Tropical Storm Karen closes in on Puerto Rico as threat grows for Bahamas, Southeast U.S.

Tropical Storm Karen, newly revived after weakening to a tropical depression on Monday, is set to impact Puerto Rico Tuesday with flooding rains and gusty winds. As of Tuesday morning, Karen was packing sustained winds of 40 mph. Because Karen is a weak tropical storm that is experiencing wind shear, or winds changing speed or direction with height, along with bursts of heavy thunderstorms that can cause its center of circulation to jump around, predicting its precise movement is tough.

A tropical storm warning is up for Puerto Rico, including Culebra and Vieques, as well as the U.S. and British Virgin Islands. It's important to emphasize that Karen's main impact in these areas will likely come in the form of heavy rains of 3-5 inches with localized 8-inch amounts, enough to spark flooding concerns in the mountainous terrain. Heavy rainfall amounts are likely to encompass the San Juan metro area, as well as other parts of the island.

Heavy rains present a threat for landslides across the entirety of Puerto Rico, and the U.S. Virgin Islands is also under a flash flood watch through at least Wednesday afternoon.

The winds, while not overly impressive in their own right, could cause some issues with Puerto Rico's fragile power grid, left vulnerable in the wake of Category 4 Hurricane Maria's 2017 assault on the island. In addition, the heavy rains will pose challenges for those still living in makeshift shelters after losing their homes to the 2017 storm.

On Monday, Karen was struggling to survive as a tropical cyclone in the face of intense shear. In fact, its structure was so limp and ragged that it was downgraded to a tropical depression, and the National Hurricane Center raised the possibility that it could disintegrate altogether. Wind shear can easily disrupt a cyclone's structure, a bit like blowing on a spinning top can cause it to teeter into oblivion. But as daylight dawned Tuesday, Karen was straddling the edge of a much more favorable region of upper-level winds; that may encourage Karen to track just 10 or 20 miles farther west than originally anticipated.

Tropical Storm Karen will attempt to scoot northward after passing near or over Puerto Rico. But on that path, Karen can only go so far. A piece of the Bermuda High, a semi-stationary dome of high pressure anchored between the Azores and Bermuda, will force Karen to slow down, stop, and potentially turn west. The time frame for this to occur would be Friday into the weekend.

From there, uncertainty escalates.

Think about the storm as a Roomba vacuum cleaner, with its random turns and varying movements. If you turned it on and then donned a blindfold, after 10 seconds you'd have a good idea as to where the vacuum cleaner was.

But if you waited an hour or two, the number of subtle random jaunts it could have taken would result in a massive amount of uncertainty.

The most likely scenario calls for Karen to move west, while possibly intensifying some. This would put people in the Bahamas and perhaps the southeastern United States on edge.

However, Karen's exact strength at that point is key, since the alignment of its mid-level circulation and low-level spin affect how steering currents would affect the storm.

Tropical Storm Jerry is the second tropical cyclone to impact Bermuda in a week, after Hurricane Humberto's eyewall brought 130 mph wind gusts onto the island Wednesday night. Jerry will pass very close to the island, but unlike Humberto, it is not a major hurricane. Instead, Jerry is a more ragged tropical storm with 60 mph sustained winds. It has no deep convection left, leaving its low-level center exposed beneath a blanket of innocuous sunshine. Bermuda is under a tropical storm warning, and gusty winds and heavy rainfall will be possible.

And then we have Lorenzo, a tropical storm southwest of Cape Verde. Lorenzo’s not going to stay well-behaved for long. Forecast to become a hurricane by late Tuesday, Lorenzo is set to strengthen into a major hurricane by the end of the week. Lorenzo will largely mind its own business as it stays over the open ocean for the next several days.