Tropical Storm Ian was strengthening Sunday night, and it is forecast to intensify more rapidly Monday and Tuesday — possibly into a high-end Category 4 storm as early as midweek this week.

State of play: Tropical Storm Ian was some 390 miles southeast of the western tip of Cuba at 11pm ET, and its maximum sustained winds had strengthened to 65 mph, up from 45 mph Sunday afternoon, according to the National Hurricane Center. It was moving to the northwest at 13 mph.

• A tropical storm warning has been issued for the lower Florida Keys and a tropical storm watch has been issued for the west coast of Florida.

The big picture: 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.

What to watch: In its 11pm update, the National Hurricane Center said Ian was expected to become a hurricane Monday and a “major hurricane” on Tuesday before hitting western Cuba.

• The National Hurricane Center forecast two to four inches of rainfall from the Florida Keys into the southern and central Florida Peninsula from Monday through Wednesday morning.

Threat level: 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 they’re saying: Even if the west coast of Florida doesn’t sustain a direct hit from Ian, “it doesn’t take an onshore or direct hit from a hurricane to pile up the water,” acting NHC director Jamie Rhome said in a Sunday briefing.

• He urged Florida residents to find out if they’re in a likely evacuation zone at FloridaDisaster.org in case evacuations are ordered.

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

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.

This story has been updated with the storm’s strengthening and the latest estimates of when the storm is expected to become a hurricane.