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  • Sailing is all about the Weather.

    Big into the exploration of Atlantic Hurricanes since Katrina came uninvited into his world, James (Jim) Gurley (JamesG161) has followed every Tropical Storm birthed in Atlantic, Gulf and Caribbean waters since. Being a boater, he knows that we often need more time to prepare than we get from the TV weather folk. Jim relies on the science of storm development to share early warning info with friends and fellow boaters.

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Sheer vs steering currents

May 17, 2004
3,424
Beneteau Oceanis 37 LE Havre de Grace
As I watch the discussions of tropical systems like Grace and Henri one thing is not that clear to me - what is the difference between a weather feature that causes sheet to a tropical system versus one that steers the system? For example, the Tropical Tidbits discussion of Henri for the last few days has been talking about northerly sheer from a ridge, causing a tilt of the rotation. But now a set of low and high pressure systems over the East coast will be steering the system as it begins to travel north. What causes some of these systems to sheer Henri and others to move the whole system?
 
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May 24, 2004
6,765
CC 30 South Florida
I think you mean "SHEAR" like break off or cut off. There are winds aloft that will shear the top of some of these storms interrupting their ability to intensify. They do not steer the storm as the bulk or engine of the storm remains at a lower altitude. The steering currents or steering winds operate at the surface and have the ability to block or redirect the storm. The "Tilt" that you refer to is just a common term as the storm heels like a boat when hit at the top by winds aloft. In a high pressure system air moves (wind) away from the center at the surface. The rotation of the Earth causes clockwise rotation of the high pressure area in the Norther hemisphere; thus the push to the North induced to Henri.
 
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May 17, 2004
3,424
Beneteau Oceanis 37 LE Havre de Grace
I think you mean "SHEAR" like break off or cut off. There are winds aloft that will shear the top of some of these storms interrupting their ability to intensify. They do not steer the storm as the bulk or engine of the storm remains at a lower altitude. The steering currents or steering winds operate at the surface and have the ability to block or redirect the storm. The "Tilt" that you refer to is just a common term as the storm heels like a boat when hit at the top by winds aloft. In a high pressure system air moves (wind) away from the center at the surface. The rotation of the Earth causes clockwise rotation of the high pressure area in the Norther hemisphere; thus the push to the North induced to Henri.
Thanks. Yes, I definitely meant “shear”, not that the storm is becoming a very fine fabric or like a steep cliff :) . I guess I’m still confused because the Tropical Tidbits discussion from yesterday, for example, refers to the Low and High pressure systems now steering the storm as “upper level”. Is he just using that to mean they’re at all levels including the upper level, or is there something else going on that makes those upper level systems capable of steering instead of shearing?
 
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May 24, 2004
6,765
CC 30 South Florida
Thanks. Yes, I definitely meant “shear”, not that the storm is becoming a very fine fabric or like a steep cliff :) . I guess I’m still confused because the Tropical Tidbits discussion from yesterday, for example, refers to the Low and High pressure systems now steering the storm as “upper level”. Is he just using that to mean they’re at all levels including the upper level, or is there something else going on that makes those upper level systems capable of steering instead of shearing?
David, the forecast reports provided to the public are usually a simplified explanation provided in layman's terms. Don't know what the Tropical Tidbits meant by what is indicated.-, but let's just say the upper level winds are both capable of Shear and Steering. Strong winds aloft can shear the top of a storm but as the storm approaches a high pressure area the winds aloft can also help steer it. The winds at the upper levels change velocity and altitude in response to atmospheric conditions. Winds blowing aloft over a strong high pressure area tend to slow down as air is forced downward to spread out at the surface. Like I said these reports are intended for the public and can be more descriptive than anything detailed.
 
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Feb 14, 2014
5,624
Hunter 430 Waveland, MS
Wind Shear to a Tropical storm is a Flow Direction and Speed of air at the 500 kPa pressure range. [upper level]

That 500 kPa [ or millibars pressure] of winds is the called the Steering currents.

When Levi Cowen of Tropical Tidbits refers to Shear he means this...

Tropical Storms spin Counter Clockwise and the Storm is Sheared by opposite flowing Wind's Force and Direction.

Levi used the correct term. Note: Shear is a term meaning opposed to direction.

He also refers to Dry Air Shear.

How can you get Dry Air over the Atlantic ocean?

Ans: Lack of HEAT to evaporate a lot of water into the air.

Levi did note that Hurricane Henri will stall and fall off in intensity by passing into ...
Cooler waters , after Passing the warm Gulf Stream.

Please note @jssailem and I always talk about HEAT of the waters. Stored Solar Energy = Heat

The intent of this forum is focus Sailors and SBO community on Weather.

Thank you @Davidasailor26 and @Benny17441 for the discussion here.

Jim...
 
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Oct 22, 2014
15,815
CAL 35 Cruiser moored EVERETT WA
In the latitudes that tropical storms usually form, the Trade winds in the vertical column are frequently consistent at multiple levels.

But not always.
Vertical wind shear can occur in three ways. One is pure speed shear, where the wind speed changes with height (see illustration below). The other is pure directional shear, where the wind direction changes with height. Finally, there is a combination of both, which is the most frequent situation in the atmosphere.



As wind sheer affect the storm it can disturb the storms development. With no vertical sheer the storm can develop vertically intensifying as it draws the warm moist air up from the ocean into the storm center. As it builds the wind speed around the core builds vertically.

1630858523995.png


With upper level wind sheer and the possible inclusion of dry air the top of the storm gets pushed - tilted. The storm cannot build up so it spreads out. Susceptible to further degradation as it spreads out across the surface.
1630858567506.png