On Monday, South Florida experienced very heavy rainfall and some strong winds. In fact, it may have been considered by some to have been the worst weather over South Florida of the entire 2007 hurricane season – so far. Some people may wonder why the weather disturbance wasn’t designated a tropical cyclone.
Monday’s weather was due to a combination of three things: an upper-level low pressure system over the southeastern Gulf of Mexico, a weak low pressure system at the surface off the southeastFlorida coast, and a nearly stationary frontal trough in our vicinity. There was also a very strong high pressure system centered off the Northeast U.S. coast.
The pressure gradient (the difference in pressure between this high and the lower pressure over South Florida) resulted in some strong winds (mainly in and north of the frontal cloud band). In fact, there were gusts to tropical storm force offshore and occasionally along the Florida east coast due to this pressure gradient.
The heavy rains resulted primarily from the convergence of air at low levels and were likely enhanced by diffluence aloft (meaning that the winds over South Florida were going different directions).
We must remember that the definition of a tropical cyclone has nothing to do with rainfall. We can get heavy rains from a variety of systems (cold fronts, individual thunderstorms or thunderstorm clusters, upper-level lows, tropical waves, etc.) not associated with tropical cyclones.
See my blog of Sept. 6 on “When Are Tropical Cyclone Advisories Initiated?”
Tuesday the weak low pressure system at the surface has moved into the southeastern Gulf of Mexico. The old frontal boundary is still analyzed over South Florida and the upper-level low over the southeastern Gulf is still evident in satellite imagery.
We are still monitoring this complex weather system for possible development and the National Hurricane Center is saying that it could become a tropical or subtropical (hybrid) cyclone.
See my blog of Sept. 19 “What is a Subtropical Cyclone?”
Most of the computer models suggest the surface low pressure system will strengthen over the next couple of days as it heads in the vicinity of Texas and/or Louisiana.