Isabel Weakens
Isabel is weakening and Isabel will likely continue to weaken for at least twelve hours more. The National Hurricane Center has blamed its usual culprit: unfavorable eyewall thermodynamics. I have a different theory. A hurricane is basically a well-organized cluster of thunderstorms that seeks to keep its center of circulation symmetric within a globule of deep convection. Isabel has been doing this beautifully for several days. However, in order to sustain the thunderstorms, Isabel requires a convectively favorable environment not only near its center of circulation but within its outflow. Satellite pictures show signs of impeded convection (almost certainly not due to wind shear which just blows thunderstorms away.) On its northern side, the hurricane's convection has been noticeably reduced, forcing the hurricane to try to re-organize by developing stronger convection to the southeast, giving the hurricane a stretched appearance. On its western side, the eye is protected by less and less deep convection. The western side's outflow has a most interesting appearance. Although it is broken, one can trace a circular curve from one end of the break to the other. In other words, the circulation remains strong, but convection is increasingly unfavorable along the hurricane's western side. The hurricane might be moving into a more stable environment.
According to upper level sounding data, instability has generally decreased from the coast of the Bahamas to Cape Hatteras during the last 36 hours. The odd building of convection to the southeast suggests that Isabel might be fighting convection in eastern Pennsylvania and Maryland and another cluster off the coast of the Delmarva for moisture. These cells developed from remnant circulation and moisture of Tropical Storm Henri, and may have drained available moisture farther down the Gulf Stream. Isabel's weakening seems to have begun about the time, it entered the axis of the monsoonal moisture plume that is still fueling heavy rain to the west of Philadelphia. Forecast models indicate that instability will increase ahead of Isabel and Isabel may escape the draining moisture plume. If Isabel's current structure escapes too much harm, it could regain much of its strength tomorrow and early Wednesday.
ESA(20030915.1)
Isabel is weakening and Isabel will likely continue to weaken for at least twelve hours more. The National Hurricane Center has blamed its usual culprit: unfavorable eyewall thermodynamics. I have a different theory. A hurricane is basically a well-organized cluster of thunderstorms that seeks to keep its center of circulation symmetric within a globule of deep convection. Isabel has been doing this beautifully for several days. However, in order to sustain the thunderstorms, Isabel requires a convectively favorable environment not only near its center of circulation but within its outflow. Satellite pictures show signs of impeded convection (almost certainly not due to wind shear which just blows thunderstorms away.) On its northern side, the hurricane's convection has been noticeably reduced, forcing the hurricane to try to re-organize by developing stronger convection to the southeast, giving the hurricane a stretched appearance. On its western side, the eye is protected by less and less deep convection. The western side's outflow has a most interesting appearance. Although it is broken, one can trace a circular curve from one end of the break to the other. In other words, the circulation remains strong, but convection is increasingly unfavorable along the hurricane's western side. The hurricane might be moving into a more stable environment.
According to upper level sounding data, instability has generally decreased from the coast of the Bahamas to Cape Hatteras during the last 36 hours. The odd building of convection to the southeast suggests that Isabel might be fighting convection in eastern Pennsylvania and Maryland and another cluster off the coast of the Delmarva for moisture. These cells developed from remnant circulation and moisture of Tropical Storm Henri, and may have drained available moisture farther down the Gulf Stream. Isabel's weakening seems to have begun about the time, it entered the axis of the monsoonal moisture plume that is still fueling heavy rain to the west of Philadelphia. Forecast models indicate that instability will increase ahead of Isabel and Isabel may escape the draining moisture plume. If Isabel's current structure escapes too much harm, it could regain much of its strength tomorrow and early Wednesday.
ESA(20030915.1)
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