But each passing year eddies and their southerly flow have become increasingly common. So now we spend lots of time each day trying to forecast the location and timing of eddies. However, eddies by their nature are pretty unpredictable and if they die midday or bulk up the wind pattern can abruptly change.
Much of this season we have seen an almost endless parade of eddies making some of the usually most reliable sites iffy (unless you were really good at slogging way outside). While other sites that thrive on SW flow have had their best season in decades. Hence Bay Area sailors and kiters have a love-hate relationship with eddies depending upon the waters they ply.
Today have a surface eddy that is strongly supported by a counter-clockwise spinning low-pressure system that extends from the surface to over 18,000 feet. Looking at the satellite animation below you can see the tracers of SW flow in the marine layer clouds and some of the sites that benefit from this flow. Of course, local topography controls the actual wind direction at the surface.
With all this southerly flow you might think the North Pacific High and its NW winds have vanished. But actually all this time the NPH has actually been unusually large. Today it is 3500 miles wide and extends from just west of the Golden Gate to way past Hawaii. And that size plays a role in the endless eddies on our coast.
So when do we see the return of the NW wind? Well, the eddy pattern begins breaking down Friday and the North Pacific High's surface NW winds begin to ramp up on the coast Friday afternoon and come on solid Saturday. Then Sunday... guess what returns!
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