What is a ridge and how does it impact Bay Area winds?
by Mike Godsey, firstname.lastname@example.org
Several times each season, especially in the fall, the forecast will mention something like this:
“The North Pacific High has pushed a RIDGE into Northern California and the wind at the Bay’s surface and just aloft is weak NE in the AM” See image #1
“The Central Valley thermal low has expanded over the coast so the SFO-SAC pressure gradient is weak.”
“The North Pacific High’s surface NW winds are held by the NE flow mostly out at the ocean buoys until MID afternoon.”
“Then in the afternoon: the NE winds fade. The Central Valley thermal low retracts to at least the East Bay hills. So the pressure gradient goes up and a combo on NW ocean wind and W to WSW thermal wind ramps up inside the bay.”
So what causes this wind pattern? Normally we think of the North Pacific High producing the NW California winds and the trade winds of Hawaii. How does it do this? The NPH is a huge flattened dome of high pressure air that spans much of the ocean between the west coast and Hawaii in the summer. The winds spiral out from this high pressure dome in a clockwise fashion. Hence NW winds for California and NE trade winds for Hawaii.
As fall approaches upper troughs at ≈ 18,000 ft. pass more often and further south over the Pacific Northwest. Sometimes they cause the NPH dome to elongate and push into far Northern California or even the Gorge. You can see this has happened today in IMAGE #1. Notice the extension of the highs isobars into Northern California. This produces weak NE winds at the surface over the Bay Area. Images #3,4. This NE flow also tends to push the NW wind away from shore in the AM.
The ridge also allows the Central Valley thermal low to move over the Bay Area coast (see image #1) so the pressure gradient to the valley drops and the winds are very weak in the AM.
Typically the North Pacific High’s ridge shrinks in the early afternoon so the NE winds fade away, the Central Valley thermal low retracts towards the valley so the pressure gradient goes up. This allows NW ocean wind to move to the coast and causes a thermal wind to develop. Unfortunately the timing of all of this is tricky and as I type this at 11:54 Sept 18 the process has yet to start so Ben, who is forecasting today, is probably sweating it a bit. Like Ben, you should be watching the Blunt Point and the Treasure Island sensors for a hint of these winds ramping up. But if you are really a wind geek you know to look at our sensor atop the S. tower of the Golden Gate. When you see the NE winds DIE there you know the “good” will ramp up. And guess what! I just checked that sensor and 10 minutes ago it switched from NE to W!