11:20 p.m. — Isaias forms, earliest 'I' storm on record

The National Hurricane Center declared the large area of thunderstorms south of Puerto Rico a tropical storm Wednesday night, as it finally detected a center of circulation. Named Isaias, the storm packs maximum sustained winds of 50 mph and is zipping west-northwest at 20 mph.

Tropical storm warnings cover an enormous areas from the Virgin Islands to the southeastern and central Bahamas, including Puerto Rico and Hispaniola.

Isaias is the earliest 9th Atlantic named storm on record, more than a week ahead of the next earliest (Irene, from Aug. 7, 2005).

According to Phil Klotzbach, a tropical cyclone researcher at Colorado State University, 2020′s five named Atlantic storms during July ties 2005 for the most storms during the month on record.

The Hurricane Center forecast Isaias to weaken some over the next day as it passes over Hispaniola but wrote: “Tropical storm conditions are likely across portions of the Virgin Islands, and Puerto Rico through tonight and will spread westward to portions of the Dominican Republic, Haiti, and the southeastern Bahamas and Turks and Caicos and the Central Bahamas on Thursday and Friday.”

These conditions include “heavy rains and potentially life-threatening flash flooding and mudslides.”

By the weekend the storm could impact Cuba and Florida. And there is some chance Isaias comes up the East Coast early next week.

For more specifics on the evolving forecast for the storm through the weekend, see below.

Original article from Wednesday morning

An increasingly organized mass of thunderstorms swirling over the Leeward Islands is likely to get a name on Wednesday as it whirls closer to Puerto Rico. Gusty winds, heavy rainfall and mudslides are possible as the developing cyclone lashes the U.S. territory. The storm is expected to affect the Dominican Republic, as well, and while there is high uncertainty, there are signs the southeastern Lower 48 may be in play.

It’s set to be the ninth named storm to form in the Atlantic so far this season, and is set to earn the name “Isaias.” It would overtake the record for the earliest “I” storm, held by Irene, which was named on Aug. 7, 2005.

The hyperactive season to date has also featured the earliest C, E, F, G and H storms on record — Cristobal, Edouard, Fay, Gonzalo and Hanna.

Tropical storm warnings are up for all of Puerto Rico, Vieques, Culebra, the U.S. Virgin Islands and the British Virgin Islands. The Turks and Caicos, southeastern Bahamas, parts of the Dominican Republic and northern Haiti are also under a warning.

The system’s intensity will largely hinge on where it tracks.

“[T]his system could bring some rainfall and wind impacts to portions of Cuba, the central and northwest Bahamas, and Florida later this week and this weekend,” the National Hurricane Center warned.

Tracking the system

On Tuesday, the strong tropical wave was labeled a “potential tropical cyclone” by the National Hurricane Center. That terminology enables the center to issue storm-related alerts before the system has actually received a name.

By Wednesday, the system — “potential tropical cyclone 9,” or PTC9 — had winds sustained up to 45 mph. That exceeds the 39 mph threshold needed for a system to be classified as a tropical storm. But intensity is just one side of the coin. The system must also be organized.

A glance at visible satellite imagery shows that PTC9 is elliptical, an oval of swirling clouds and thunderstorms. But the system shows no clear center of circulation. “[A] recent Air Force Reserve reconnaissance aircraft was still unable to find a well-defined circulation,” the National Hurricane Center wrote in its 11 a.m. discussion.

Until a cohesive, well-defined center forms around which the system can orbit and organize, its ability to strengthen is limited.

However, a burst of convection — shower and thunderstorm activity — near the system’s axis of spin may unlock the key to organization. If the rising motion of a thunderstorm updraft can capture and vertically stretch a column of that spin, its rotation will intensify. (Think of the spinning ice skater pulling in their outstretched arms.)

There’s a good chance that will happen by late Wednesday or early Thursday, allowing winds to coil about a clear-cut center of circulation.

When Air Force Reserve Hurricane Hunter reconnaissance missions into the storm find evidence of a center, the system can be named Isaias.

PTC9 has been slow to organize due in part to an influx of hot, dry air at the upper levels associated with the Saharan Air Layer. That acted to cap its upward development of thunderstorms. However, PTC9 has nearly outrun that air mass and could intensify more before affecting Puerto Rico from Wednesday evening to early Thursday.

Puerto Rico impacts

The system is expected to be classified as a tropical storm when it makes its closest pass to Puerto Rico beginning Wednesday evening. Doppler radar showed heavy downpours beginning to rotate southeast from near the U.S. Virgin Islands through Puerto Rico on Wednesday morning. Those tropical downpours are predicted to increase in areal coverage and intensity throughout the day.

Winds will gradually build throughout the afternoon, with gusts topping 50 mph overnight, especially in the southwestern part of the territory, including Ponce. Parts of the Lajas and Rojo Cabo municipalities may encounter a few gusts over 55 mph. The National Hurricane Center expects the storm’s center to pass offshore to the southwest, with maximum sustained winds nearing 60 mph in its core.

Heavy rainfall is possible, which could lead to flooding and mudslides. A flash flood watch is in effect for all of Puerto Rico and the U.S. Virgin Islands, where a widespread 3 to 6 inches with localized 10 inch totals is possible.

The rains could be beneficial to the commonwealth, more than half of which is considered to be in drought by the U.S. Drought Monitor. Nearly a quarter of Puerto Rico, particularly in the island’s east, is experiencing a severe drought.

In addition to wind, rain, flooding and potential mudslides, a few of the downpours and thunderstorms associated with soon-to-be Isaias could produce isolated waterspout or tornado activity.

Encounter with Hispaniola

After leaving Puerto Rico during the first half of Thursday, PTC9 will probably hit Hispaniola. The National Hurricane Center is predicting the cyclone to make landfall in the Dominican Republic on Thursday as a tropical storm.

Haiti and the Dominican Republic probably will see very heavy rainfall, with a few double-digit rain totals possible. That will be especially problematic in Haiti, where rainfall frequently interacts with deforestation and loosened topsoil, increasing the risk of mudslides.

When PTC9 hits Hispaniola, the terrain will probably weaken it significantly, the mountains up to 10,000 feet tall disrupting the system’s structure.

Florida/Gulf threat

The National Hurricane Center estimates PTC9 will emerge north of Hispaniola at low-end tropical storm strength on Friday, slipping north of Cuba. The cyclone will slowly begin regaining strength over the lukewarm waters southwest of the Bahamas, but its progress may be impeded by a brief increase in wind shear — a change in wind speed and/or direction with height.

There is an increasing probability that PTC9 may wind up spending time in the Gulf of Mexico before potentially hitting Florida. However, there is a high degree of uncertainty both in track and intensity.

PTC9 has shown a tendency to remain slightly to the south of the “guidance envelope” — the range of weather models that predict the cyclone’s track.

If PTC9 does end up farther southwest than predicted to be late Saturday into Sunday, its time over the Gulf could prove concerning and problematic — a reservoir of toasty ocean waters at the ready to fuel speedy strengthening.

Those with interests in Florida and the eastern Gulf of Mexico should remain abreast of the latest forecast.

Enhanced East Coast rainfall chances

Wherever PTC9 ends up, there is a growing chance its remnants could bolster downpours along the East Coast early to mid- next week.

An upper-level low pressure system to the west, heralded by a cold front, will act as a western guardrail, ahead of which warm southerly flow will scoop PTC9′s leftover moisture north. The slow-moving cold front, which will help focus shower and thunderstorm activity, could enhance rainfall and tap into the system’s moisture.

The National Weather Service Weather Prediction Center has already begun highlighting this possibility in its outlooks, although confidence remains low.