Freezing fog will settle over approximately 250,000 square miles of the interior western United States into this weekend.
"Clear, calm and moist conditions will allow the fog to form and linger for days east of the Cascades and Sierra Nevada and west of the Rockies," according to AccuWeather Meteorologist Maggie Samuhel.
The persistent fog will be just a nuisance to some, but it will lead to slow, dangerous travel on area highways and city streets. Significant airline delays are also possible.
Fog will not be a problem in coastal areas, including cities such as Seattle, San Francisco and Portland, Oregon, through this weekend. However, motorists venturing eastward and cross-country on Interstate 80, I-84 and I-90 should be prepared for travel problems.
The foggy conditions will be extensive and affect vast portions of Washington, Oregon, Nevada, Idaho, Utah, Montana and Wyoming.
"Even much of California's Central Valley will be affected by dense fog during at least the morning hours," according to AccuWeather Senior Meteorologist Kristina Pydynowski.
Cities likely to be affected by days of fog include Spokane, Washington; Boise, Idaho; Pendleton, Oregon; Elko, Nevada; Missoula, Montana; Jackson, Wyoming; Salt Lake City and Sacramento, California.
"Recent rainfall and moist soil could enhance the fog situation enough so that fog fails to burn off in parts of the Central Valley during the afternoons," Pydynowski said.
The fog can reduce the visibility to near zero at times, especially during the late-night and morning hours.
Over the interior West, temperatures will struggle to climb out of the 20s F during the day. Temperatures will dip into the single digits and teens in many locations at night.
These cold, moist conditions will allow ice to build up on exposed, untreated surfaces, which can be dangerous for motorists and pedestrians. The icy conditions will not be limited to bridges and overpasses due to the long-duration event.
While temperatures will be held back in the Central Valley as well, fog should lift enough to allow temperatures to climb above freezing in most areas during the day.
In addition to the dangerous conditions, poor air quality will be a concern for those with cardiovascular and respiratory problems in heavily populated areas. People with such health problems should remain indoors in a climate-controlled environment during the foggy pattern.
"The same mechanisms creating the fog will trap pollutants in the lowest levels of the atmosphere," Samuhel said.
The fog is generally a low-elevation phenomenon. Those in good health wanting to escape the foggy conditions may want to consider a day skiing. Conditions over much of the high country in the region will be bright and sunny during the pattern.
In some cases, the fog may continue into the first part of next week, when winds will pick up over much of the interior West.
Meanwhile, in coastal areas of Southern California, it will be gusty winds that can cause problems late this week.