by Mike Godsey, iwindsurf.com/ikitesurf.com, mikeATwindsurf.com

I took the helicopter photo in my banner above on a day when it was nearly calm at Pt. Diablo just outside the Golden Gate’s north tower. But the strong NW aloft were buffeting the copter and there were strong surface winds inside the Golden Gate off Crissy. This morning a customer mailed me about yesterday’s winds which sound like a similar day.

“Everyone was asking me today why outside the GG bridge was nearly zero, just inside the bridge was solid 25 gusting higher, Anita was gusting to 30. When I went down to Blunt it was much lighter than I was expecting and much lighter than it has been for the past week. I can’t explain it … Would make for some interesting forecast comments and maybe even a blog post. MS”

Here is my response (slightly elaborated for clarity):

Hi MS,

I have not forecast since last Friday so I am a bit out of the loop. However, I have seen the pattern you are describing so I can take a somewhat educated guess.

The Marin ridges NW of the Central Bay are one of my favorite for meteorology hikes since I can both see the Central Bay water, feel the nature of the wind above the Bay and, politically incorrectly,  sometimes release Helium filled toy balloons. From the high ridges above the Marin Headland, I could watch the balloons erratic vertical flight pattern as it headed above the Golden Gate.  (in recent years I have done the same in the Hwy. 92 gap trying to understand the SW eddy flow that impacts 3rd…. not much luck there…yet)

The hikes and balloon play suggested the following hypothesis.

This first image shows the 449 Mhz. Bodega profiler from yesterday. Note how Bodega, which is upwind of the Golden Gate,  had very strong WNW to NW wind just aloft from 200 meters to over 1000 meters.

At this elevation these winds would crash into the Marin coast range and which creates a lot of NW to WNW moving turbulent wind that would end up several hundred feet above the Marin Headland and Sausalito hills inbound to the north tower Golden Gate to Point Blunt area.  

Now, look at the insert above which shows the Mt. Tam winds yesterday at about 2572 feet.  Since Mt. Tam is way taller than nearby hills you would expect steady winds unless there was turbulence. But the data clearly shows there you can see the Mt. Tam wind is extremely gusty.

Meanwhile, at sea level, you have NW surface wind that is curving in through the Golden Gate as WSW wind at Crissy and to Treasure Island. This is a cool dense wind and as such, it tends to hug the surface of the water. And just above that WSW wind you have turbulent warm air moving from the WNW-NW. What I have observed that when this happens the WSW wind mass is pushed slightly southward leaving a much lighter wind area outside the Golden Gate, near the north tower and near Point Blunt.

A similar pattern happens frequently to Pt. Isabel with strong NW winds aloft and at the surface as you can see in this blog and the following animation.

https://blog.weatherflow.com/why-did-i-forecast-strong-winds-everywhere-except-pt-isabel-and-berkeley/

Looking at the animation notice how the WSW wind inbound from the Golden Gate is diverted away from Pt. Isabel and since Brook’s Island blocks any NW wind people are slogging outside.

So, looking at the animation, mentally just shift that diverting flow over the Central Bay to over the Marin Headland and you can see how that might weaken the Point Blunt,  north tower and winds outside the Golden Gate.

Incidentally, my experience is that the WRF greatly overestimates the impact of this pattern on Treasure Island and somewhat overestimates it for Point Blunt. Tell me what happens today.

This pattern is rarely as pronounced as you describe. But when I think I see this pattern I mention in my forecast something like: “Strong winds inside at Crissy and Treasure Island but fading towards the north tower and Point Blunt”

Let me know if the above makes sense. I will probably do a blog suggesting this as a possible explanation for what people observed. (And here is the promised blog)
Mike

by Mike Godsey, mikeATiwindsurf.com

Hi Mac,

Simple answer… This season we have often had:

1. El Nino conditions that make it more likely that part of the North Pacific High will be pushed into far Northern California creating NNW ocean winds so eddies become more likely. Eddy induced southerly winds “push” through the gaps in the coast range rather than being “pulled” smoothly by the pressure gradient to the Central Valley. These pushed winds are less stable and interact more with topography and become turbulent hence gusts. Have you noticed that during eddy winds you often do not see the smooth “fogfalls” of fog flowing like water over the Marin Headlands like we see most summers?

2. The North Pacific High has been severely disrupted by the El Nino so it has elongated south and especially way north even into the Gulf of Alaska. Normally the NPH is centered west of the Gorge by June but this season part of it is still very near the Bay Area. So in recent days, we have seen our typical WSW surface winds but ALSO strong NW wind just aloft. This also creates turbulence and wind shifts. Hence gusts and sudden fades.

3. El Nino also means a more southerly storm track. So many areas in California had more rain and snow this winter. But this also means that we are seeing a more southerly track of low-pressure system this summer. This brings unstable air over the west coast. Unstable air does not hug the terrain and lifts from the surface easily. So gusty winds.

All this means some sites often have been very gusty.

For details see an upcoming blog.

The image below shows how the NPH is unusually far North and the weird location of a small low pressure. Plus, see that huge storm west of the Aleutian islands? You will probably be hearing more about this storm as it swings far more south this El Nino year.

by Mike Godsey, mike@iwindsurf.com

So it was mid-day and hour after my 11:30 AM forecast update for the San Francisco Bay Area when the messages from customers started arriving: The first was from boggsman1

From:	boggsman1
To:	windfind
Posted:	26 Jun 2019 19:16
Subject:	Re: FOG!
Mike, Its practically raining in the city today, thickest fog this year…Doubt you get clearing at crissy today.

This was interesting since I had specifically mentioned in the forecast that an inbound upper trough had mixed out the marine layer clouds which would mean early but somewhat unstable winds and here was Boggsman saying we had the thickest fog of year!

This is when you are tempted to turn Chicken Little and issue a special update saying in effect that “The fog is coming, the fog is coming” and drop the wind forecast for the coast and Crissy and many other sites near the coast. And watch in this first animation how incredible fast the fog came in! I could almost feel feathers growing as I watched the cams.

Fortunately, I had been following the cams, the Bodega Profiler and the RAWS hill top sensors. So I was pretty sure that without an inversion of warm air above the fog it would lift and mostly evaporate FAST. So I stuck to my guns. And almost every site reached its forecast wind value although Crissy was weaker than forecast.

These animations tell the story. Notice how incredibly fast the fog developed in the first animation and how fast it disappeared the second animation.

by Mike Godsey, mikeATiwindsurf.com

The battle is on! There is a tiny eddy near Stinson and a larger ribbon like eddy running from the Golden Gate to Santa Cruz creating southerly ocean winds near shore.

Meanwhile, very strong NW winds from the North Pacific High are roaring just west of our ocean buoys.

At dawn NNE winds at 1000 – 2000 feet aloft are holding the NW surface winds away from the Bay Area.

You can see these wind aloft in the 2nd image.

If you have ever sailed Sherman Island on northerly winds you know that these winds always fade as the morning progresses.

So as the NNE winds fade midday the NW wind moves over the coast.

Then as the pressure gradient towards Bakersfield goes up these winds roar through W and NW facing gaps in the coast range.