A large portion of Illinois, including Lake and McHenry counties, are expected to get severe thunderstorms Monday into Thursday. | Graphic Provided by AccuWeather.

Thunderstorms capable of producing isolated tornadoes, 70 mph winds, hail and flooding are expected Monday evening in northeastern Illinois, including Lake and McHenry counties.

The National Weather Service (NWS) said the storms will begin late Monday afternoon into Monday evening across much of the Chicago area.

The primary threat is strong, damaging winds up to 70 mph, the NWS said.

The thunderstorms will also be capable of producing large hail up to half-dollar size, a tornado or two, and downpours and localized flooding.

According to the NWS, approximately 4-5 inches of rain has already fallen in some spots since early Monday morning, which has led to localized minor flooding of streets, fields, and ditches, mainly in portions of Boone and McHenry counties.

A Hazardous Weather Outlook was issued by the NWS around 12:10 p.m. Monday warning about the expected Monday evening storms.

The alert also said that stronger and more severe thunderstorms are possible Tuesday evening in the area. Periodic thunderstorms are expected through at least Thursday.

Warm and very humid conditions will also persist through Thursday, the NWS said. Heat indices of over 100 are expected in most areas during the afternoon hours, with dangerous heat index values nearing or possibly surpassing 105 Tuesday afternoon.

Those who hear thunder are recommended to move indoors and have multiple ways to receive weather warnings, the NWS said.