5 p.m. - Tropical storm warning issued for northwest Bahamas

The northwest Bahamas, recently ravaged by Hurricane Dorian, are now under a tropical storm warning. Tropical storm conditions could begin on the islands as soon as Friday morning and the National Hurricane Center predicts up to 2 to 4 inches of rain could fall before the potential storm exits Saturday. Isolated amounts up to 7 inches could cause flash flooding.

Islands included under the warning are: Abacos, Berry Islands, Bimini, Eleuthera, Grand Bahama Island, and New Providence. The Abacos islands and Grand Bahama Island, in particular, were heavily damaged by Hurricane Dorian.

The National Hurricane Center projects the area of disturbed weather centered over the central Bahamas to become a tropical depression by Friday afternoon and a low-end tropical storm by Friday night as it tracks northwestward. If it becomes a tropical storm, it would be named Humberto.

The conditions forecast for the northwest Bahamas will in no way resemble those they endured during Dorian but would lead to additional suffering for the many residents without access to shelter and food.

After exiting the Bahamas, the Hurricane Center’s track forecast brings the potential storm near Florida’s east coast. “The system could bring tropical-storm-force winds and rainfall to portions of the Florida east coast over the weekend,” the Hurricane Center wrote. “Residents there should monitor the progress of this system.”

As described below, there is significant uncertainty with respect to the potential storm’s exact track and areas along the Southeast coast north of Florida should also monitor the storm.

Original article from 11 a.m.

The tropics have been relatively quiet for the past week, but they’re not going to stay that way for long. A tropical storm — which would be dubbed Humberto — is likely to form between the Bahamas and Florida’s east coast and could affect parts of the southeastern United States in the coming days. In addition, an area much farther east in the tropical Atlantic also bears watching.

Disturbance over Bahamas is likely to become Tropical Storm Humberto, its destination highly uncertain

A disturbance over the central and southeastern Bahamas has become better organized but is a very challenging system to forecast. Its ragged, sloppy structure right now makes it difficult for computer models to project its future path.

We do know it’s favored to develop. In fact, the National Hurricane Center gives it a 70 percent chance of becoming a tropical depression or storm in the next 48 hours and an 80 percent chance in the next five days. A Hurricane Hunter aircraft is scheduled to investigate the system Thursday afternoon, and the Hurricane Center may begin issuing regular advisories later today.

The most immediate concern is that, according to the Hurricane Center, the disturbance will bring heavy rains (1 to 3 inches or so) and gusty winds in portions of the northwestern Bahamas that were hammered by Hurricane Dorian.

There are many possible paths for this disturbance, including one that wasn’t looking like a possibility yesterday: a track up the Florida east coast.

Possible scenarios

Scenario 1: If this disturbance tracks up Florida’s east coast, the southeastern part of the Sunshine State would get some rain and rough surf along the coast, but the worst of the storm would initially remain offshore. However, this track could put northeast Florida, coastal Georgia or the Carolinas in play for heavy rain, gusty winds and perhaps some coastal flooding if it bends to the west on its northward jog, as some models suggest. This scenario favors a stronger storm system, as the fledgling storm would spend more time over bathlike ocean waters.

Scenario 2: The system could track up the Florida peninsula, riding through the middle of the state like it’s a train track. While this would bring heavy rain to Florida sometime in the Friday/Saturday time frame, wind and storm surge would be a lesser threat. There would be a risk of isolated tornadoes in this second scenario, however, particularly from the Treasure Coast up to the Space Coast.

Scenario 3: It could cross the Florida peninsula, producing heavy downpours Friday and Saturday, and end up in the northeast Gulf on Sunday. That would lend a narrow window of opportunity for it to develop, likely into a tropical storm. If this were to occur, it would likely meander westward only a bit, impacting either coastal Alabama or the Florida Panhandle with tropical-storm-force winds and heavy rainfall (Sunday-Monday). However, Gulf intensification beyond that level of strength would not be favored given the incipient storm’s short life span over water.

Scenario 4: The storm passes over the northwest Bahamas and tracks out to sea, offshore the East Coast.

While models favored Scenario 3 on Wednesday, there is diminishing support, and the “East Coast skimmer” (scenario 1) seems more likely at this point. But anyone from Alabama through the Carolinas, and particularly the east coast of Florida, should monitor this disturbance in the coming days.

Tropical wave in eastern Atlantic could develop next week

While not an immediate threat at this point to any land masses, this disturbance located a few hundred miles west of Cabo Verde has roughly a 40 percent chance of development, according to the National Hurricane Center. Where its track ultimately takes it is unclear at this point, but it should continue meandering west. The National Hurricane Center does advise that “conditions appear conducive for development,” however, and that “a tropical depression could form early next week.”

Long-term outlook

Beyond that, it’s looking like a flare-up of activity will occur toward the end of the month into early October. Signs point to a broad-scale rising motion in the upper atmosphere in the tropics around that time, which would kindle tropical growth and foster hurricane development.

Close to a half-dozen systems will have tried, and failed, to develop since Dorian. But a return to dangerous activity is possible. Using some tropical meteorology, we can estimate a return to an active pattern in these regions beginning sometime around the following dates (within a few days):

Gulf of Mexico: In about a week

Caribbean: In about 10 days

Tropical Atlantic: In about 10 days to two weeks

Why? It has to do with something called the Madden-Julian Oscillation, or MJO.

The MJO is a large overturning circulation that propagates east across the globe at about 30 mph. In practicality, it’s just a giant, slow, invisible wave that lifts air to the west and drops it to the east. On the leading edge of these waves is the “suppressed” phase of sinking air. To the west (on the wave’s backside), the air is more prone to rising — favorable for hurricane activity.

You can see that here. Pay attention to the areas of brown and green. Notice what happens in Phases 1, 7 and 8. You see a lot of green across the Atlantic. That indicates rising air:

Now we can look at what weather models estimate the MJO to do. Almost all models indicate a return to Phases 1 or 8, the phase that favors hurricane activity over the Atlantic, within the next two to three weeks. The upward motion would arrive from the west over the Gulf of Mexico first before overspreading the rest of the tropical Atlantic.

In addition, there’s another mechanism — called a Convectively Coupled Kelvin Wave — wrapped up inside the larger MJO envelope that will do the same thing. It, too, will work toward fostering hurricane production at the end of September into early October.

Even if things seem quiet in the wake of Dorian, they may not be for long.