A big warm-up will send temperatures toward 100 degrees Monday through Wednesday, forecasters say, the hottest weather of summer so far.

The National Weather Service says not to expect much of a cool-off until next weekend, which means the heat wave could last more than the typical three days. And that can lead to more than just uncomfortable sleeping, weather experts say. "We may not see more than two or three days of 100 degrees, but we're looking at an extended period of 90 degrees or above, and that's a long stretch when it comes to health concerns," said Steve Todd, meteorologist-in-charge of the National Weather Service in Portland.

Play it cool

• Drink plenty of fluids, especially water, even if you are not thirsty. Caffeinated drinks are diuretics and will work against you. Drinking ice-cold drinks causes the body to work harder than if you drink merely cool drinks.

• Eat light. Bananas are a good choice; their potassium helps the body maintain normal electrolyte balance, reducing the likelihood of muscle cramps.

• Get in the shade when possible. Wear brimmed hats or use an umbrella.

• Reschedule sports activities. Plan them early in the day or in the evening to avoid intense sun exposure.

• Wear sunscreen.

• Do not leave animals in your car. Temperatures inside can swiftly kill them.

Todd said a long string of 90 degree days with little nighttime relief can exacerbate health problems -- especially for senior citizens without air conditioning or adequate ventilation.

He said the weather service issues heat advisories and watches based on mortality rates linked to heat by geographic region.

"A 100-degree day in Phoenix is a nice day," he said. "Up here in the Northwest, a lot of us don't have air conditioning and people are not acclimatized."

The record for days at 90 degrees or higher is eight, from Aug. 12 to 19, 1967. The longest period of 100 degrees or higher came during five days, July 13-17, 1941.

Portland's all-time record high -- one that may not be broken for a long time -- is 107 degrees, and it occurred twice: July 30, 1965, and Aug. 8, 1981. Record highs for Monday through Wednesday, are, respectively, 102, 101 and 100 degrees.

For those without air conditioning, agencies have announced the opening this weekend and early next week of cooling centers.

• Clark County Fire District 6 will open a cooling center at its Felida fire station, 11600 N.W. Lakeshore Ave., Vancouver, on Monday, Tuesday and Wednesday.

• In Multnomah County, the Loaves & Fishes Center at 1032 S.W. Main Street will be open from 10 a.m. to 8 p.m. Saturday and Sunday; and the Hollywood Senior Center, 1820 N.E. 40th Ave., will be open from 3 to 8 p.m. Saturday and Sunday

• Portland Parks & Recreation issued an online reminder of cool facilities and activities it oversees, including 13 public swimming pools, spray features in 11 parks and more than 10,000 acres of parks and natural areas in which to find shade.

Other public safety agencies probably will open cooling centers if the heat settles in for a long stay.

For more details on the effects of heat on and aging, visit the National Institute of Aging's Web site.

The weather service released this list of late July statistics for the Willamette Valley:

-- Stuart Tomlinson;

stuarttomlinson@news.oregonian.com