The instigator in this forecast: Onshore flow


Details for Tuesday, August 31, 2021

PORTLAND, Ore. (KOIN) – Well, well, well, we have another 70-degree afternoon coming our way. We’ve had plenty of those over the last few weeks (8 observations in the 70s). Not much of a change in the forecast from Monday, outside of the potential for maybe a few more sun breaks.

I would expect clouds in the forecast for Tuesday morning and on and off in the afternoon, with temps starting in the lower to mid 50s. Onshore flow is the instigator in this forecast, ushering in the cooler air. It is also going to be the reasoning for transporting any wildfire smoke in our region off to the east.

Going back to the morning temperatures, notice that temperatures are going to be cool even out near Pendleton. Some of that heat was still holding across the Lower Columbia Basin 24 hours ago, now much cooler to start the day. You can see that temperatures are rather uniform Tuesday morning, with expectations for a similar outcome in the afternoon as most communities only warm up 15 degrees or so.

This is what the afternoon should feel like. Temperatures expected to hit the lower 60s for the Oregon coast. Another day where highs are floating around 70 degrees for Portland. Temperatures slightly warmer if you have more sunshine and are down south in counties like Marion or Linn.

Will we see an 80 degree temperatures on Tuesday? I think it is actually going to be pretty tough for areas to warm up to that mark. With that area of low pressure moving through and the cooler air from the north right over the top of us, we won’t have much of that summer heat around. The temperatures are going to be below average Tuesday for most locations. Hermiston has a chance, but you folks won’t spend a lot of time in the 80s if you happen to get there. It’s a pleasant day to get some work done outside.

I mentioned that the area of low pressure associated with this trough will be moving slowly east through the day. The image below gives an idea of that trough, or dip in the jet stream, setting up directly over the Pacific Northwest (PNW). Height contours down to 558 over Washington, as colder air is located in this part of the country. Cold air can be hard to budge, so it will take another 24 hours before we see this shift to the east. Behind this trough is a weak ridge which will allow for warmer air to fill in by Thursday. If you like a small taste of fall, this is your type of forecast.

Sometimes these events bring in more moisture around here. We can’t rule out a few spotty showers, but most should remain dry Tuesday.

We will talk more about our local rain chance in a minute. First, check out the rain total forecast through the day for the United States as a whole. Rain totals are still astounding around the southeast on Tuesday. Largest totals still in areas of Mississippi and Tennessee today. That heavy rain will start to creep into Kentucky and portions of southern Illinois and Indiana.

This is the moisture coming in from Hurricane Ida, which is traveling slowly to the north and east. A smattering of green in the PNW, by night (9:30 p.m.), but we aren’t looking at much. Any sort of rain that comes down Tuesday will most likely be up in Washington or out near the Oregon coast.

Here is a look at the rain threat for the morning hours and heading into the evening. It’s not much, right?

Overall, scarce around the state of Oregon. There is a spot here or there where we may have a shower develop in the early hours. I would say the chance is highest around the northwest Oregon coast and into Wahkiakum County in Washington. Low clouds aren’t out of the question, but the rain definitely is in question. It’s easy to tell that the rain is in more abundance to the north, especially close to the core of the area of low pressure in Canada.

If you shift the graphic over to the afternoon, the rain threat is nearly gone. I would say there is probably less than a 10% chance for rain around the region by afternoon. This doesn’t mean the clouds are going to be totally gone, but the moisture is sure limited. A northwest flow still continues into the afternoon, but it won’t be that strong around the valley. Again, a nice day to go for a bike ride. More sunshine to the south, with clouds likely around Portland.

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