SEATTLE — A Pinpoint Alert Day is up for Friday as an unusual “atmospheric river” brings a stream of rich moisture across the Pacific right into Western Washington.
This is a fairly common feature in our fall weather, but happens this time of year perhaps once in a decade.
The last time we had an inch or more in the first part of August in Seattle (through Aug. 16) was indeed ten years ago — in 2015!
And as compared to fall or winter atmospheric rivers, which can cause all kinds of problems like flooding and landslides, this one will be generally beneficial.
However, as we get into Friday afternoon and especially Friday evening, the weather will turn a bit more disruptive across the area.
Rain will increase in intensity from midday Friday through Friday night. While the rainfall totals will be impressive — one half to more than one inch or a little more across the lowlands — most soils will be able to handle that rain if it is spread out evenly.
But new data show that some of the downpours Friday afternoon and Friday night could be heavy enough for the dry, parched soil to not absorb the moisture quickly enough, leading to some sudden runoff.
This could be seen in the form of isolated spots of urban flooding, or standing water on roads and intersections.
It will be breezy on Friday with some wind gusts mainly near the water in the 20-30 mph range in gusts. Unusual for August, but should be of little significant impact. However, it will feel like fall!
In the mountains, one to four inches of rain will fall which is excellent news to help extinguish our local fires. There’s a slight potential for rainfall to cause flooding near burn scars, but most areas will just get benefit from this.
Some west facing slopes could get even more than five inches from this event. Rivers will rise to unusual or even record levels for August, but no flooding is expected.
Rain will end and taper to spotty showers Saturday morning, with showers continuing to taper Saturday evening.
We’ll be mainly dry and mild from Sunday through early next week.
-Chief Meteorologist Morgan Palmer
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