If you absolutely must get around D.C. this weekend, Metro wants you to know that you should likely expect delays.

As the District government prepares for the area’s first major snowstorm of 2016 (a “potentially historic” one, some forecasts predict), the region’s most heavily used transit system is coordinating a “severe-weather response.” Metro will in all likelihood activate its Emergency Operations Center during the storm and immediately after it, such that the agency’s executive leadership from all key departments will work 12-hour shifts to help ensure safe operations.

“It’s just too soon to know, but anyone who relies on bus service should expect there to be changes,” Metro spokesperson Dan Stessel explains in an email to City Desk, noting that riders can sign up for systemwide alerts.

In fact, Metrobus has a tiered-approach to handling snowstorms. In the case of “severe snowfall and ice,” service is limited to “major roads only”; it’s also possible that all service gets suspended “if road and travel conditions become unsafe” (see if your regular route or routes would be cut under severe weather here). For “moderate snowfall,” the agency outlines, “service is suspended on some routes and buses are detoured” around potentially hazardous roads.

On Metrorail, there’s no precise threshold for when outdoor service would be cut, although Metro notes that when “snowfall approaches eight inches,” operations slow. Below is a map of the system showing outdoor rails as faded:

“There is not a hard and fast rule—but when total snow accumulation begins to interface with the electrified third rail, that’s when aboveground service may need to be temporarily suspended,” Stessel says.

“For safety reasons, as well as to preserve our railcars and allow for faster recovery after the storm, Metro may suspend aboveground rail service in a major snowstorm and serve only underground stations,” the agency explains on its website. “In some cases when snowfall is not equal throughout the region, rail service may operate in some aboveground areas, but not in others.”

Frigid temperatures are also a risk for the integrity of the rails themselves, which can crack because of fluctuations in degrees. On Monday morning, when temperatures in the region were well below freezing, a rail cracked near the East Falls Church station in Virginia, causing delays on the Orange and Silver lines. The incident was “probably” temperature-related, Stessel says.

“Weather-related delays are possible—and even likely—in a storm like this,” he says. “But more important, you should only travel if you absolutely have to.”