Steve Cosby wrote:Something just hit me - if the shear is 20 knots from the south and the hurricane is moving NW at 17 knots would that not mean, and I'm not good at trigonometry, something likely "only" about 10 knots of effective shear?
Shear doesn't work that way... it's a difference between winds at different levels. 20 kts of shear is still 20 kts of shear, whether the difference is between an 80 kt wind aloft and a 60 kt wind below, or the difference between a 30 kt wind aloft, and a 10 kt wind below, and so on.
A hurricane moves with the steering flow, which is some weighted average of winds over a depth of the atmosphere, and is different for every system. A hurricane that is stationary and getting sheared from the south, if it started to move north, would experience less shear, not because it was moving "with" the shear, but because the low-level environmental winds have begun to line up more with the upper-level winds, both starting to move the hurricane north, and decreasing the shear at the same time.
I hope that makes sense, and any other met on here who wants to chime in with corrections or additions, please do so..
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