The record-setting heat wave that has been broiling the Pacific Northwest will finally envelope portions of the Northern Rockies beginning Tuesday through the weekend.

According to the National Weather Service in Missoula, dangerously hot daytime and overnight temperatures are expected for much of the region throughout the week.

A high pressure centered over the coastal northwest is to blame for this rare early summer meltdown. The heat bubble that pushed temperatures to 112 in Portland, Oregon, on Sunday will slowly drift eastward into Montana on Tuesday.

In advance of the dangerous weather, a heat advisory was issued for the Flathead and Mission valleys and into West Glacier, with high temperatures up to 100 degrees expected across all areas through Friday.

It's worse for locations farther west.

An excessive heat warning is in place for the Lower Clark Fork region and toward Libby and Troy where temperatures will eclipse triple digits for at least six consecutive days. Some parts of the Kootenai could hit 110 on the hottest days.

The Weather Service's "heat index" gauges the effect of extraordinarily high temperatures on the body. During an excessive heat warning, people are cautioned that the human body cannot maintain proper internal temperatures and may experience heat stroke.

"We cannot stress enough what kind of toll the impending heat will have on individuals spending time outside throughout the next week," the Weather Service warned in its forecast discussion Monday.

"If you remember the heat wave that was July 2007, then you will know what kind of pattern the Northern Rockies will be stuck in for a least this coming week."

Thunderstorms are possible this week as well, but the Weather Service noted that it's difficult to predict when and where they might occur.

PEOPLE ARE advised to take this severe weather event seriously. Drink plenty of fluids, and when possible reschedule strenuous outdoor activities to early morning or evening.

"Don't be mowing the lawn at 2 p.m., or gardening at noon," warned Logan Health trauma coordinator Joy Fortin.

Fortin said people should seek out air-conditioned buildings if possible.

"Air condition is key to thriving in these condition," she added. "Seek AC when you can, in your own home or a public space."

Fortin said the most vulnerable populations for heat exhaustion or heat stroke are children under 4 and people over 65.

For the elderly, many prescription drugs are diuretics which can dehydrate people. The elderly tend to not compensate for the loss of water with enough fluids.

Likewise, Fortin said young children also don't drink enough fluid, and they have yet to develop the communication skills to let adults know if they're overly hot.

"If parents aren't paying attention they can get overwhelmed [by heat]," she said.

Adults should also be wary of drinking alcohol while in the heat, Fortin warned.

According to a Logan Health fact sheet, common signs of heat exhaustion are heavy sweating, paleness, muscle cramps, tiredness, weakness, dizziness, headache, nausea or vomiting and fainting.

Heat stroke is indicted by an extremely high body temperature (above 103 degrees); red, hot and dry skin with no sweat; rapid, strong pulse; dizziness; confusion; and unconsciousness.

Anyone overcome by heat should call 911 or get to a hospital immediately, and cool down with whatever methods are available until medical help arrives.

People are also encouraged to check on relatives and neighbors, especially those who live alone and those who don’t have air conditioning, Fortin added.

Children and pets should never be left unattended in vehicles under any circumstances.

THE HEAT wave was also affecting Idaho, where temperatures above 100 are forecast in Boise for at least seven days starting Monday. The National Weather Service in Coeur d'Alene said this week's weather "will likely be one of the most extreme and prolonged heat waves in the recorded history of the Inland Northwest."

In Portland, authorities said the high heat was straining the power grid and overhead wires that propel the MAX trains, so service was being suspended through Tuesday morning.

In eastern Washington, the Richland and Kennewick school districts halted bus service for summer school because the vehicles aren't air-conditioned, making it unsafe for students.

Cooling centers were opened across the region and people flocked to them in Seattle, where only about 44% of households have air conditioning, according to U.S. census figures.

In Issaquah, Washington, an outage Sunday afternoon left thousands without power, and some went to a community center cooling station.

The Associated Press contributed to this report.