Heat wave coming to Modesto. How long will the triple-digit temperatures last?

It’s going to be a scorcher in Modesto next week, and the National Weather Service has issued an excessive heat watch for the Valley and foothills.

The weather service is forecasting daytime highs in Modesto reaching at least 100 degrees Monday through Friday, with Wednesday the hottest at 107 degrees. The lows will be in the low 70s.

The upcoming Fourth of July holiday means picnics, parades and other outdoor fun. And the Stanislaus County Fair starts its 10-day run on July 5.

But the excessive heat watch is in place Tuesday morning through Saturday night, and the weather service is advising people to limit their time outdoors to the coolest part of the day or evenings, keep in the shade as much as possible and stay hydrated.

Stanislaus County residents without air conditioning and other means of staying cool can use the cooling centers throughout the county and its cities. The centers are primarily public libraries, senior centers and community centers. The complete list is at stanemergency.com/pdf/cooling-locations.pdf.

And for people who are heading out to rivers and other waterways, the Modesto Fire Department has free, loaner life jackets at fire stations in Modesto, Turlock, Ceres, Waterford, Riverbank, Oakdale and Knights Ferry. More information is available at modestogov.com/3112/Life-Jacket-Loaner-Program.

NWS meteorologist Eric Kurth said the temperatures will be about 10 to 12 degrees above normal for this time of year.

The heat wave can be blamed on the merging of two strong high-pressure ridges — one over the eastern Pacific Ocean and the other over the desert in the Southwest. Ridges are characterized by sinking air that warms as it falls. Ridges also trap heat over an area.

Kurth said the high-pressure ridges mean there will not be much cloud cover or moisture, just lots of sunshine that will heat up everything.