A season of atmospheric rivers and our current rain deficit


PORTLAND, Ore. (KOIN) — We’re cranking away through our month of April and you could say it has been a rather dry month. When we put it into a calendar, you can really get a grasp of how much dry time we’ve been able to stack up this month.

We have had measurable rain since April 10, with a 10-day dry stretch currently in the works. I want to set the tone before we go back and discuss some atmospheric river data, which I think you will find very interesting. If you’re interested in that section of the article, you can scroll down to the category titled “Atmospheric Rivers.”

Right now, we are 10 days from the end of April with only 0.09 inches of measurable rain at the airport. What can we really do in 10 days? We know that we are finishing the current week with no rain, so that means we have about seven days to try to make up for a month’s worth of rain. With a current departure of -1.82 inches, we have some work to do. An atmospheric river would certainly extend a helping hand.

Atmospheric Rivers

An atmospheric river is a long ribbon of moisture, or the horizontal transport of water vapor, that usually packs extratropical or tropical level moisture. This usually results in heavy precipitation and steady rain for the areas that are impacted by an atmospheric river. It’s a term we are familiar with here in the Pacific Northwest because it is the cause of our heavy rains and at times, the reason for local flooding.

A collection of data from the Center for Western Weather and Water Extremes (CW3E) provides a breakdown of our atmospheric rivers that made landfall over the West Coast during the water year 2021 (through March). There is a lot of science behind this, but what you can take away from the graphics below is where those atmospheric rivers landed, their strength (based on a scale that you can find in the graphic) and how many occurred.

SOURCE Center for Western Weather and Water Extremes

According to the information from the CW3E, there were 49 atmospheric rivers that made landfall, 48 of which impacted Oregon. A majority of the atmospheric rivers actually made an impact on the Pacific Northwest and not so much California. A few of those atmospheric rivers were in the extreme category. You can see the date, strength and path of some of those atmospheric rivers going back to October.

I decided to pick out one extreme event, which was the January 11/13 event. You can see the path of that and the landfall at the Oregon Coast. We started that week with 2.51 inches of rain in Portland. We came in with 1.94 inches of rain on January 12, which was a record rain event for us. That was also the stretch of time that we were in the lower 60s. That tells you how warm the air was and where that source of moisture was coming from. I also checked out the December 20 event, which was considered a strong atmospheric river, which brought 1.43 inches to Portland — also a record for the day.

January 12 – 1.94″ Extreme AR Event

We need our spring rain

Our forecast is finally calling for rain come this weekend. Are we expecting an atmospheric river in our neighborhood? Not at this time. However, we do have moisture showing up and that is going to help us get out of this dry rut that we have going on in the region right now. Weather models have rain moving in by Saturday morning. This will start slow and then pick up at times throughout the day. This will be the wettest day that we’ve had this April, which isn’t saying much. It will be by far the wettest weekend that we’ve had all month too.

Rain totals still coming in at nearly 1 inch of rain from the coast to the valley. Some neighborhoods a bit more than others, depending on the setup of this system. It is still a good sign that the moisture will reach central and eastern Oregon. Those areas need the moisture even more than we do west of the Cascades. It still feels like the totals are a bit aggressive, but we will watch the models shape the forecast as we near the weekend. I think we are closer to the half-inch to three-quarters-inch mark around Portland. That will kick us out of the running for the driest April on record. Can we top an inch by the end of the month? Right now, it is actually looking likely. Those aforementioned seven days that we have to work with may turn out to be fruitful.

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