Tropical Atlantic begins to awaken as October looms

Twenty-three named storms have whirled their way across the Atlantic basin so far this season, exhausting the naming list used by meteorologists and forcing then to dip into the Greek alphabet for only the second time in history. While the majority of systems have been relatively weak and unremarkable, some, like Category 4 Laura and Category 2 Sally, have caused significant damage.

Now, October promises to bring another round of weather that must be watched, as a combination of large-scale atmospheric circulations overlap to enhance tropical weather activity. The focus is already on the Caribbean and Gulf of Mexico, which tend to prove to be autumn hot spots for storms. In fact, one or two weather systems there could already be in the works.

System to watch for the Yucatán Peninsula; heavy rainfall possible in Florida

On Wednesday, a strip of low pressure oriented from south to north was sauntering westward across the eastern Caribbean. Satellite imagery reveals a few thunderstorms along that axis of low pressure, and computer models indicate that one clustering of downpours may eventually acquire some spin and serve as the impetus for tropical formation.

It’s a good bet that something will form or at least try to form, but the wild card exists in where along that low pressure “trough” it will be. Some models, like the American GFS and the Canadian model, show a slightly weaker, southerly solution, with a low pressure system clipping or passing over the northern Yucatán Peninsula this weekend. That would bring 3 to 6 inches of rain, with localized eight-inch amounts, to northern Quintana Roo and the Yucatán states of Mexico.

Other models, including the European model, fringe the Yucatán Peninsula with that lobe of spin. The German ICON model depicts an even farther north solution, which would yield a stronger system and probably a tropical storm. That sort of scenario would allow any eventual system to pass between Punta Cana and western Cuba and enter the southeastern Gulf of Mexico.

The National Hurricane Center estimates a 60 percent chance of tropical development with this system. It’s unlikely the system would become more than a tropical storm given its path and increasingly disruptive shear, or a change of wind speed and/or direction with height, ahead of a cold front approaching from the northwest.

Moisture from whatever system does develop could pool along the cold front, and is likely to bring heavy rainfall to central and southern Florida next week.

A second system down the road?

In the world of tropical meteorology, trying to accurately storm development beyond a week in advance is a bit of a stretch. That said, there are signs that point to a second system that may develop in the Caribbean.

After the first system dissipates, a trailing clump of vorticity, or spin, could begin to consolidate in the Caribbean. That would first be evident this weekend.

Models then indicate that any system that does develop could meander northwest into the Gulf of Mexico, where it could then affect land areas. However, at this time range, any details would be speculation.

The broad pattern

Viewed more widely, the broad weather pattern across the tropical Atlantic will become increasingly favorable for tropical development in the weeks ahead, with the pattern possibly lasting through October.

An area of rising air over the Pacific will shift east into the Atlantic beginning a little over a week from now, and that extra vertical oomph should help any fledgling tropical systems to develop.

Meanwhile, an uptick in organized thunderstorm activity over the Atlantic can be anticipated thanks to enhancement by the Madden-Julian Oscillation, or a region of enhanced showers and thunderstorm that cruises across the global tropics every 30 to 60 days.

While the heart of this storminess won’t park itself over the Atlantic, a chain reaction of rising and sinking air masses over the Indian and Pacific Oceans will eventually reduce disruptive wind shear over the Atlantic. That makes it easier for thunderstorm clusters to gather together to form tropical storms and hurricanes.

Moreover, the Gulf of Mexico, a known breeding ground for tropical systems in October, remains anomalously hot — save for cooler sea surface temperatures along the continental shelf, where cooler subsurface waters churned up by Sally and rainfall draining offshore have brought water temperatures down a bit.

Putting it all together, tropical activity may be quiet for the next few days, then climb to average and eventually above average in the 10 to 12 days. Thereafter, the remainder of October looks to be above average, especially toward the second and the third week of the month.

Philip Klotzbach, a hurricane researcher at Colorado State University, published his own outlook for the next few weeks on Wednesday. In it, Klotzbach and his team write “we expect that the next two weeks will be characterized by above-normal amounts of hurricane activity.”

The pendulum should swing the other way as we enter a more tranquil period once November nears, which is around the same time as the climatological waning of hurricane season anyway. But for the time being, we’re not done — we’re just enjoying intermission.

Source Article