Meanwhile, Hurricane Norma is forecast to unleash heavy rain, flooding, mudslides and dangerous surf in Baja California. The storm had weakened slightly, to a Category 3 Friday, after having rapidly strengthened into a Category 4 on Thursday. Norma, which is forecast to come very close to landfall near Cabo San Lucas, is the strongest of three tropical systems that forecasters are watching.

Tammy becomes a hurricane

Hurricane Tammy was about 90 miles northeast of Barbados on Friday morning with maximum sustained winds of 75 mph. The Hurricane Center wrote that the storm had “become much better organized” as it churned to the west and northwest. Tammy was forecast to gradually strengthen, with peak winds reaching 80 to 90 mph this weekend into early next week.

Tropical storm warnings were in effect for Dominica, Guadeloupe, Antigua, Barbuda, Montserrat, St. Kitts, Nevis, Anguilla, St. Barthelemy, Saba, St. Eustatius and St. Martin. Some of those same islands were also under a hurricane watch, while Barbados and Martinique were under a tropical storm watch.

A storm surge, or sudden rise in sea level, of up to a foot to 3 feet above normal tide, could occur near where the center of Tammy crosses the Leeward Islands. Portions of the islands could also experience damaging hurricane-force winds.

“Heavy rains from Tammy will begin to affect the Leeward and northern Windward Islands today, spreading into the British and U.S. Virgin Islands and Puerto Rico over the weekend,” the Hurricane Center said Friday. “This rainfall may produce isolated flash and urban flooding, along with isolated mudslides in areas of higher terrain.”

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Tammy was predicted to produce 4 to 8 inches of rain with localized amounts of 12 inches across the Leeward Islands, 2 to 4 inches of rain with localized amounts of 6 inches across the northern Windward Islands, and 1 to 2 inches of rain with localized amounts of 4 inches across the Virgin Islands into eastern Puerto Rico. Life-threatening swells and rip currents were expected to continue for portions of the Lesser Antilles over the next few days.

The Hurricane Center’s forecast shows Tammy moving north and northeast into the open ocean early next week, where it will be unlikely to threaten the U.S. mainland. Tammy is the 20th named storm in the Atlantic this hurricane season, marking only the fourth year to record at least 20 named storms in the Atlantic by Oct. 18, according to Philip Klotzbach, a researcher on the Tropical Weather and Climate Research team at Colorado State University.

Norma closes in on Baja California

Norma’s maximum sustained winds nearly doubled between early Wednesday morning and early Thursday morning, jumping from 65 mph to 120 mph in just 24 hours. The storm joins a growing number of tropical systems that have met the criteria for rapid intensification in recent years, which is defined as a storm that undergoes a 24-hour wind increase of at least 35 mph.

Norma’s peak winds reached 130 mph Thursday before dropping to 110 mph as of 11 a.m. Friday; the storm was centered about 245 miles south of Cabo San Lucas. Norma was forecast to gradually weaken by Friday into the weekend.

Norma was forecast to track northward into Saturday and still be a hurricane as it approaches the southern tip of Baja California on Saturday, possibly making landfall near Cabo San Lucas. There was still some uncertainty whether the eye of Norma would make landfall in Baja California or veer just to the east before weakening to a tropical storm. A hurricane warning was in effect for Baja California, from Todos Santos to Los Barriles, while tropical storm warnings were in effect from north of Los Barriles to La Paz and from north of Todos Santos to Santa Fe. Las Islas Marias was under a tropical storm watch.

“Heavy rainfall from Norma will begin to impact the far southern portions of California Baja Sur later today, continuing through Sunday,” the Hurricane Center said Friday. “This rainfall may produce flash and urban flooding, along with possible mudslides in areas of high terrain.”

General rainfall amounts of 5 to 10 inches were predicted across the far southern portion of Baja California Sur, with localized amounts up to 15 inches. Swells generated by Norma were already impacting the coast of southwestern Mexico and were forecast to spread north along the western Mexican coast and toward Baja California. “These swells are likely to cause life-threatening surf and rip current conditions,” the Hurricane Center added.

Moisture from Norma could eventually enhance rainfall next week in the Southern Plains, where some areas are experiencing extreme to exceptional drought, according to the latest U.S. Drought Monitor.

To the southeast of Norma, another disturbance was showing signs of development. That system, several hundred miles south of the Gulf of Tehuantepec, now has an 80 percent chance of becoming a tropical storm in the next week and a 50 percent chance in the next two days. It’s too soon to say whether it could become a threat to land.