Tropical Storm Gabrielle
Gabrielle has been sputtering as an intermittent tropical cyclone since midmorning last Wednesday. It shredded into a remnant low Friday but soon sprang back to life as a tropical storm. Since then, Gabrielle’s been meandering around the north-central Atlantic, packing central sustained winds of 50 mph. Over the open ocean, Gabrielle will transition from a tropical to extratropical storm soon as it makes a run toward the northern United Kingdom late in the week. Ireland and Scotland could see breezy rain showers Wednesday night into Thursday.
System No. 1
Meanwhile, our attention turns to the Gulf of Mexico, where a system could approach late in the week. Right now it’s centered just north of Hispaniola, producing sporadic showers and thunderstorms. In the next few days, the disorganized blob of thunderstorms will scoot up toward the southern Bahamas, giving it an inch or two of rain while spilling heavier afternoon thunderstorm activity over the Florida peninsula Thursday.
Friday into Saturday, the wave will move into the Gulf of Mexico, straddling the gulf’s northern periphery. That would put gulf coastal Florida, Mississippi, Louisiana and, yes, Alabama in line for showers and thunderstorms at times this weekend.
If the system remains offshore long enough, there’s a chance it could develop into a tropical storm; the National Hurricane Center gives this possibility a 30 percent chance over the next five days, but those odds could be raised.
Conditions are conducive for development, but any potential system’s intensity would be “capped.” Whether it organizes enough to get a name, a plume of very moist air will arrive along the Texas Gulf Coast on Sunday night into Monday. This increases the chance of heavy downpours and potential flooding from Houston to Lake Charles.
System No. 2
This underachieving tropical wave is about 900 miles east of the Windward Islands right now. Its prospects to develop into a tropical depression or storm are limited to 20 percent, according to the Hurricane Center. While some organization is possible in the next day or so, it’s not long for this world. The Hurricane Center says “upper-level winds are forecast to become unfavorable for tropical cyclone formation” by Thursday.
System No. 3
It’s very early to make a call on this system. The wave recently exited the African coastline, south of Cape Verde. There’s nothing to encourage it or kindle it for the time being, but that may change next week as the system draws nearer to the Lesser Antilles. How it evolves thereafter remains to be seen, but, for the time being, its odds of development are rather low.
However, don’t rule it out. “A handful of European model ensemble members have been bullish on it,” wrote Brian McNoldy, CWG’s tropical weather expert. Most of the Atlantic is warmer than average, he writes, except the waters around Cape Verde. That’s “the first environment that fledgling African waves encounter upon leaving the nest,” McNoldy writes. We won’t have a better grasp on this system until it approaches 50 degrees west longitude.
However, this one could prove mischievous down the line; the Hurricane Center gives it a 20 percent chance of developing into an organized system.
End of September/early October
Despite the lull in hurricane activity, Tuesday (Sept. 10) marks the peak of Atlantic hurricane season. If you were to pick a random day out of a random year, Sept. 10 would have the greatest probability of there being a tropical cyclone in the Atlantic.
There are signals that tropical activity will ramp back up toward the end of September into early October. So while systems before then remain possible, the atmosphere will be fired into a second round of “turbo boost” mode in about two weeks’ time.
The reason? Much of it has to do with a convectively coupled Kelvin wave. These are very sluggish, large-scale “waves” in the equatorial atmosphere that, in essence, feature sinking motion on their leading edge and rising in their wake. They move east, unlike the vast majority of tropical weather systems, which go west.
A convectively coupled Kelvin wave is not a storm system. Instead, it’s sort of an overturning circulation that can either suppress or encourage rising air. From that, we know that tropical cyclones, which thrive from rising air, will have a much greater chance of developing on the backside of a passing convectively coupled Kelvin wave.
This potential is elevated further by the Madden-Julian Oscillation, which will additionally enhance rising motion over the Atlantic come very late September into early October.
Michael Ventrice is a tropical meteorologist at the Weather Company and specializes in work surrounding convectively coupled Kelvin wave. He has proved them to be a useful predictor of periodical upticks in hurricane activity.
“In the longer term, the European model and its 51-member ensemble are predicting the [Madden-Julian Oscillation] to make a quick shift to phases 1 & 8 starting in the middle of next week,” McNoldy writes. Simply put, those phases favor increased hurricane activity.
What will the period from Sept. 26 to Oct. 10 feature? Stay tuned.