March 18: powerful winds today but…..

How can the pressure gradient be so massive and the bay winds so weak?NEwindsNPHanim

by Mike Godsey,

Benjamin is forecasting a massive .25 inch pressure gradient today from SFO to Winnemucca, NV  and there powerful winds roaring just west of the ocean buoys today at dawn. So why is the wind forecast for the Bay Area for weak winds?

First let’s look at a video of the wind patterns over California and the Pacific. Find the Bay Area in the lower right corner. Then locate the center of the North Pacific High in the upper left corner. Notice how the winds spiral clockwise out from the center of the North Pacific High. This creates the strong NW wind that is so critical to spring and early summer Bay Area winds. Find those on the video. However today most of that NW wind is several hundred miles west of the Bay Area. Why? Notice the powerful N to NNE winds closer to the California shore. Due to the angle of the California coast near the Bay Area these winds are somewhat offshore. So all the hills and ridges of the coast range block most of this wind from ZZ1FA803C1reaching  into the bay. Also notice the wind shadow from Pt. Reyes to about Año Nuevo.

In the next image notice the weak N to NE winds hitting out sensors around the bay.  Since our prevailing winds range from NW to SW it seems weird to have NE winds inside the bay. To understand the genesis of these winds let’s look at the behavior of the North Pacific High.

In the next image we are looking at the isobars, or lines of equal pressure, from the North Pacific High and the Central Valley thermal low. Notice the North Pacific High on the left side of the image. Picture the NPH as a low dome like mountain of high pressure air. At times the mountain of air develops an extension or ridge just like a rocky mountain. In this case the ridge has pushed into far Northern California, Nevada and Oregon. This has the effect of creating a high pressure zone NE of the Bay Area. 

Now find the isobars from the low pressure in the Central Valley and the Great Basin in the lower right of they image. When the North Pacific High pushes a ridge into far Northern California and the Pacific Northwest the valley low pressure expands over the Bay Area and out into the pacific. This has THREE implications:

1. It means the SFO-SAC (Sacraemento) pressure gradient is near zero in the AM and most of the PM so the thermal winds will be very weak.  

2.  It means there is a strong pressure gradient from far Northern California to just south of the Bay Area. This creates NE winds over the Bay Area.

3. And this NE winds tends to push the North Pacific High’s surface NW winds away from shore.

All three of these situations tend to weaken in the LATE afternoon so as the low retracts eastward  and the NE winds fade both the pressure gradient and the NW wind ramp up. But today it will be too little too late.

But why are the NNE winds so weak at our sensors? The NE winds tend to be air warmer that the surrounding air so when the NE winds hit a hill they lift from the surface and and may not return. Since the Bay Area is surrounded by hill tops very little of the NE wind reaches the surface. But looking at the 3rd image notice the strong wind on the hilltops about the Bay Area. Then look at the wind and direction data from the predawn Oakland Airport weather balloon today. Note in the last column how the N to NE wind is weak at the surface but strong as the ballon reaches the hill top levels.

It is this amazingly persistent ridge of the North Pacific High that continues California’s drought since it tends to divert storms northward into Canada.  Unless this ridge departs it we will continue to see this pattern of the NW wind barely reaching the shore.

Now of today’s question: Why is Benjamin forecasting a chance of upper teens at Waddell today despite the NE wind. Hint… look eastward from the Waddell beach.

Posted in Los Angeles, San Diego, San Francisco