Cut-Off Low

Cut-Off Low compacts North Pacific High against coast setting the stage for strong wind in the Bay and the Gorge.

by Mike Godsey,

In this video BELOW we are seeing satellite imagery from 7AM this morning. The sensors on this satellite are set to detect water vapor and clouds ABOVE 10,00 feet so you are NOT seeing the marine layer clouds or the North Pacific High which are far below these clouds. Why do we care about water vapor so far above us? Basically because the movement of that vapor allows us to visualize the upper troughs and upper ridges that steer and control the wind and weather at the surface.

Now look west of California and you can see a counter-clockwise spinning Cut-Off Low at about ≈ 18,000 ft. WNW of the Bay Area. According to the models this huge puppy is heading towards to Gorge with an ETA of Sunday and should create great winds there.

As the Cut-Off Low moves towards to Gorge it pushes the North Pacific High towards the California coast. Since the North Pacific High is a surface feature it resists going over the coast range. So its isobars are compacted against the coast and the large scale pressure gradient goes up. That is why you are seeing the Bodega hitting 22 knots at dawn. Later today as the pressure gradient from the ocean to the Central Valley goes up those NW winds will zoom through the gaps in the coast range to hit many of the Bay Area launch sites.

But why will the pressure gradient from SFO  to Sacramento, Redding & Bakersfield be so strong today. NE of California is a small upper level high pressure. This is an area of descending air and as it reaches the surface it compresses and heats. So the Central Valley the temps go up and the pressure gradient drops. So there is a stronger pressure gradient across the Bay Area and the winds are pulled deep inside the bay.

A somewhat similar process will happen Sunday in The Gorge. The upper level high pressure actually overlaps the Gorge today. That feature will retreat today to the deserts of the far eastern Gorge so the pressure gradient goes up Saturday and Sunday. Then the Cut-Off Low moves over the Gorge Sunday creating strong WSW wind aloft and leaving high pressure to the west. But remember that Cut-Off Lows are often unpredictable so all of this could change fast.

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Posted in Columbia River Gorge, San Francisco