Baltimore fireworks

The 4th of July fireworks explode as the moon rises over the Inner Harbor as seen from Federal Hill in 2012. (Kim Hairston, Baltimore Sun / July 4, 2012)

A burst of heat on top of the humidity that's already here is expected for the Fourth of July, and that could fuel scattered afternoon or evening thunderstorms, according to the National Weather Service.

About a 30 percent chance of storms is forecast in the Baltimore area. The strongest chances, however, are expected west of the mountains, according to weather service forecasters.

Humidity is expected to remain at the high levels the region has been suffering through this week, with dew points in the upper 60s to lower 70s. Temperatures, meanwhile, are expected to be hotter than they have been.

Highs are forecast in the lower 90s, with the humidity bringing the heat index up to the mid- to upper-90s.

Such heat and humidity can cause convection, when differences in air temperature and moisture levels can cause updrafts and downdrafts that produce storms.

Even if storms don't come, it could be an uncomfortable and stifling evening for watching fireworks outside.