Temperature inversion mixes out as next rain system moves in

Weather Blog

PORTLAND, Ore. (KOIN) – Have you noticed how cold it has been the last few mornings? How about the fog that’s been preventing the morning sunrise from gracing the picturesque sky of Portland?

This was the combination of high pressure and a building temperature inversion from the weekend, delivering some of our coldest mornings this fall so far. It felt more like winter than a fall morning.

On Monday, we hit 33 degrees in Portland, which is just a degree from freezing — and enough to get the ground frosty and to warrant heavy sweaters and winter garb. Meanwhile, temperatures dropped to the teens over in Madras and out towards Baker City, with temperatures actually warmer up on the mountain last night. We now have had three mornings in a row where we were in frost territory. To put this into perspective, about a week ago we had a morning low that was at 60 degrees.

I mentioned that the mountain was warmer than the valley last night. This is due to something called a temperature inversion. The high pressure will tame the wind and it allows for the sky to be clear and the heat to escape out to the atmosphere. Eventually, the temperatures reach the point of dew, leading to fog in the valley. If you just focus on the red line below, which represents temperature, you can see that there is a large shift to the right as the line grows up. That is warmer air, compared to where the line starts to at the bottom of the image, which is the surface.

Swipe over to the next set of graphics and you can see the temperatures at 6540′ and 5380′. That is much higher in elevation compared to Portland and the valley. At one point in time, it was 45 degrees up on the ski slopes at midnight. At that point in time, Portland was in the mid-30s. You will notice that the cold air is mixing with the warmer air this afternoon, allowing for the temperatures to increase near the surface today. As of 1 p.m., the temperature was in the upper 30s on the mountain, and down near the base of Mt. Hood Meadows, it was in the mid-40s. That temperature inversion is fixing itself.

Below is a generic idea of what an inversion will do around the state of Oregon and across other valleys in the United States. This type of event can make it tough to forecast the daytime high because if the air doesn’t mix out, you can find a day where the temperature is 10 to 15 degrees below normal. This is more common in the winter. Notice the blue sky coming from Mt. Hood Meadows this afternoon. It was crystal blue in the morning before some high clouds returned to the scene today.

What will mix out the temperature inversion? It usually takes a system to come in to stir conditions up. Allowing for a full day of some sunshine to help warm up the surface is also an option, but we don’t have much of that sun as we near the winter. The wind profile is picking up today, which has allowed for the southern Willamette Valley to mix out of the fog and cold air today. It will work north today as our next disturbance arrives by the dinner hours. The wind is coming out of the southwest aloft, but running out of the south near the surface. This will try to bring some uniformity to the atmosphere.

Look at the temperature discrepancy from Eugene to Portland! The warmer air has found a place to the surface down south. Once an inversion breaks, you watch the temperatures at the surface jump. We are even colder than the communities out in central Oregon this afternoon. Once this breaks, we will have more clouds streaming in for the rain that is going to be here by tonight. We won’t be able to enjoy the warmer air, but this will at least get our morning temperatures back near average.

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