Rogue Valley meteorologists could be spending Fourth of July weekend updating their almanacs thanks to a forecast that calls for five days of high temps expected at least to tie — if not outright break — heat records set decades ago.

The National Weather Service in Medford has issued an excessive heat watch for a period of triple-digit temperatures starting Thursday, July 4, and lasting through Monday, July 8.

Officials in Medford were in the process of arranging a cooling shelter but had not immediately confirmed hours, days or location as of 1 p.m. Tuesday, citing volunteer availability. Ashland is activating a cooling center from noon to 7 p.m. Thursday, July 4, to Sunday, July 7, at 2200 Ashland St. The cooling center may extend to July 8 and 9, depending on the heat wave and volunteer availability to staff the shelter, city officials said Tuesday. See ashlandoregon.gov or text 97520SHELTER to 888777 to be notified about upcoming shelter activations.

The apex of the heat wave will be Saturday, when the forecast calls for a record-setting high of 111 degrees. It will shatter the previous record of 104 degrees, set on July 6, 1922, according to almanac data provided by Dan Weygand, lead meteorologist at the local Weather Service.

Reached by phone midday Monday, Weygand said the agency was in the process of considering whether to extend the heat watch based on updated data and forecasts.

Weygand said that factors behind an excessive heat watch include the severity of the heat during the day, as well as a lack of nightly cooldowns. Friday and Saturday’s overnight lows, for instance, are forecast to be in the upper 60s.

More than 60 million residents are under heat-related watches and warnings across the West Coast and south-central United States, according to a nationwide short-range forecast discussion issued Monday by the Weather Service.

Along the West Coast, the excessive heat warnings, watches and advisories are going into effect stretching from the low desert of southeastern California to Southern Oregon.

For Medford, the multiple days of highs at or near records are expected to begin July 4, which calls for a forecast of 105 degrees, according to Weygand. The previous record for the date was 103 set in 2015.

Friday’s forecast calls for a high of 110 degrees, which will smash the record of 102 degrees set July 5, 1926.

“111 is the peak on Saturday,” Weygand said.

Sunday’s forecast calls for a high of 105 degrees, which will break a record of 102 set in 2014, and Monday’s high is forecast to be 104, which matches a record set July 8, 1952.

During an excessive heat watch, the Weather Service advises that people take precautions to avoid heat-related illness, such as staying hydrated and indoors in air conditioned spaces when possible.

According to Weygand, the extended forecast so far shows only mild relief next week. He described it as “only a modest cooldown,” likely by the middle of next week, to highs in the lower 90s.

Weygand said there was “some uncertainty” in the extended forecast, but the lower temps will likely prevent the area from breaking its 10-day record for consecutive 100-degrees in July set in 1967.

It depends on when and whether the high-pressure ridge leading to the current high temperatures starts to break down.

“If it doesn’t break down, we could start approaching that record,” Weygand said. “It’s a lengthy period of extreme heat nonetheless.”

The heat would also be a strong start for breaking Medford’s record for the most 100-degree days in July — 13 — set in 2014.