As discussed in previous blogs (see July 23, 2010 for example), the NHC track forecast cone depicts the probable track of the center of a tropical cyclone, and is formed by enclosing the area swept out by a set of circles along the forecast track. The size of each circle is set so that two-thirds of historical forecast errors over the most recent 5-year sample fall within the circle. The current cone covers the Florida Keys and portions of the South Florida peninsula.
It is important to note that the cone only shows the most likely path of the track of the center of the tropical cyclone. It says nothing about the impacts. One cannot discern from the cone how far out the tropical storm force or hurricane force winds extend from the cyclone center, how high the storm surge will be, rainfall amounts, or the number and location of any associated tornadoes.
A reasonable question, based on the current track and cone, is “will any portion of South Florida be placed under tropical storm watches or warnings?” That will depend, in part, on how far out the tropical storm force winds extend to the north of the center as Paula passs to our south. At the present time, NHC is forecasting the tropical storm force winds to extend out 100 nautical miles to the northeast of the center and between 50 and 75 nautical miles to the northwest of the center as Paula makes it’s closest point of approach to South Florida.
The water vapor image above shows a mid- to upper-level low centered over Arkansas with a broad trough extending southward into the Gulf of Mexico. As this trough moves eastward, it should 1) turn Hurricane Paula more toward the northeast and eventually east and 2) increase the wind shear in the vicinity of the hurricane that will lead to weakening. Of course, the timing of the eventual eastward turn and weakening will be critical. The NHC foercasters will evaluate the confidence they have in the track, intensity, and extent of tropical storm/hurricane force winds before issuing any watches and warnings for South Florida.
Hopefully, the current forecast will be close to being correct and the tropical storm force winds will remain to the south of South Florida. LaNiña winters (which this is forecast to be) typically result in drier than normal conditions over Florida. The rains that we will receive over the next couple of days on the fringe of Paula should be welcomed.