CHICAGO (CBS) — It will be a hot and muggy start to the week for Chicago, with temperatures reaching the low- to mid-90s, and the heat index soaring past 100, prompting a heat advisory on Monday.

The National Weather Service has issued a heat advisory from 11 a.m. to 7 p.m. for most of northern Illinois and northwestern Indiana.

Temperatures were already at nearly 80 degrees at 6 a.m. Monday, heading to a high of around 93 in Chicago.

High humidity levels were expected to push the heat index to peak levels between 103 and 109 for the Chicago area.

People should keep cool by drinking plenty of cold water and staying in air conditioning as much as possible.

Monday night won’t bring much relief to the Chicago area, as temperatures are expected to drop only to about 78 degrees overnight.

The hot weather will continue on Tuesday, with temperatures again reaching the low 90s, before thunderstorms cool things off.

Temperatures on Wednesday and Thursday will top out in the low 70s, and then return to the 80s on Friday ahead of more storms next weekend.