As Washington, D.C., prepares to host a parade celebrating the U.S. Army’s 250th birthday on Saturday, weather forecasters are warning of the potential for showers and thunderstorms that could disrupt the day’s events, including a scheduled appearance by President Trump, whose 79th birthday coincides with the celebration.

A significant number of demonstrations, from supporters and critics, are also expected across the country. For all involved, the weather may prove less celebratory, as forecasters noted some risk of severe storms in a region from Montana into Texas and of extreme heat in the South and Southwest.

Across a wide section of the Eastern and Central United States, a slow-moving weather system is expected to keep conditions unsettled through the weekend, with showers and thunderstorms across portions of the Gulf Coast up through the Ohio Valley, the Mid-Atlantic States and the Northeast. While forecasters say widespread flooding is unlikely, downpours could cause localized flooding in some areas.

The National Weather Service issued a broad “marginal” risk for excessive rainfall on Saturday, the lowest level on its four-tier scale, for much of the eastern third of the country through the weekend. A slightly higher risk was forecast for parts of Kansas, Oklahoma and Virginia.

In the nation’s capital, the likelihood of rain is high on Saturday. Conditions are expected to be cloudy and humid, with a high temperature of about 85 degrees Fahrenheit. Events on the National Mall are scheduled all day long, and the parade is scheduled to begin at 6:30 p.m. A fireworks display is planned for three hours later.