All Weather News

Karen Versus Wind Shear

3 Oct 2013, 4:51 pm

Afternoon of October 3rd, 2013

Guest blog from WFOR CBS4 Meteorologist Craig Setzer.

As of Thursday afternoon Karen is struggling against moderate upper level wind shear. And as long as this level of shear remains, it would be difficult for Karen to intensify much, if at all. A tropical cyclone needs to have a coherent vortex to intensify. In other words, it needs a tight circular tube of rotating winds that ideally extend vertically straight up into the atmosphere. That vortex also needs to be supported by strong thunderstorms releasing enormous amounts of energy into the atmosphere and the vortex is best maintained when the thunderstorms are spread out on all sides of the circulation center.


Source:Prof. M.K. (Peter) Yau, McGill University

Shear, in the tropical sense of the term, is wind apart from the tropical storm blowing against the tropical storm. It is considered hostile to the storms ability to survive and intensify and here’s why.  Winds that blow against the vortex tend to tilt it sideways. That causes thunderstorms to be displaced away from the low level center. It can also cause drier air to be pushed into the cyclone. If strong enough, it can even tear the tube of rotating winds we call the vortex in half, resulting in a swirl near the surface and another in the middle parts of the atmosphere. Remember, it’s the surface circulation that gives the storm its identity and that’s the feature we track. If the surface and mid level swirls come apart or decouple, the storm will likely decay unless new thunderstorms can develop around the surface swirl.

So what is going on with Karen Thursday afternoon? Well the shear is fairly formidable and acting to tilt the vortex and tear thunderstorms away from the surface center. As long as this process continues, it is unlikely Karen could strengthen much. The shear is forecast to remain nearly constant over Karen up until the time of landfall late Saturday or Saturday night. That being said, because Karen will be traversing very warm Gulf water, any slight relaxation in the shear could allow for better organization and intensification. It wouldn’t take much strengthening to bring it to minimal hurricane status as it approaches the coast. And we are always cautious when making intensity forecasts because we have don’t have the best skill in that area of tropical cyclone prediction

Cooperative Institute for Meteorological Satellite Studies, Univ of Wisc

The satellite image with shear values indicates southwesterly shear of around 20 knots blowing into Karen. The shear has resulted in the center being located southwest of the worst weather and highest winds. This orientation of the storm is likely to continue with areas near and east of the center feeling the worst effects of Karen while areas immediately west of where the center crosses the coast only experiencing strong winds but not much rainfall. By Friday evening we should begin to have a better idea where that center and bad weather will make landfall along the Gulf coast.



Meteorologist Craig Setzer is the longest on-air degreed meteorologist at CBS4 and myTV33 news. His experience in the #1 hurricane market in the country makes him uniquely qualified to cover them and inform you. Few meteorologists in the South Florida market have the education and experience to cover hurricanes and severe weather like Craig does.

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