Tropical Storm Alpha (search) drenched the Dominican Republic and Haiti with heavy rains after sweeping ashore Sunday, forcing the evacuation of 30,000 people from flood-prone areas.

Forecasters warned that deadly flash floods and mudslides were possible as days of rain have already swollen rivers and saturated the soil.

The two countries, which share the Caribbean island of Hispaniola (search), were under a tropical storm warning as the record-setting 22nd named storm of the Atlantic season made landfall with maximum sustained winds of 50 mph.

Alpha had no clearly defined center as it began its passage across the southernmost tip of Hispaniola, said Ron Goodman, a forecaster with the U.S. National Hurricane Center (search) in Miami. It was not considered a threat to the United States, forecasters said.

"It's just a big mass of thunderstorms," Goodman said.

Alpha was expected to weaken over land, and it could dissipate in the mountains of the Dominican Republic, the Hurricane Center said. At 11 a.m. EDT, its rough center was 175 miles west-northwest of Santo Domingo and moving toward the northwest at nearly 15 mph.

Maximum sustained winds had weakened to nearly 35 mph.

Heavy rains were reported throughout the Dominican Republic, and authorities with megaphones walked through low-lying neighborhoods of San Juan de Maguana — which was badly damaged by Hurricane George in 1998 — to urge people to leave. Moderate flooding was reported in several low-lying communities in the south.

There were no immediate reports of deaths or injuries but the country was in a high state of alert, said Jose Luis German, spokesman for the country's Emergency Operations Committee.

In Haiti, authorities closed the airport because of heavy rain but had no immediate reports of deaths or injuries — though it was not possible to contact remote communities, said Abel Nazaire of Haiti's Risk and Disaster Management agency.

A rain-swollen river overflowed its banks in the southern town of Jacmel, flooding some areas and forcing an unknown number of residents into shelters, said civil protection director Maria Alta Jean-Baptiste.

Meteorologist Ignacio Feliz of the Dominican weather service said authorities were especially concerned since heavy rains — in part due to Hurricane Wilma — already had drenched the island in recent days. In Haiti, 12 people died from rains and flooding caused by Wilma.

The U.S. National Hurricane Center warned that Alpha could bring 4 to 8 inches of rain across the island — and as much as 15 inches in some places — "with a strong likelihood of life-threatening flash floods and mudslides."

Both nations were vulnerable, but the danger is particularly high in Haiti because of extensive deforestation and the millions of people who live in flimsy homes on river banks and mountain sides. The storm could drench the city of Gonaives, where 1,900 people were killed and 900 went missing after Tropical Storm Jeanne hit last year.

A tropical storm warning was posted for the southeast Bahamian islands and for the Turks and Caicos.

The storm formed Saturday in the Caribbean, and marked the first time forecasters had to turn to the Greek alphabet for names. The previous record of 21 tropical storms and hurricanes had stood since 1933. The hurricane season doesn't end until Nov. 30.