Alabama twisters cause deadliest tornado day in U.S. since 2013

• The Lee County tornadoes devastated rural communities in that state, with more than 100 rescue crews continuing to search for survivors on Monday morning.
• Other severe thunderstorms and possible tornadoes struck in Georgia and Florida as well, as a severe weather outbreak swept through the region.
• The storms were well-forecast, including more than 90 tornado warnings issued across the Southeast — and two dire "tornado emergency" alerts denoting an imminent threat of loss of life and/or "catastrophic damage."

The big picture: National Weather Service forecasters are surveying damage across at least 3 states to determine how many tornadoes touched down and how strong they were. One of the Lee County tornadoes was given a preliminary rating of an EF-4, with estimated maximum winds of 170 mph.

• The storms resulted from a clash in air masses as cold, dry Arctic air pushed southward out of Canada, while warm, moist air and a fast-moving flow of air raced up the East Coast in association with a storm system that rippled along this boundary.

• The low-level winds provided plenty of wind shear, which allowed the severe thunderstorms to develop the persistent rotation necessary for producing tornadoes.

Go deeper: California's "fire tornado" had 143 mile-per-hour winds