Hurricane threat to U.S. may increase next week
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11:15 a.m. update Sunday (originally posted at 12:55 p.m. Friday): Earl has strengthened into a hurricane and the latest track guidance has shifted slightly to the west -- with the North Carolina outer banks on the periphery. Though Earl will likely stay off the coast , we are watching it carefully and will post a full update tomorrow.
The "Cape Verde" portion of this year's hurricane season has come alive. Thunderstorm clusters associated with tropical weather disturbances moving westward off of Africa -- and close by the Cape Verde Islands, hence the name -- are now showing an ability to remain intact as they trek across the Atlantic.
Now appearing with near regularity, roughly one every 5-6 days, many of the recent systems have been highly suited for further development. In fact, the last couple have evolved into significant tropical cyclones: Hurricane Danielle (category 4 as of Friday morning), Tropical Storm (soon-to-be-Hurricane) Earl, and another disturbance that has just recently moved off the African continent that might very well become Tropical Depression #8 in a matter of hours. And this may just be the beginning...
Fortunately, in the last couple of weeks or so, the global weather pattern has favored the accumulation of upper-level troughing, or a dip in the jet stream, near the East Coast of North America and over the western Atlantic. This resulting tendency toward a counter-clockwise flow along the Eastern Seaboard has created a persistent wind from the southwest at high altitudes over the western Atlantic.
So far these offshore winds (shown to the right), recently penetrating as far south as Florida, have been able to shove approaching storms back out to sea. This will likely be the case with Danielle and Earl, as these systems are projected to remain over the Atlantic and not threaten the mainland U.S.
One of the mechanisms nudging the atmosphere toward this state is the MJO (Madden-Julian Oscillation). The MJO is a tropical weather system the size of an entire ocean basin. It propagates eastward around the tropics and is intricately involved with varying the wind, sea-surface temperatures, cloudiness and rainfall in the hurricane development regions.
Right now the MJO is in a place that tends to encourage East Coast troughing at upper levels of the atmosphere. However, the latest forecasts for the MJO suggest that it will revert back to a configuration that favors the opposite -- East coast upper-level ridging, or a northward displacement of the jet stream, a pattern we've seen many times in the last several months.
By Monday, the clockwise flow associated with the East Coast ridging is expected to bring upper-level winds that are blowing toward the Southeast U.S. from the Atlantic Ocean (shown at right). This setup offers less protection to the U.S. coast from tropical cyclone strikes (particularly of the Cape Verde type) by allowing the storms to move farther westward at tropical latitudes, rather than trying to recurve them on the heels of a high-altitude offshore flow.
So if indeed the Cape Verde disturbances continue to show the potential they have recently, an MJO that tries to coax the global weather pattern into East Coast upper-level ridging is something we need to watch carefully.
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