EARLY ACTION FORECAST IN THE TROPICS

A combination of wintertime weather systems and a weak disturbance in the tropics are forecast to combine into an organized low-pressure system over or near the Bahamas over the weekend. Most of the effect on South Florida’s weather will come before the system organizes, when tropical moisture spreads over the region from the south.

The ingredients for this system are coming from three different directions. There is a mass of tropical moisture related to an old cold front across Cuba and the Straits of Florida. In addition, an unusually strong high-pressure system is centered over Virginia and covers the entire East Coast. It’s already creating the strong wind coming off the Atlantic, and is currently holding the moisture to the south. A third system coming from the west will upset that balance.

A wintertime-like disturbance embedded in the strong jet stream that stretches from the Pacific, across the Gulf and Florida, and into the Atlantic will reach Florida late tomorrow. Its main initial effect will be to lift the tropical moisture north, ramming it into the strong high. The outcome is expected to be windy, raining weather spreading from south to north later tomorrow into Friday.

Over the weekend, the high-pressure system will retreat to the east toward the central Atlantic, and leave an area of somewhat favorable conditions east of Florida and northeast of the Bahamas.

Since the low-pressure system that is expected to form would be a combination of the weak disturbance from the tropics and the wintertime-like disturbance in the jet stream, it won’t be a pure tropical system. We call these hybrids “subtropical.”

And just because a low-pressure system develops, that doesn’t automatically mean it meets the National Hurricane Center’s criteria to call it a Subtropical Depression right away. We have to see how well it’s able to organize. The computer forecast models, mostly show a system with decent organization moving away from Florida Sunday and Monday.

The ocean water off the Southeast coast is cooler than we would normally expect to support a tropical system, but with the extra boost from the wintertime disturbance, there’s a decent chance the system can overcome the slightly cool water.

This hybrid type of weather system is not uncommon at the beginning and the end of hurricane season. It says nothing about how the rest of the season will play out. In fact, the windy weather over the Gulf and the near Atlantic will tend to cool both areas by stirring up the water, which can’t hurt.

The subtropical system will be called Arthur if winds reach 40 mph in the circulation. Ironically, the stronger it is, the better South Florida’s weather will be because a stronger system would have the oomph to pull drier air down over the state.

Looking statistically, there is a small peak in tropical (or subtropical) activity in late May and early June because we can get these interactions of winter and summer weather systems. By late June, the wintertime systems generally migrate north, so on average things calm down while we wait for the purely tropical season to begin in August.