Tropical Storm Ian intensifies as it threatens Florida

Tropical Storm Ian intensified Saturday night, and is forecast to rapidly strengthen through Monday, possibly into a high-end Category 4 storm.

The big picture: Hurricane Ian could hit western Florida or another part of the Gulf Coast as a powerful hurricane as early as midweek this week.

State of play: As of 11 am ET on Sunday, Tropical Storm Ian was located about 570 miles southeast of the western tip of Cuba, and contained maximum sustained winds of 50 mph, and was moving to the west-northwest at 14 mph.

• President Biden declared a federal state of emergency for multiple Florida counties on Saturday night and Florida Gov. Ron DeSantis has declared a state of emergency for the entire state.
• The National Hurricane Center is forecasting two to four inches of rainfall from the Florida Keys into the southern and central Florida Peninsula from Monday through Wednesday morning.

What to watch: Ian is moving toward an area with extremely favorable conditions for it to rapidly intensify.

• In its 11 am update, the National Hurricane Center said Ian could become a hurricane Sunday night or Monday and reach "major hurricane strength" by Monday night or early Tuesday before hitting western Cuba.
• Studies show an increase in the occurrence of rapid intensification due to human-caused climate change.
• The western Caribbean Sea is a powder keg for hurricanes right now, with high ocean heat content and weak upper-level winds.

What's next: The key questions facing forecasters, public officials and tens of millions of residents along the Gulf Coast is where the storm will head once it becomes a hurricane, and how strong will it be once it gets there.

• The computer models have been diverging, with some showing a landfall in northwestern Florida or perhaps southeastern Alabama. Others show a hit much farther east, closer to Tampa.
• Forecast trends since Friday have nudged the most likely track of the center of Ian to the west, closer to the Panhandle region of Florida.
• While the likelihood of significant impacts in South Florida has decreased, it has not entirely disappeared, and the Hurricane Center is urging all Floridians to prepare for storm impacts.

• "It cannot be overstated that significant uncertainty remains in Ian's long-range prediction," the Hurricane Center stated in an online forecast discussion Sunday morning.

Between the lines: The differences between computer models stem from incomplete information about the strength and placement of a dip in the jet stream, also known as a trough, forecast to develop across the Central and Eastern U.S. early in the week.

• The European model has consistently shown Ian gaining strength and curving to the north-northeast over time.
• The main American model, known as the GFS, has been depicting another scenario entirely, with Ian missing the express lane of the trough, and instead threatening areas further to the north in the eastern Gulf of Mexico, but as a weaker storm.

Context: Human-caused climate change is altering the characteristics of nature's most powerful storms.

• For example, sea level rise from melting ice sheets makes a hurricane's storm surge more harmful.