In the hot desert, you have a lot of warm air rising into the upper atmosphere. That creates lower pressure near the ground as you now have less air around.

This is a near-daily occurrence in the summer in the Desert Southwest and California. However, if strong high pressure moves south out of Canada and into the Rocky Mountain states, that can actually push the thermal trough north into the Pacific Northwest.

At this point, the placement of the trough is key to the temperatures around the area. In some sense, it might be best to think of the trough as a valley, and air will want to pour into the valley to fill it.
 

A few times a year, the trough will actually come north either along the Pacific Coastline or even out over the ocean. When that happens and the trough is to our west, you have air now rushing from the east out toward the offshore trough (See "What Is Offshore Flow" for more information there.). That offshore flow will bring us very warm temperatures -- probably some of the warmest of the season.

The trough can stay offshore for a few days, bring an extended heat wave to the area. But eventually, the predominant westerly flow will win the battle, pushing the trough east into Eastern Washington and beyond.

As the trough pushes east of your location, the winds will shift from east to west, bring a rush of cooler, ocean air into the region, most likely triggering a marine push (See "What Is A Marine Push?" for more information there.) and bringing an abrupt end to the heat spell.

For Seattle, the fun begins as soon as the trough passes over the Cascade mountain range.

(You've heard the saying, "The bigger they are, the harder the fall." Usually, the stronger the thermal trough, the greater the cooling. It's not unusual to have one day where it is 93 degrees outside, have the trough move east, and then have the next day be about 67 degrees under a heavy fog.)

Do It Yourself Forecasting / Other Notes

Thermal troughs are some of the most interesting weather patterns to track, because you can watch its movement in real-time, and the pattern follows a general script.

In troughs that begin offshore, you can track their eventual demise by looking at local weather observations: (Washington Currents  -- Oregon Currents) and a visible satellite.

Usually, on the visible satellite, you can see fog moving north from the California Coast into Oregon as the trough moves north and east of those areas, thus bringing the cool ocean breeze behind it.

You can also keep tabs on the Oregon and Washington Coast cities' current temperature. As the trough passes each city, the temperature will take a huge drop (usually into the upper 50s or mid 60s), the wind will shift to a west/southwest direction, and the fog will roll in.
 

As it comes up the Oregon Coast, that's a 1 day warning that the heat wave is coming to an end for the Washington Coast, and a 2-2.5 day warning that the end of the heat wave is coming to Seattle.

By the next day, the trough has probably moved over the Puget Sound area, and the fog line usually has moved up to the tip of the Washington Coast, keeping them in the mid 60s while Seattle bakes in the 85-95 degree range.
 

The day the trough moves overhead is usually the warmest day of the heat wave (and you can also watch the trough pass Port Angeles as their winds shift from east to west and their temperature will plummet.)

As soon as the trough passes over the Cascades, the rush of cool (refreshing?) marine air will rush into the Puget Sound area. This normally happens around 10 p.m. to midnight, but can happen anytime. You can usually sense it. The wind shifts from north to south, and you can begin to hear the wind chimes going and puffs of cool air move in.
 

It is something that boaters need to watch carefully. If we get a very strong push, winds can go from calm to 35-40 mph winds in a real hurry -- especially in the Strait of Juan de Fuca and Northern Inland waters.

For More Information:

What Is Offshore Flow
What Is Onshore Flow/Marine Push