Memorize this: Here's your Spring!

[kirk]

[shreddbob]

Hey Mike G.,

Sorry to horn in on the West Coast forum but it seems like your valuable insight on NW winds might apply to Mass. Buzzards Bay and Cape Cod.

Over here it seems the best SW days are often not forecasted well because they occur when dry air Northwesterlies are dramatically/suddenly turned around into SW. May/June and Sept/Oct are prime periods for this to happen. And I think having the NW flow be "medium" strength is also key here--enough wind energy around, but not too much, which allows the local thermals to assist in bending the wind around. This phenomenon is perceived in the isobars as "troughing" in the NW flow. Do you have insight as to what makes this "troughing" more likely to occur?

Thanks for all you do.

[msamols]

Mike,

Now that it has blown NW relentlessly with presumably mega, uber upwelling, are we looking at a good wind/kitesurfing season? How is the long-term crystal ball FX looking for the Bay Area? I'm pretty impressed by the NPH myself.

Thanks,

Michael

[kevinkan]

Hi Mike,

Do you have an explanation for why NW wind occasionally hits Berkeley/Alameda.

Seems like Alameda only picks up the NW in the spring... also, sometimes Berkeley will pick up the NW wind w/o it bending and going WSW. Those days are best to launch off HS Lordships or Ashby Beach.

A more common pattern is when there is strong NW wind, Berkeley picks up early WSW and abruptly dies right around 2pm as the wind shifts back to NW. Any explanation for this?

Thanks in advance!

[windfind]

Hi Kevin and Michael (M, I thought you had departed to New Zealand)

See graphics below for short answer.

First Michael: Yep, in theory this long NW blow should result in good thermal winds this summer compared to the weak winds we would have had if the El Nino warm water had stayed west of the Golden Gate.

Powerful NW wind like we have had recently blows the relatively warm surface waters away from our coast. This forces relatively cold water to "upwell" to the surface from deeper levels. This cold water then cools the surrounding marine air making it denser creating a local thermal high pressure just west of the Golden Gate. Then when the Central Valley thermal low develops in the summer this high and low act to create our famous Bay Area thermal winds.

If you check out the buoy records you will see that in the last 10 days both the Bodega buoy and S. F. buoy have shown a significant temperature drop. Without this NW wind induced upweling it would have been a weak wind summer with little fog.

Kevin asked "when there is strong NW wind, Berkeley picks up early WSW and abruptly dies right around 2pm as the wind shifts back to NW. Any explanation for this?"

Kevin: (Below are graphics that answer your question so you can skip this text if you want.)

Actually it is amazing that Berkeley, and especially Alameda, blow at all. For thermal wind to blow you need a pathway for wind induced by the pressure gradient to get to the Central Valley. If you look eastward from the Berkeley all you see is relatively high ridges blocking the pathway to the valley. But in certain conditions the wind coming in from the Golden Gate does reach Berkeley as it curves from WNW or W to WSW seeking the break in the coast range on the way to Sherman Island.

To answer your question I have to discuss some of the wind patterns that influence Berkeley winds.

Pattern #1 below occurs on days that look perfect for Berkeley but then mid morning an eddy forms west of the Golden Gate. You can see this in the top satellite image. This gives a strong SW cant to the wind coming into the Golden Gate. If you look carefully you can see a streamer of fog heading towards Pt. Isabel from the Gate. This is NOT a good set up for Berkeley except <b>way outside</b>. It took me years of observation and finally a boat trip in the fog out to the S.F. buoy to figure out this eddy pattern. This pattern is most common in the summer.


Pattern #2 Below is the perfect Berkeley pattern with relatively weak NW wind on the coast and a good SFO-SAC pressure gradient. This pattern is most common in the summer.

Pattern #3 is the one which has plagued Berkeley frequently the last 7 days. You look at my forecast of "mid to upper teens and the UP AND DOWN and FADE warning" then you see the sensor go over 20 knots and you launch and sometimes find an abrupt wind fade. This sort of thing is invisible to the models. I basically figured it out by hiking up to hills in Marin and especially at the top of Mt. Livermore in the middle of Angel Island while monitoring the East Bay wind. From those ridges it became clear that when the NW ocean wind exceeded a certain velocity the wind no longer simply flowed thought the Golden Gate and other gaps in the coast range. Rather it was climbing over the Marin ridges and taking a direct pathway towards Berkeley.

Unfortunately going over all of these ridges creates a lot of turbulence so the wind reaching Berkeley would be especially up and down. But worse this UP AND DOWN NW wind coming over Angel Island the sometimes blocked the west wind curving in from the Golden Gate heading towards Berkeley (see 3rd image) When this happens the wind in the Berkeley area abruptly weakens from upper teens/low 20's to the low teens. This pattern is most common in the Spring.

This is all oversimplified but hopefully you can get the idea studying the graphics.

What makes Berkeley and Pt. Isabel forecasting truly difficult that subtle changes in this curving wind pathway can occur randomly during the afternoon causing one or the other site to lose it's wind. This typically occurs long after the forecast is issued and if somehow I don't notice my bad forecast then Andy will remind me.

And if ever I think I got it all down pat there is even a song to remind me that my name is a misnomer:

http://www.youtube.com/watch?v=hOY1VBfO-aU

Time to go sailing so I am out of here. Hope the forecast was decent.

Mike
iwindsurf.com

[NickB]

This is extremely helpful, thanks!

Would you have such patterns for Alameda?

Alameda is so hard to predict. Where does that wind come from? the outskirt of what hits the Stick or Coyote? I just noticed that I have a lot more chance of planing winds if I see the fog roll-over Twin Peaks. Also that the harbour-bay ferry channel (couple miles off-shore) gets a good 3-5mph more wind usually.

Thanks in advance, you're doing a terrific job!