Published: 2007-10-02 10:53:26
Updated: 2007-10-02 10:53:26
Posted October 2, 2007
By Birgit Andersen
MIKE MOSS SAYS: Birgit, The typical west to east movement of weather systems is a midlatitude phenomenon that is related to an average westerly flow that prevails in the middle and upper levels of the atmosphere north of about 30 degrees or so. During the warmer half of the year, this prevailing pattern shifts north somewhat and weakens, which is one reason we don't have many summertime cold frontal passages and why summertime weather is often dominated by rather similar conditions day after day.
To our east and southeast, there is a semi-permanent subtropical high pressure ridge that stretches across the Atlantic, with a prevailing westerly flow across its northern periphery and a prevailing easterly flow across the south (the "trade winds"). The southern side of this high often corresponds to the formation regions for tropical cyclones, so they are steered toward the west along the south side of the high until they reach either a disruption in that flow caused by a transient low pressure area (in which case they may turn northward into the central Atlantic), or reach the western end of the subtropical high (we commonly refer to the western end of that ridge as the "Bermuda High"). Since the western end of the high often terminates near the Bahamas and the southeastern U.S., that is an area where the storms often begin to "recurve," or turn to the north. Once they move across the axis of the high pressure ridge, they then turn toward the northeast and into the northern Atlantic.
Of course, this is an oversimplified explanation, and there is enough variability in the structure of the western end of the subtropical high that some storms will be forced farther west into Central America, some others will turn north through the Gulf of Mexico, and others will follow the paths I mentioned above. In all cases, though, the general idea is that the prevailing steering currents at lower latitudes where the classic storms develop are actually from the east rather than the westerly currents we're used to seeing in the States. Also, especially during the early and late portions of the hurricane season, storms may form initially just off the southeastern U.S. coast or over the Gulf of Mexico, where they may already be under the influence of either the western edge of the high, or of other systems that may move them in almost any direction (but frequently with a component toward the north).
For a schematic illustration of this pattern, see this link from the Hurricane Research Division FAQ site: