The mercury hovered in the mid-90s this afternoon, heralding what the National Weather Service expects to be the hottest day of the summer, with a heat index of over 100 in the region.

The Weather Service issued an excessive heat warning, meaning that "a prolonged period of dangerously hot temperatures" can be expected, with hazards to man and beast alike. As of about 4 p.m. the temperature in Washington was 96 degrees, with a heat index of 101 degrees , the weather service reported.

Wednesday is expected to be hot as well, with the break likely to come Thursday.

The weather service cautioned residents to stay out of the sun, drink plenty of water and protect children and pets. The heat forced the D.C. Department of Motor Vehicles to close its inspection station on Half Street SW at 1 p.m. today. Pepco issued an alert asking customers to use electricity wisely, so as to avoid blackouts.

The source of the discomfort, apart from the fact that it's Washington in the summer, is lethal hot air from the West and Midwest slogging eastward, where it is meeting up with a moist and soggy air mass arriving from the South, all of which is trapped here by the jet stream across the northern section of the country, which is acting as a barrier against the heat's escape.

The warmest day so far this year recorded at Reagan National Airport was July 21, when it hit 95. Today's reported temperature tops that record by one degree. The record high for July 26 at National is 103 in 1930.

The city and counties are making facilities and assistance available for those who need it throughout the area.

The District is implementing its heat emergency plan while the heat index stays at 95 degrees or above. Key components of the plan include: activation of street showers in selected locations; opening of cooling centers in senior citizens facilities, District government buildings and other locations; extension of public swimming pool hours; and distribution of fans to special-needs residents. People who need transportation to cooling centers can call the hypothermia hotline at 202-399-7093.

Prince George's County is opening cooling centers at the Cora B. Woods Senior Center in Brentwood from 9 a.m. to 2 p.m.; the New Carrollton Municipal Center in Lanham from 10 a.m. to 2 p.m.; and the Camp Springs Senior Center from 9 a.m. to 3 p.m. The Department of Family Services has electric fans available for senior citizens. Call 301-265-8450 for more information.

Montgomery County doesn't have cooling centers but is opening two of its homeless shelters to anyone who needs to cool down. The sites are the Montgomery Avenue Women's Center in Rockville, open from 8 a.m. to 5 p.m., and Progress Place in Silver Spring, open from 8:30 a.m. to 5 p.m. Elderly or low-income residents can get electric fans by calling 240-777-3000. Residents can also call the Montgomery County Crisis Center 24 hours a day at 240-777-4000. The county also recommends that people seek shelter in public places such as shopping malls, recreation centers and senior centers.

As the heat moved eastward, a cold front brought rain Monday to parts of Iowa, Minnesota and Wisconsin, and was expected to cross Illinois, Missouri and Indiana on Tuesday, Ed Shimmon, a weather service meteorologist in Lincoln, Ill., told the Associated Press. Eastern states should feel relief by Wednesday.

Further west, authorities said about two dozen people have died from heat-related illnesses in the Phoenix area since July 16. Most were homeless. Some of the others were elderly or were inside without working air conditioning, wire services reported.

Mesa police reported one person dead, as did Tempe police.