MIAMI (BNO NEWS) -- Tropical Depression Thirteen formed over the central Gulf of Mexico on late Thursday evening, forecasters said, prompting tropical storm warnings and a state of emergency for the U.S. state of Louisiana.
Forecasters at the U.S. National Hurricane Center (NHC) have been following the weather system since Tuesday afternoon when it emerged as a tropical wave over the northwestern Caribbean Sea in the company of disorganized cloudiness and showers. It became better organized throughout the week.
"Satellite, surface, and NOAA reconnaissance aircraft data indicate that the low pressure area over the central Gulf of Mexico has acquired a closed circulation that is defined enough to be considered a tropical depression," said NHC senior hurricane specialist Daniel Brown.
As of 1 a.m. CDT (0600 GMT) on Friday, the center of Thirteen was located about 240 miles (385 kilometers) southwest of the mouth of the Mississippi River. It is currently nearly stationary, but a slow northwest drift is expected to start later on Friday, followed by a turn toward the north on Saturday.
Maximum sustained winds of Thirteen are near 35 miles (55 kilometers) per hour, with higher gusts. "A shear analysis from [the University of Wisconsin-Madison Cooperative Institute for Meteorological Satellite Studies (UW-CIMSS)] shows about 20 knots (23 miles or 37 kilometers per hour) of westerly shear over the cyclone," Brown said on early Friday morning.
"This shear is forecast to relax during the next 12 to 24 hours as an upper-level anticyclone builds over the Gulf. This is expected to result in a favorable environment for intensification," the forecaster said. "However, given the large size of the circulation, strengthening is expected to be gradual."
Thirteen is expected to reach tropical storm strength on Friday afternoon before heading to the Louisiana coastline, resulting in a tropical storm warning for Pascagoula, Mississippi westward to Sabine Pass, Texas. The warning area includes the city of New Orleans, Lake Pontchartrain, and Lake Maurepas.
"The slow motion of the cyclone during the next five days is likely to produce a prolonged period of tropical storm conditions and heavy rainfall over portions of the northern Gulf coast," Brown warned. He said total rainfall accumulations of 10 to 15 inches (25 to 38 centimeters) is expected over southern Mississippi and southern Alabama through Sunday, with possible isolated maximum amounts of 20 inches (50.8 centimeters).
Thirteen is forecast to make landfall as a tropical storm on the coast of Louisiana on late Saturday evening or early Sunday morning. The storm will then move into Mississippi and possibly Alabama as it weakens.
As a result, Louisiana Governor Bobby Jindal has declared a state of emergency. "We're closely monitoring the weather in the Gulf and our agencies are on alert and stand ready to assist coastal parishes if the system strengthens," Jindal said. "We know from experience that it's best to prepare for the worst and hope for the best, and since we're at the height of the hurricane season, now is a good time for Louisianians to make sure they have a gameplan to protect themselves and their families if a major storm approaches our coast."
Thirteen is the thirteenth storm of the 2011 Atlantic hurricane season, following Tropical Storm Katia which formed in the far eastern Atlantic earlier this week and is expected to become a major hurricane in a few days. If Thirteen becomes a tropical storm as forecast, it will be given the name Lee.
According to figures released last month, NOAA's Climate Prediction Center is expecting an above-normal hurricane season in the Atlantic this year. The outlook calls for 14 to 19 named storms, with seven to ten becoming hurricanes and three to five expected to become a major hurricane (category 3 or higher).
An average Atlantic hurricane season produces 11 named storms, with six becoming hurricanes and two becoming major hurricanes. The Atlantic hurricane season runs from June 1 through November 30, with peak activity in September.