On the first day of summer Friday, the National Weather Service in St. Louis issued its first heat advisory this year, as the heat index is set to reach 105 degrees Saturday. It also issued a flash flood watch through Saturday morning, before the day heats up.

Temperatures will soar to the lower 90s and reach a heat index up to 105 degrees from noon to 8 p.m. Saturday.

The NWS also issued a flash flood watch for parts of Illinois and Missouri, including St. Louis County and St. Louis city, from 9 p.m. Friday through 7.a.m. Saturday morning.

After thunderstorms Friday, the NWS expects another round of thunderstorms late tonight. That could lead to another inch or two of rainfall on top of mostly saturated grounds from the rainfall that already happened Friday. That could lead to dangerous flash flooding, the weather agency warned. Rivers and creeks could see significant rising.

The heat and humidity during a heat advisory may cause heat stress during outdoor exertion or extended exposure. The very young, the elderly, those without air conditioning and those participating in strenuous outdoor activities will be the most susceptible. Also, car interiors can reach lethal temperatures in a matter of minutes.

The areas affected by the heat advisory are concentrated in the center of the St. Louis region in St. Louis County, St. Louis city, Madison County in Illinois and St. Charles County in Missouri.

The flash flood watch affects a broader area of the St. Louis region. Beyond St. Louis County and city, affected counties include St. Charles, Madison, Bond, Monroe, Lincoln, Pike, St. Clair, Montgomery, Macoupin, Clinton, Jersey, Marion, Fayette, Shelby, Brown, Audrain, Calhoun, Greene, Knox, Washington, Warren, Adams, Callaway, Marion, Boone, Lewis and Ralls.