Five days after the demise of Isaias, there’s a new tropical wave that bears watching swirling across the Atlantic. It’s the latest sign of activity in an ocean basin that has already featured a relentless spate of early-forming storms which have claimed numerous records. Now, another system could be forming one to two weeks ahead of a possible tropical cyclone barrage.
On Monday, a tropical wave was about 600 miles west-southwest of Cabo Verde as it churned across the open Atlantic. While its window to develop may begin to close by late in the workweek, modestly conducive conditions will be at its disposal through at least Wednesday. The National Hurricane Center assigns it 60 percent odds of becoming a tropical depression or storm.
If this system earns a name, “Josephine” is next.
The earliest J storm on record was Jose, which formed Aug. 22, 2005. Katrina formed two days later.
The exceptionally busy 2020 season has also featured the earliest C, E, F, G, H and I storms on record.
If this system does not develop, in one to two weeks a much more favorable environment for robust tropical development looks likely to take place.
The forecast for Invest 95L
The tropical wave trekking westward Monday appeared on satellite with mixed signals regarding its growth prospects. While it did have a reasonably well-defined circular spin, the system — dubbed Invest 95L — lacked heavy convection, or shower and thunderstorm activity, toward its center. That’s paramount to its growth, and until more towering clouds brew near the middle, Invest 95L is unlikely to strengthen much.
The system is moving into an area characterized by limited wind shear, though, which could improve its prospects for development. Wind shear, or a change in wind speed and/or direction with height, can play tug of war and tear apart nascent cyclones as they develop vertically.
Most computer models indicate that 95L will probably skirt the northern Leeward Islands late in the week. It could clip Puerto Rico with some heavy downpours before recurving north over the open Atlantic and potentially influencing Bermuda’s weather.
How a Kelvin wave is playing a role
Part of what may hinder 95L’s development is the broad upper-atmospheric sinking motion from what’s known as a convectively coupled Kelvin wave, or CCKW. Such waves feature a zone of subsidence, or sinking motion, that inhibits hurricane growth in the suppressed branch of their large overturning circulation. On the other side, rising motion can enhance storminess.
While 95L is under the rising branch of a convectively coupled Kelvin wave right now, it is a rather weak one — and 95L will soon encounter the subsiding branch of said wave moving east over the Caribbean. The Atlantic may be rendered virtually silent by this upcoming weekend, but the quiescent lull will not last long.
Looking at where we sit with Kelvin waves, there's a weak active Kelvin wave currently passing the invest, shifting upper-level winds to easterly. But there's another suppressed phase on it's backside that will interact with the invest, possibly fizzling it out. We'll have to 👀 pic.twitter.com/gV2hyEYQQO
— MJVentrice (@MJVentrice) August 10, 2020
A particularly potent convectively coupled Kelvin wave and its associated lift will approach into early next week, and potentially wake up the Atlantic by mid- to late next week. That hefty enhancement of upward motion looks to team up with a similar feature associated with the Madden-Julian Oscillation, or MJO. The MJO is composed of an overturning circulation on a broader scale.
Ominous signals
With all the parameters lining up constructively, the overlapping ingredients are cause for concern. Assuming enough African easterly waves, or the fledgling “seedlings” of tropical cyclones, move west over the Atlantic, the set of ingredients would favor heightened tropical storm activity.
Steering currents, moreover, could help shunt them farther south and west toward the Caribbean. That’s because the Bermuda High, a high-pressure system parked over the Atlantic that acts as a bumper for tropical cyclones, is more swollen on its western limb than usual.
That enhanced branch of the Kelvin wave is over the eastern Pacific, where Tropical Storm Elida is making a run toward hurricane strength as two additional systems brew.
Anomalously warm ocean waters, driven in part by climate change, will also increase the potential for storms to intensify and become wetter.
Regardless of whether Invest 95L evolves into Josephine, an active storm period is probable in the weeks ahead.