Forecasters say Western states will sizzle this week amid excessive heat watches

Aug. 15 -- Summer heat is set to build across the West as a gradual shift in the weather pattern occurs this week, with forecasters saying temperatures could climb almost 20 degrees above average in some areas over the coming days in cities like Seattle and Portland, Ore.

The influx of hot weather will expand across Northwest states like Washington, Oregon, Idaho and Montana. The heat will also grip portions of Central California and Nevada, throughout the week.

An expansive swath of excessive heat watches have been issued from north-central California, through the San Joaquin Valley. The watches cover numerous counties through the latter half of the week and span from Redding to Bakersfield in California.

In the above image, excessive heat watches are seen in red and span several counties in north-central California and the San Joaquin Valley.

"Much like recent hot stretches, this will be caused by a large bulge in the jet stream, acting to keep the storm track lifted north and allowing temperatures to surge in the coming days," explained AccuWeather Meteorologist Andrew Johnson-Levine.

Temperatures are expected to peak around midweek across many locations in north-central California, Nevada and Oregon, before gradually sinking closer to average. Farther north into Washington state, the heat will likely persist into the weekend.

AccuWeather meteorologists say that the upper-level pattern across the country's eastern half will play a prominent role in how many days the heat lingers over the West.

"The dip in the jet stream and cooler temperatures in the eastern U.S. will act as a roadblock in the atmosphere, which should keep the hot conditions around the West for much of the week," stated Johnson-Levine.

The hottest regions outside Central California are likely to be southern Oregon, the Willamette Valley and areas east of the Washington Cascades. By Wednesday, temperatures are forecast to be around 100 degrees Fahrenheit across most of these locations.

Outdoor plans and activities may need to be altered as the hot conditions set in across the West to limit the risk of heat-induced illnesses. Residents are urged to drink plenty of water and utilize air conditioning if it is available.

For those without access to air conditioning, experts recommend wearing breathable, light-colored fabrics and cold washrags on your neck and wrists. Some tips to keep your home cooler include keeping doors shut and closing curtains and blinds.

Portions of Oregon and California are facing moderate to exceptional drought levels from a lack of rainfall. Many locations have observed 25% or less of their average rainfall since July 1. Olympia, Wash., has only received 15% of its average rainfall since the start of July and Seattle stands at 23%.

By mid- to late week, some moisture from the North American monsoon is expected to push northward this week.

"While the monsoon is bringing heavy rain and flooding to the Southwest, areas farther north will have a bit less moisture to work with, meaning less rain from any thunderstorms that form. This may lead to dry thunderstorms, which can quickly start wildfires when lightning strikes dry vegetation," explained Johnson-Levine.

Wildfires have peppered the dry Northwest landscape over the last couple of weeks. The most expansive fire still ongoing is the Moose Fire in central Idaho. As of early Monday morning, the Moose Fire had burned over 78,000 acres and was at 34% containment.

The Significant Wildland Fire Potential Outlook for August, projected by the National Interagency Fire Center, depicts that portions of Oregon, central and eastern Washington, Idaho, Montana and California, will face a higher-than-average wildfire risk.

Although the peak of the heat is expected around midweek, conditions will likely remain hot through at least the weekend. However, an uptick in rainfall chances may provide some relief to the Northeast by the start of the following week.