By Mike Godsey

Watch as an upper trough buds off a Cut-Off Low

by Mike Godsey,

Cut-Off Low formingMost sailors, kiters and windsurfers have come to know that a Cut-Off Low is a forecasters woe. One wag on the forums once said of Cut-Off Lows that he suspected we made them  up on days when we did not have a clue what would happen with the wind. And actually he was right about the “not a clue” part of this statement. But if Cut-Off Low are not the product of clueless forecasters… where do they come from and why do they make forecasting so difficult?

To answer those questions let’s first look at a simple graphic. In the top Yesterday image first find California and the S. F. Bay Area. Notice the clockwise rotating upper level high pressure that is creating the current heat wave.

Now look to the west and find the upper trough at ≈ 18,000 ft. Notice how the upper level winds from a loop. This loop in the upper winds is an upper trough. Typically upper troughs are much broader than this one and you have SW wind on the east side and NW wind on the west side. These upper troughs typically move from west to east around the world and act to guide and enhance the surface weather systems like the North Pacific High and storm systems.  This large scale (synoptic) movement is critical in forecasting weather in advance since the movement is fairly predictable using the laws of  thermodynamics and plasma physics.

This particular upper trough is so tightly looped that the very bottom portion is about to pinch off from the upper trough. If you can visualize that pinch off you are seeing a counter-clockwise spinning mass of air. That is a Cut-Off Low as you can see in the Today image at the bottom of the graphic.

Now you are ready to see the video at the bottom and make sense of all the action. But the big question is “what will this due to the wind” And the answer is “I don’t have a clue….yet” The Cut-Off Low is still too far away to impact the California wind and it may die a lonely death a 1000 miles to our west. Of it may move to the Central California coast and plague Peninsula and coast sailors while jacking up East Bay  and Sherman Island winds. Why am I so clueless? Because such is the nature of Cut-Off Lows.

Cut-Off LowFormsAnim