LAFAYETTE -- The weather is supposed to be cool and clear Sunday morning as the sun comes up and thousands of runners (and walkers) stretch, drink coffee and find a port-a-potty for a last-minute bathroom break.

More than 4,000 runners, walkers and casual observers are expected to descend on downtown Lafayette for the 2013 Lafayette Reservoir Run, the 21st annual run, taking place Sunday morning Oct. 27, starting and ending on Mount Diablo Boulevard near the Metro Lafayette restaurant and lounge.

The "Res Run" attracts athletes from across the Bay Area for its combination of small town feel, beautiful scenery, challenging 5-K competition and the 10K course that circles the reservoir.

As always, there are several options, including the 2-mile Fun Run (with an 8 a.m. start), the 5K (8:30 a.m. start, west along Mt. Diablo Boulevard to Acalanes Road and back) and the 10K, west from downtown and around Lafayette Reservoir, including the challenging "heartbreak hill" from the boulevard up to the reservoir. That starts at 8:45 a.m. The 5K and 10K races have certified electronic timing.

A pancake breakfast, sponsored and prepared by the Lafayette Rotary Club, will be served starting at 6:30 a.m. and goes until the batter runs out, probably around 10:30 a.m. Musicians from Stanley Middle School will perform through the start of the major races.

Pre-race (and this year for the first time, post-race) warm-up exercises and Health Expo vendors will all be part of the scene, too.

And this year, a free post-run concert featuring "Diamond Dave" Hosley will be held at the reservoir's new stage, starting at 1 p.m. Sunday. The concert is sponsored by Lafayette-based Trust in Education, in celebration of the opening of the Girl's School in Farza, Afghanistan. Trust in education provides educational, economic and health care assistance to villages in Afghanistan.

The Lafayette Reservoir Run is a Lafayette Chamber of Commerce event sponsored by Sutter East Bay Medical Foundation and Oakwood Athletic Club of Lafayette.

Adult registration fee is $30, or $35 on race morning; for elementary school participants, it's $22, or $27 the day of the race. Race-day registration starts at 6:30 a.m.

Proceeds go to benefit local school programs and the chamber's community activities.

To register online or for more information, go to http://www.lafayettechamber.org and click on "events."