* Monster tornado part of storm system across US South
* Floods a concern with rising rivers, saturated soils
* Obama declares federal state of emergency in Alabama (Updates Mississippi death toll)
By Peggy Gargis
BIRMINGHAM, Ala, April 27 (Reuters) - Tornadoes and storms killed at least 45 people in Alabama on Wednesday as severe weather lashed the southeastern United States, bringing down power lines, destroying buildings and triggering flooding.
The storms were part of a series that have struck half a dozen southern states over the last few days, killing at least 72 people.
"State (emergency officials) can confirm 45 storm-related deaths today in Alabama," said Yasamie August, spokeswoman for the Alabama Emergency Management Agency.
Fifteen people were killed when a tornado tore through Tuscaloosa, a city of 93,000 that is home to the University of Alabama, she told Reuters.
The thunderstorm that spawned a tornado there triggered more twisters in Georgia three hours later.
"This could be the worst tornado in Alabama's history," said meteorologist Josh Nagelberg of AccuWeather.com.
Marshall County in northeastern Alabama had six fatalities, five of them in one house, Sheriff Scott Walls told Reuters.
"That house was in the direct path of the tornado. We had homes and businesses that took direct hits. Every community in the county has suffered damage," he said.
U.S. President Barack Obama declared a state of emergency for Alabama and ordered federal aid for the stricken state.
"While we may not know the extent of the damage for days, we will continue to monitor these severe storms across the country and stand ready to continue to help the people of Alabama and all citizens affected by these storms," Obama said in a statement on Wednesday night.
FLOODS AND POWER OUTAGES
Floods were a big concern throughout the storm-hit area, where rain compounded with melted snow to cause rising rivers and saturated soils.
Several states suffered power outages as well as property and infrastructure damage that could prove costly to repair.
The storms forced the Tennessee Valley Authority to close three nuclear power plants in Alabama and knocked out 11 high voltage power lines. [ID:nN27183193]
Alabama Governor Robert Bentley mobilized around 1,400 National Guardsmen and said they would be on the ground early on Thursday to help with search and rescue, logistics and debris removal, a statement from his office said.
Bentley and governors in Arkansas and Tennessee declared a state of emergency. In Mississippi, Governor Haley Barbour declared a state of emergency for 39 counties.
Two deaths were reported later on Wednesday after a tornado hit Georgia near the border with Tennessee and Alabama.
Mississippi authorities said on Wednesday night that the storms had caused at least 11 deaths in eight separate counties in the state in the last 24 hours.
Eleven deaths have also been recorded this week in Arkansas, two in Louisiana and one in Tennessee. Authorities expect the toll to rise as emergency workers attempt rescues and recovery in the storm's path.
Residents in Tennessee are coping with flooding, power outages and blocked roads. Flooding became more widespread in Arkansas on Wednesday after several days of intense storms.
Violent weather has pummeled much of the U.S. South this month. Two weeks ago, at least 47 people died as storms tore a wide path from Oklahoma to North Carolina. (Additional reporting by Verna Gates in Birmingham; Writing by Matthew Bigg, Editing by Doina Chiacu and Paul Simao)
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