Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather
By: Dr. Jeff Masters , 1:18 PM GMT on July 07, 2006
An area of disturbed weather in the western Caribbean is expected to move onshore the Yucatan Peninsula tonight with no development. We'll have to watch this disturbance when it crosses into the southern Gulf of Mexico Saturday. The computer models are not predicting development, though.
The area of disturbed weather south of the Carolinas is associated with a front, and no tropical development is expected. The computer models continue to show an area of low pressure developing in the vicinity of this front tonight. It is now apparent that this low will be extratropical, since it is expected to form where there is a strong front with contrasting temperatures on either side. The low is expected to move north and bring strong winds to the Outer Banks of North Carolina on Saturday, and possibly to Cape Cod on Sunday. Elsewhere in the tropics, there is nothing of note today.
Figure 1. Current weather map for the Southeast U.S. shows a front and plenty of rain south of the Carolinas.
Tropical, subtropical, extratropical?
It is often difficult to tell from looking at forecast model data this time of year whether a low that is expected to develop near the U.S. coast will be tropical, subtropical, or extratropical. The difference is important, since tropical systems have the potential to quickly grow into hurricanes, while extratropical or subtropical storms do not. So, here's a quick meteorology lesson on the differences. We talk about three main types of large-scale storms (also called cyclones):
Tropical cyclones. These include tropical depressions, tropical storms, and hurricanes (which are called typhoons the Western Pacific). Tropical cyclones have warm air at their core, and derive their energy from the "latent heat" released when water vapor that has evaporated from warm ocean waters condenses into liquid water. Tropical cyclones form only over waters of at least 80 F (26 C). One does not find warm fronts or cold fronts associated with a tropical cyclone. Tropical cyclones regularly become extratropical cyclones when they get close enough to the pole to get caught up in a front.
Extratropical cyclones. These include blizzards, Nor'easters, and the ordinary low pressure systems that give the continents at mid-latitudes much of their precipitation. Extratropical cyclones have cold air at their core, and derive their energy from the release of potential energy when cold and warm air masses interact. These storms always have one or more fronts connected to them, and can occur over land or ocean. In winter, extratropical cyclones over water can grow as strong as a Category 3 hurricane.
Subtropical cyclones. These storms occur over the oceans, and are a mix between a tropical cyclone and an extratropical cyclone. Subtropical cyclones get their energy from latent heat like tropical cyclones, and from potential energy of contrasting air masses, like extratropical cyclones. A subtropical cyclone typically has an exposed center of circulation, with very heavy thunderstorm activity in a band removed at least 100 miles from the center of circulation. The difference between a subtropical storm and a tropical storm is not that important as far as the winds they can generate. It is common for an extratropical cyclone to form over cold waters, move Equatorward over warmer waters, and gradually acquire a warm core and enough deep thunderstorm activity to be classified as a subtropical storm. Eventually, many of these will become full-fledged tropical storms if the deep thunderstorm activity can move all the way to the center, and the core becomes warm from the surface to the upper atmosphere. Subtropical cyclones very rarely attain hurricane strength.
I'll be back tomorrow with an update on the tropics. On Monday I plan to discuss the long range outlook for July, and when we might start to see some action in the Atlantic. I really don't see much to be concerned about in the next few days, and the long range outlook--so far--is for typical July weather. This is not the Hurricane Season of 2005!
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