Updated at 9:10 a.m. ET
A complex set of weather systems appears likely to evolve into a tropical depression or tropical storm in the Caribbean over the weekend or early next week. Currently, the atmospheric environment is hostile to tropical development, although it is conducive to producing bands of heavy rain, which have been affecting Puerto Rico and the surrounding islands.
A strong dip in the jet stream has been scooping moisture from a weak, elongated tropical disturbance and spreading it north toward the islands. The upper winds related to the jet-stream dip wouldn’t allow development of the disturbance, however. This weak disturbance is being designated Tropical Disturbance #1.
Over the next few days, the jet-stream dip is going to pull away to the north, leaving a conducive atmospheric environment, and another weak tropical disturbance will enter the Caribbean. The old disturbance plus the new disturbance plus the conducive conditions are forecast to come together to produce a tropical system.
The National Hurricane Center is now giving the system a high chance of developing into at least a tropical depression over the weekend or early next week.
On Puerto Rico and the Virgin Islands, the very rainy period is likely to continue until the system clears the area by the middle of next week. Flash flooding will be possible when tropical downpours focus on one area.
The exact nature of the new system is unknowable, but there is high confidence that it will generally move west in the Caribbean. High pressure across the Gulf of Mexico and Florida will keep it bottled up to the south.
Although the atmospheric conditions appear conducive for development, the system will be battling dry air that has pushed south from the continental U.S. The arid atmosphere should help limit how strong it can get.
An important rule is that forecasts for just-developing systems are prone to high errors and are likely to change. So everyone around the Caribbean should stay informed over the next several days.
Disturbance #2 is related to the same elongated disturbance and upper-level jet-stream dip as Disturbance #1. A separate low-pressure system might develop from the northern end of the disturbance and drift north into the waters west of Bermuda.
It will only have a brief window of time to develop, however. The National Hurricane Center is now putting the odds as low. In any case, it does not appear to be a significant threat, but it could bring some squally weather to Bermuda.
Elsewhere, a wintertime pattern is slowly taking over.