(CNN) -- Mexican authorities issued a hurricane warning along parts of the country's eastern coast Wednesday as Tropical Storm Arlene approached, the National Hurricane Center reported.
The storm's speed has slowed, but forecasters said it was expected to start speeding up as it moved west Wednesday night.
Arlene, the first named storm of the 2011 Atlantic hurricane season, was moving at about 4 mph, packing sustained winds of 60 mph.
Authorities in the Mexican states of Tamaulipas, Veracruz, San Luis Potosi and Nuevo Leon braced for heavy rains. Forecasters from the National Hurricane Center warned of possible flash floods and mudslides.
"With any storm of this size, it's easy to overlook it and call it insignificant," said CNN meteorologist Chad Meyers. "The major effect with this storm will not be wind damage, but will be inland flooding from up to 12 inches of rain in the mountains of Mexico."
The hurricane warning in Mexico stretches from the eastern coastal town of Barra de Nautla north to La Cruz. Authorities have also issued a tropical storm warning from La Cruz north to La Pesca, and from Palma Sola south to Veracruz.
Meyers added that while Mexico's major northeastern city of Monterrey should be just north of the maximum rainfall, Victoria "may be right in the middle of the bull's-eye."
Arlene is expected to make landfall early Thursday very near hurricane strength.
It is expected to produce 4 to 8 inches of rain in some areas, while higher amounts of isolated rainfall will likely fall in mountainous terrain and could cause life-threatening flash floods and mud slides.
Tropical storm force winds are expected to reach the east coast of Mexico on Wednesday evening, which could make outside preparations difficult or dangerous, the Hurricane Center said.
When Arlene makes landfall, the storm surge could raise water levels up to 2 feet above normal tide markers.
The Atlantic hurricane season spans June 1 to November 30.
CNN's Chad Meyers, Mari Ramos and Sarah Dillingham contributed to this report.