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Re: The factor of wind speed plus forward speed

Posted by Chris in Tampa on 10/7/2017, 7:07 pm

The advisory wind speed includes the forward speed already. On the forward right quadrant of a storm, you will see the strongest winds. If the storm is moving toward the NW, the NE quadrant will usually have the strongest winds. If the storm is moving due north, the eastern quadrant will likely have strongest winds. If the storm were moving due south, then the western quadrant would probably have the strongest winds.

Whatever the highest winds are in the storm at sea level, that's what the NHC will put in their advisory. (they do that for gusts too, which are in the Forecast Advisory) That's why usually most people don't see the strongest winds in a hurricane that the NHC has in their advisory. On the other side of the storm, you can subtract up to the amount of the forward speed usually. In this storm it was even more than that earlier, though I don't know the reasons for that. I don't know if that is still the case. With 90mph 1 minute sustained winds, the winds might be as low as 67mph in some areas of the other half of the storm, such as perpendicular to the forward motion.

So in the image above, red dot might be around where you see strongest winds and the blue dot directly opposite, perhaps as low as 67mph. Of course as I note in image, this is not always how it will work. And even in the quadrant where you see the highest winds, it will vary based on the vector of the wind. And of course there are other factors, like an area of more intense convection that might have higher winds. Maybe shear is contributing to either increasing or decreasing winds on the 'weaker' side of the storm. Maybe another low to the SW in the example above would make the winds higher than the 67mph.

In that image above I say "Gusts as low as 92mph". I don't know how to word that. Basically I am saying 115mph - 23mph = 92mph. In most all areas of the quadrant, it would be higher than 92mph if this was a perfectly ideal example. (which won't happen in the real world) Other factors could make it a lot less than that. If there is more intense convection in the forward right quadrant, gusts might simply be higher there than if there is less intense convection in area directly opposite the forward right quadrant. So gusts might be a lot lower in some areas of the other side of the storm, well beyond just subtracting the forward motion.

It's just a very, very rough guide.


In this thread:

The factor of wind speed plus forward speed - LawKat, 10/7/2017, 11:16 am

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