The heat index surged to 100 degrees in Baltimore by 2 p.m., with air temperatures threatening to tie or set a record.
With dew points in the upper 60s, that made it feel like the triple digits.
The Inner Harbor meanwhile reached temperatures as high as 94 degrees early Wednesday afternoon.
The heat could fuel storms Wednesday evening and overnight. A 50 percent chance of thunderstorms is expected, with some storms possibly turning severe, with damaging wind gusts and large hail. About a quarter to half inch of rain is possible.
The Storm Prediction Center forecasts 15 percent chances of severe winds and hail across most of Maryland, including the Baltimore area. Storm threats extend from the northern Plains through the Great Lakes region and to the mid-Atlantic on Wednesday.
Temperatures dropped to a low of only 76 degrees early Wednesday morning at BWI. That is one degree warmer than a record for the highest minimum temperature in Baltimore on Wednesday's date. Temperatures climbed into the 80s soon after sunrise.
Skies will turn mostly cloudy on Wednesday night with lows again in the mid-70s by early Thursday morning.
A small craft advisory is in effect from mid-morning through the afternoon for the northern portion of the Chesapeake Bay.