Friday, September 03, 2004
Fickle Frances
Jason Samenow @ 11:48 PM
Hurricane Frances weakened for the second straight day yesterday, and is now down to Category 2 intensity. Maximum winds have dropped to 105mph, and the pressure has continued to rise. What's going on? The answer: some entrainment of dry air into the storm and, importantly, wind shear (click on the link for a nice explanatory graphic). Essentially, shear acts to rip the tops off of developing thunderstorms, damaging the storm's structure and weakening it.
Look at the satellite image of Frances from Friday evening (directly to the right). The red arrows represent upper level westerly winds that are disrupting the development of storms on the western side of the hurricane. (The green arrows depict the weak mid level steering current) As a result, the storm is assymetric with more convection (i.e. thunderstorm activity) east of the center of the storm and no eye. Under these circumstances, the storm cannot grow.
Contrast that to the image of Frances from Tuesday evening (on the right) when there was little shear (and when she was a Category Four storm). Notice how the storm was nearly perfectly symmetrical. At the time, the storm was a much more efficient "engine", able to efficiently pull in warm, humid air from the lower atmosphere and exhaust dryer air into the upper atmosphere (click here for explanation). Accordingly, the storm was able to sustain itself and intensify.
During the next 24 hours, the shear is forecast to relax a bit and Frances will be over warm waters. So I would not be surprised to see her intensify a bit, maybe back to a Category 3.
As far as landfall, I still think the corridor from West Palm to Vero Beach is most likely. Given the slow movement of Frances (currently only 4mph), landfall may not occur until midnight or so (given the erratic motion, some uncertainty here). (For information about the impacts I'm expecting, see yesterday's post).
After Frances moves through Florida, I'm still expecting heavy rains in Georgia and Alabama Monday and Tuesday and into the Tennessee/Ohio Valley/Appalachians on Tuesday and Wednesday (per Tropical Projection center projections). Frances' influence on DC is still tough to call, so stay tuned.
Lastly, Tropical Storm Ivan formed yesterday, and may become a powerful hurricane as it crosses the Atlantic into the Caribbean. More on that in the coming days.
Look at the satellite image of Frances from Friday evening (directly to the right). The red arrows represent upper level westerly winds that are disrupting the development of storms on the western side of the hurricane. (The green arrows depict the weak mid level steering current) As a result, the storm is assymetric with more convection (i.e. thunderstorm activity) east of the center of the storm and no eye. Under these circumstances, the storm cannot grow.
Contrast that to the image of Frances from Tuesday evening (on the right) when there was little shear (and when she was a Category Four storm). Notice how the storm was nearly perfectly symmetrical. At the time, the storm was a much more efficient "engine", able to efficiently pull in warm, humid air from the lower atmosphere and exhaust dryer air into the upper atmosphere (click here for explanation). Accordingly, the storm was able to sustain itself and intensify.
During the next 24 hours, the shear is forecast to relax a bit and Frances will be over warm waters. So I would not be surprised to see her intensify a bit, maybe back to a Category 3.
As far as landfall, I still think the corridor from West Palm to Vero Beach is most likely. Given the slow movement of Frances (currently only 4mph), landfall may not occur until midnight or so (given the erratic motion, some uncertainty here). (For information about the impacts I'm expecting, see yesterday's post).
After Frances moves through Florida, I'm still expecting heavy rains in Georgia and Alabama Monday and Tuesday and into the Tennessee/Ohio Valley/Appalachians on Tuesday and Wednesday (per Tropical Projection center projections). Frances' influence on DC is still tough to call, so stay tuned.
Lastly, Tropical Storm Ivan formed yesterday, and may become a powerful hurricane as it crosses the Atlantic into the Caribbean. More on that in the coming days.
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