Get your rain jackets ready: Rainy days are on the way


PORTLAND, Ore. (KOIN) – Likely the wettest weather that we’ve seen this fall is going to approach us this week and upcoming weekend.

We’ve had some thunderstorms in September that produced some hefty totals, but this will be more consistent with the ribbons of moisture that arrive in the fall months that we are used to. Below you can see a large mid-latitude cyclone brewing over the Pacific Ocean (what looks like a comma). This is going to be the first of our moisture-heavy systems to impact the Pacific Northwest this week. In fact, some of the moisture that’s attached to this system is coming from Typhoon Hagibis, although I must admit that it’s just a small piece that is hitching a ride.

Incoming System Tuesday Night – Wednesday (Satellite)
Courtesy: College of Dupage Next Generation Weather Lab

Regardless, this is going to be a moderate moisture event that will likely push rain totals around Portland to an inch or more by the time we reach the weekend. Below you can see the precipitable water chart, which shows that ribbon of moisture stretching straight to the PNW by Wednesday morning. That is when the rain will begin, and the showers will be on and off from that point on until the weekend.

What exactly is precipitable water? An easy way to gauge it is the amount of water that can squeeze out from the atmosphere to the surface. As we analyze this graphic, you can see from the key above, about 1″ or more of water will be available. Because of the complicated variables attached to our atmosphere, it doesn’t directly translate to observed precipitation amounts, but the precipitable water is a strong start to gauging a forecast.

That being said, weather models are pushing about .50″ – 1.00″ of rain for Portland by the time we wrap up Wednesday and finish off the week. Check it out:

Locations that will likely see the highest rain totals will be the Oregon coast, the Coast Range, and the Cascades. For you folks east of the Cascades, you will also see some light rain totals from this system, but the moisture will mainly stay to the west.

Lastly, this event that starts Wednesday will just be the door opening to what we may see come this weekend. Right now, I am forecasting measurable rain for four to six days. The weekend could potentially be a very wet few days, but that forecast is always open to change. If you were to make some plans now, I would be wary of some heavy rain on the weekend. Again, we have a glance fo the precipitable water for the weekend (GFS weather model), and this is carrying deeper moisture that our event Wednesday.

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