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Warm PNW temperatures, West Coast Thermal Troughs

Weather Blog

PORTLAND, Ore. (KOIN) — The month of April felt like winter for a short spell as cool temperatures and rain pummeled the PNW.

In fact, most of the water that accumulated this month occurred in the first two weeks. We collected nearly 3 inches of rain before April 18th, with a measly .20 inches from April 19th on.

How about our temperatures? Well,  there was a period of time from April 5 to April 15 during which we stayed in the 50s and below average. It wasn’t until April 18 when we finally broke out of the chilly spell and our temperatures warmed into the mid-70s — about 10 to 15 degrees above average for that day in April. That was the first glimpse of a West Coast Thermal Trough (WCTT) setting up to the south of Oregon.

Credit: UW Atmospheric Sciences

Fast forward to April 25: we have a more pronounced thermal trough setting up into Oregon, which subsequently helped boost our temperatures into the mid-70s once again. During this time, many communities in Northern California were experiencing record-breaking temperatures. 

Thermal trough
A thermal trough is quite a fascinating setup that is, at times, unique to the West Coast. To put it simply, warm air in the desert southwest will heat up and rise, leaving lower pressure near the surface. This will build to the degree that it will elongate to the north through California, Oregon and even up to Washington. This will stretch right up to the mountain line and promote warmer temperatures for communities that are in the vicinity of this trough.

Additionally, high pressure will usually perch to the east of Washington and Oregon and encourage offshore winds or easterly flow. The combination of this will lead to downslope wind that will cause friction and warming, which will heat us up. Also, depending on the time of the season and the current wildfire conditions, it could cause some major issues for wildfires. 

You can almost bet that if we start seeing temperatures into the triple digits in the summer, it’s likely due to the results of a thermal trough. Although it’s not a predominant force in the spring, it sometimes makes an appearance. 

Copyright 2019 Nexstar Broadcasting, Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

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