The National Weather Service has issued a forecast for thunderstorms with potentially heavy rainfall starting Friday and continuing through the weekend of the Missoula Marathon.

Meteorologist Leanne Allegretto said a series of very wet storms is headed towards western Montana.

"In advance of all the wetness that we'll be experiencing later Saturday, Sunday and Monday, there is the chance for thunderstorms to develop from the southern Bitterroot Valley and south and east of there," Allegretto said. "For the most part, these will be very wet storms, so flash-flooding and burn scars are certainly a concern. Gusty winds and lighting always seem to accompany these storms, but we are looking for localized flash flooding with any of the storms that do develop."

Allegretto said the Missoula Marathon 5k event on Saturday morning should be fairly free of inclement weather, but not so for Sunday's half and full marathon races.

"Saturday morning should be OK if you're in Missoula," she said. "It'll be cloudy with maybe some light rain for the runners on Saturday. But, come Sunday, it'll be a different story. We're looking for prolonged heavy rainfall, very cool temperatures, and that's going to last for most of the day. It's going to start out on Saturday night, through Sunday morning and then become heavier and more widespread through most of the day on Sunday."

In addition, the National Weather Service reports that snow levels will decrease to below 6,500 feet Sunday night through Monday morning and daytime temperatures will be up to 25 degrees below normal.