by Mike Godsey, mikeATiwindsurf.com

Perhaps you noticed that our wind patterns keep on changing as the pattern of upper-level winds change.

One new feature that is appearing is nagging long thin eddies that run along the coast creating southerly flow and favoring sites from about the Stick northward.

For lack of an official label, I have taken to calling them Elongated Eddies. Today’s satellite animation shows what they look like.

This type of eddy has support aloft from 975 MB. to 850 MB so they tend to last all day.

Watch how the curve of the fog pattern swings back and forth in the Pt. Isabel to Berkeley zone. Do you see why it is so hard to forecast in advance where the winds will be strongest in advance?

by Mike Godsey, mikeATiwindsurf.com

There is nothing worse in forecasting that to get burnt missing a huge marine surge at the end of a heatwave. A typical marine surge not only brings a welcome cool down it also brings strong winds to many sites north of the Bay Bridge. And with a dry marine surge (0ne where the dew point is such that little or no fog froms) it is really hard to track the surge since it is invisible in the satellite imagery.

In tracking the arrival of a dry marine surge the sensors are really helpful but they tell you where the surge is after the fact. So in tracking a dry surge two big things I watch for is:

1.  how much of the marine surge gets diverted into Salinas Valley. That valley indirectly connects to the Central Valley in the Bakersfield area so it can suck up a lot of a marine surge blocking or at least stalling it in reading to the Bay Area. That is what happened yesterday as the forecasters on duty kept hinting at a possible marine surge but knowing that it would probably not impact kiting and windsurfing sites. 

2. The other thing I watch is any hint marine haze right at the horizon on the Mavericks, Half Moon Bay and Año Nuevo cams.

Lastly, I also watch which direction the birds at Año Nuevo are facing. Think about it…

We have gotten hugely better in doing all of this in the last 20  years. Unfortunately, we have a new generation of younger kiters who don’t remember when our forecasts were really crappy so they don’t appreciate our improving.

This imagery shows a dry marine surge. Notice the pathway of the southerly winds in the wind map and now weak the wind is at places not in that pathway.

by Mike Godsey, mikeATiwindsurf.com

In recent years counter-clockwise spinning eddies have become increasingly common west of the Golden Gate. These eddies are most likely to form when the winds of the North Pacific High are out at the ocean buoys but have an NNW direction rather than the more common NW wind  (but  several other factors also play a role)

Having an eddy just offshore means SW-SSW winds along the coast anywhere near the eddy. And this, in turn, really weakens the wind on the coast and most sites south of the Bay Bridge while greatly enhancing the winds at sites north of the Bay Bridge.

So forecasting the existence of an eddy and its fate during the day is a high priority for our forecasters.

Sometimes forecasting an eddy is a no-brainer like this May 25, 2019 eddy as seen in a satellite animation to the left.

But often the eddy is not as well organized as this or it may be obscured by mid-level clouds so you can not see the eddy at all as was the case for the May 29th, 2019 eddy on the right.

And if you miss forecasting the eddy everyone is going to be upset since the winds will be radically different than forecast.

So here are some of the tricks we use to detect an eddy when it disorganized or hidden by clouds above the marine layer clouds.

So, let’s look closer at that May 29, 2019 eddy in this third image and show you some of the clues to detect an “invisible” eddy.

The sort of haze you see just west of the Golden Gate is actually mid-level clouds above the marine layer clouds and they are blocking our view of the actual eddy.

But if there is an eddy there should be southerly flow along the coast. And that means that fog should pour through any south facing gaps in the coast range.

So one thing I look for is for fog in the Olema Valley that extends from near Stinson Beach towards Tomales Bay. See that finger of fog in the image going up the valley. That suggests an eddy at work.

The Marin sites like Larkspur and Brickyards/Peacock Gap used to blow stronger and more often decades ago when the average marine layer was deeper.

These sites need a deep marine layer since they are fed wind from the vague high elevation Muir Gap.

So these days they need strong southerly flow to make the fog pile up in Muir Gap. Note the mass of fog in the image. That strongly suggests and eddy (the same thing happens here during a marine surge)

Now notice the mass of fog in the low area in the coast range in the Hwy. 92 Gap. Lots of fog in this gap also suggests and eddy and can mean unreliable W to WSW wind inside at 3rd.

In the next image, you can see the wind patterns at 7:45 AM also suggest an eddy. Note the NW winds out at the ocean buoys but the southerly winds along the coast. Also, note the southerly winds in the Larkspur and 3rd. Ave. areas.

The last image is a relief map of the Bay Area so you can better see the topography defining these south facing gaps in the coast range.

Remember that finding an eddy is only the first half of the eddy wind equation. We also have to try to forecast how much of the day the eddy will spin before dying.

If it dies early then the coast and Peninsula may have good wind.

If the eddy endures all day the coast and 3rd. Ave. will have crappy wind inside even though there may be good wind in the channel.

And the goal of forecasting the time the eddy dies is still elusive.

Worse sometimes the eddy mostly dies but leaves some SW flow through the Hwy. 92 gap that just barely reaches the 3rd. Ave launch.

You can see the impossible forecast situation this presents in the animation below.

1. Note the favorable WNW wind from the San Bruno Gap out in the channel and extending near shore.
2. Notice the W to WSW wind in the Half Moon Bay area from a fading eddy.
3. This SW flow enters the Hwy. 92 Gap.
4. And accelerates as it flows through this vague relatively narrow gap.
5. Notice how WNW wind vs. WSW wind battle that occurs right over the 3rd. Ave launch site making the winds inside unreliable. This battle zone will move about during the afternoon so depending upon when you arrive you may find useless wind to good wind.
   There is no way, at present, that we can forecast where this battle line will be during the afternoon.
6. Notice how the two battling winds end up turning NW to NNW and head to Palo Alto and over Morgan Hill through the Pacheco Pass to San Luis

by Mike Godsey

A very atypical situation today as a storm tracks from the south through the interior of Oregon and Northern California. At 6AM the storm is centered just north of the Bay Area. Normally our storms approach from a westerly direction so the counter-clockwise spinning winds of the storm provide southerly storm winds and rain then NW clearing wind as storm exits to the east.

But this storm is approaching from the NORTH so we are on the “backside” of the storm as it passes over us heading towards Southern California. This is why Bay area sites are seeing upper-teens to mid 20’s morning winds from the NW to W. as the storm’s winds and some showers combine with the North Pacific High’s surface NW winds.

Since this is very unstable air aloft expect very UP AND DOWN winds. This wind does NOT just funnel through the gaps in the coast range like our normal wind but also blasts over the mountains making for turbulent winds. Hence the low 30’s to upper 40’s gusts raking the hilltops at dawn.

by Mike Godsey, mikeATiwindsurf.com

You may remember that 4-5 days ago I mentioned in the extended forecast that the North Pacific High would really expand this week. As you can see below that the NPH now spans the waters from Alaska to Hawaii to the tip of Baja. I also guesscast that we might see an eddy setup develop towards the end of this week.

That scenario still looks likely and we should see an AM eddy tomorrow that fades midday. Then Friday the eddy becomes more prominent.

The images below show some of the steps in my reasoning. Eddies change very fast so it is still to early to forecast how all this will impact individual sites but the area north of the Bay Bridge is more likely to benefit.

Eddy: May 8

Eddy: May 7