How an upper ridge impacts Southern California, Gorge and Bay Area winds.

by Mike Godsey, mike@iwindsurf.com

In the first video we are looking at  a model projection of the upper level winds at the 500mb level (  around ≈ 18,000 ft.)  from Friday 28 through Monday.

UpperRIdgeBajaCAheat

At the start of the video notice the very warm air (reddish color) west of Baja. Notice how these upper level winds for a bulge that each passing day this bulge moves northward. This type of bulge is called a ridge. First the ridge covers Southern California then Northern California and finally by Monday the Gorge.

The air mass within this upper ridge is warm and descending which compresses as drops towards the surface. This heats the surface air and creates a surface low pressure area. This low pressure ares then expands and moves closer to the coast. I know it seems counter intuitive that high pressure  aloft creates low pressure at the surface. But just think upper level high pressure creates heat at the surface and heat causes the air to expand so the pressure is lower. And as that low pressure increases in size to enlarges in all directions so it move closer to the coast both in Southern California, the Bay Area and The Gorge.

So what does this upper ridge do to the major kiting and windsurfing venues on the west coast?

Gorge: Note how The Gorge starts off yellowish (cooler air) and then becomes red as the  upper level ridge slides  up from Baja by Monday. Once the upper ridge is over the Gorge the winds

fade away as the surface low pressure expands over the region from the Columbia basin. With low pressure in the Gorge there is no pressure gradient over most of the launch sites. But late in the day when the low pressure retreats from the coast Jones Beach may have a decent pressure gradient.RidgeBajaCAheat

Southern California: Normally an upper level ridge means good coast winds for Southern California especially in the Cabrillo to Sunset corridor.  This happens as the surface low deepens and moves closer to the coast making for good beach winds.

And if it is really strong upper ridge and it the North Pacific High pushes a surface ridge into the Great Basin at the same time Southern California will rip. But if the upper ridge creates too much heating the thermal low pressure (aka “heat bubble”) will expand over much of the coast killing the wind as it does in The Gorge.

Unfortunately this upper ridge is too weak for this to happen plus an atypical Catalina Eddy will limit the Southern California beach wind.

Bay Area: The Bay Area undergoes the same process as the Gorge on Monday as the upper ridge causes the Central Valley thermal low to expand to the coast. But unlike the Gorge the Bay Area can still have at least upper teens wind late in the afternoon even during most heat waves. This happens for the same reason that Jones Beach in the Gorge can blow in the late afternoon. (see above)

The second video shows what the upper ridge looked like on Friday 28 as it first started its expansion  northward.