Occasionally, steering winds are weak and don't provide a clear message about storm motion. ![]() But, sometimes assessing the steering environment for a tropical cyclone isn't so straightforward. I've marked the center of the Bermuda High with a white "H", and the pattern of steering winds shows that Irene was following along with the clockwise flow around the high's periphery, as many Atlantic hurricanes do. ![]() If we look at the analysis of deep-layer steering winds (average winds through a deep layer of the troposphere) below, the reasons for Irene's northwest movement at this time were pretty clear, if you account for the fact that Irene's own circulation leaves a footprint in the wind field right in the storm's vicinity. At the time, Hurricane Irene’s minimum central pressure was 957 millibars, so it was a fairly intense tropical cyclone, which would have been largely steered by the mean winds through a deep layer of the troposphere. Here's a plot of Irene's track, and at 12Z on August 24, 2011, Irene was moving northwestward through the Bahamas. To see an example of how forecasters assess a tropical cyclone's steering environment, let's look at Hurricane Irene, which brushed the East Coast of the United States in 2011. The main takeaway is that stronger tropical cyclones are steered by the mean winds through a deep layer of the troposphere, while weak tropical cyclones tend to be steered by the mean winds in a much shallower layer in the lower half of the troposphere. Strong tropical cyclones (hurricanes) tend to move with the mean wind in a much deeper layer that spans most of the troposphere (forecasters commonly look at the layer between roughly 5,000 and 35,000 feet). Weak tropical cyclones (tropical depressions and tropical storms) tend to move in concert with the mean wind in a relatively shallow steering layer residing in the lower half of the troposphere (roughly 5,000 to 18,000 feet is a good proxy). And, while there seem to be slight differences between ocean basins, the following theme holds true everywhere: The depth of the steering layer for a tropical cyclone increases with increasing cyclone intensity. ![]() Research has shown that the relevant atmospheric "steering layer" for a given tropical cyclone depends on the intensity of the storm. To further complicate matters, tropical cyclones can actually impact their own steering environments (especially when steering currents are weak) as well as the steering environments of other nearby tropical cyclones.īut, when it comes to forecasting the movement of tropical cyclones, years of experience and research have shown that the average winds in various atmospheric layers are the dominant steering forces for tropical cyclones. If only it were that simple! Indeed, when tropical cyclones head toward the middle latitudes, mid-latitude weather systems (particularly upper-level troughs and ridges) can also steer tropical cyclones as they move poleward from the tropics. While the mid-tropospheric winds around subtropical high-pressure systems play an important role in the steering of tropical cyclones (especially Cape Verde storms), they're not the only features that steer tropical cyclones. In 2013, these islands formally changed their name to the Cabo Verde Islands, but the name "Cape Verde storm" still lives on in meteorological circles. For the record, tropical cyclones that make the long trek across the Atlantic are often called "Cape Verde storms" because they often form within 1000 kilometers of the former Cape Verde Islands off the west coast of Africa.
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