It’s like autumn, but where’s the rain?

PORTLAND, Ore. (KOIN) — One of the benefits of PNW is being able to enjoy summer without too much rain to hold you back. As summer begins to wind down, we begin to see the rain resume.

The average amount of rain for Portland in September is triple the amount for August. The stubborn summer high pressure is slowly being pushed south and the wavy jet is starting to show more signs of cooler air and more rain. We will be over halfway through the month when we reach the end of the week. Temperatures will start to look more like fall, but the rain has yet to show up. Light amounts reached the valley bottom over the past few days, concluding our 67-day rainless spell in Portland.

When next?

On average from 1991 to 2020, Portland receives about 1.5 inches of rain. This usually comes in the form of a handful of light rain events and then an additional hand of moderate to occasionally heavy showers. You can see how this breaks down in the chart below. On average, we usually have at least one day that picks up a half inch of rain or more.

Last September we ended the month with almost 4 inches of rain. You might be thinking that there’s no way we’ll end up with anything close to that based on our current situation. Well, we actually have more rain around this time than last year. September 2021 had only a trace of rain until September 17. We then had two events that brought over an inch of rain, and the rest is history. That means we can’t rule out a soggy finish this month. We have had above average rainfall in September since 2016.

Will we do it again? At this time, there are no signs of major humidity crossing our corner of the country. If you look at the graph of precipitable water movement below, you’ll see light blue trying to enter the region closer to Saturday, but that alone won’t produce a rain event worth talking about. We may not see anything significant over the weekend. That would bring us closer to the week of September 19, which would give us 11 days to catch over 1.50 inches of rain to reach the 30-year average.


There is certainly good news for our cooler air mass, regardless of the lack of rain. We are seeing lower temperatures and higher relative humidity, allowing for a major overnight wildfire progression. This at least avoids a scenario in which growth deteriorates considerably. The overnight hours allowed the relative humidity around the Central Cascades to drop to at least 80%, then in the afternoon it drops but not to levels that require alerts. The higher the relative humidity, the better it is for transporting moisture to fuels.

Here is an image of what could be calculated as hotspots from the Cedar Creek fire. Progress has been made since the easterly wind helped ignite the wildfires on Friday and Saturday of the weekend. Currently, the Cedar Creek Fire is 92,548 acres in size, growing to what would be the second 100,000-acre “mega” wildfire of the season.

However, with the type of conditions that currently exist, this should help tip the momentum in the other direction as the conditions help moderate fire activity.

About Lucille Thompson

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