"Wave" Off the African Continent
About this time of the year, forecasters begin to watch the area off of the east African coast for the next big hurricane.
“Cape Verde” storms are the result of “waves” or troughs of low pressure which leave Africa westward bound. They often will spark disorganized thunderstorm activity. As they encounter warmer water, more unstable air and less dry air from the Saharian Desert, they can develop a closed circulation, the first step to a tropical storm. Assuming the wind shear isn’t too strong, these waves often do turn into something stronger.
What steers hurricanes? Storms of the type I’ve been discussing are caught in the trade winds of the central Atlantic which blow westward off of a large high which sits north of their track. A “wave” is an area of low pressure which forms in connection with the InterTropical Convergence Zone (ITCZ), which is where the trade winds of the northern and southern hemisphere converge.
A hurricane will travel westward or west-northwestward until it encounters a dip or distrurbance in the jet stream of the middle latitudes. These winds, “the westerlies”, drive weather systems eastward. If a storm travels into this zone, it will want to curve more to the north and eventually northeast. If a storm is at a low latitude or never encounters any jet stream waves, it continues what it was doing and will likely slam into land in a zone from central America to the east coast. The confidence in timing these hurricane landfalls with precision is higher because the winds are steady and there are fewer variables.
September Hurricane Frequency and Tracks
On the other hand, when storms encounter the jet stream, or large island landmasses or other surface pressure features, they become much more unpredictable. Knowing when a storm will “do the curve” then is completely about the accuracy of the computer models in forecasting jet stream waves as they move eastward in the middle latitudes. How accurate is this? How accurate is the forecast five days or more into the future for us? Sometimes o.k., sometimes the timing is way off!
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