Hard to believe we’re only a few hours away from the first heavy rain event in Portland, since last winter!  It’s been an incredibly dry 6 1/2 months; our spring (March-May) was by far the driest on record, and we’ve seen less than one-tenth an inch of measurable rain at PDX since mid-June!  Many trees are going to be stressed by this unusually dry growing season; I can see that a number of oak and maple trees have already begun to drop brown leaves due to the drought.  In short, this kind of dryness is the kind that Central California endures in one of its “normal” springs or summers.

This rain has been well-advertised for a few days as an atmospheric river; that’s a narrow band of extremely moist air in the troposphere.  Usually they come up from the southwest and have a subtropical influence; but this one is a bit “cooler” in nature than most.  It’s also accompanied by a strong cold front, which will sweep through early tomorrow morning.

Many atmospheric river events are quite stationary and feature a relatively narrow region of extremely heavy rain; for example, Seattle may be inundated with 2-3″ of rainfall while Portland only sees a quarter inch.  But this one will sweep through from northwest to southeast along a cold front.  That means everyone gets generous rain, but nobody gets an extreme deluge.

These are the 84-hour totals for the storm (Friday morning through Monday afternoon).  Practically everyone west of the Cascades gets more than an inch (except the Umpqua-Rogue basins in southwest OR and the ‘Sequim rainshadow’ in NW Washington).  But I don’t see any really crazy totals of 5″ or more, except for a narrow spot along the Olympic Peninsula.  Portland appears set to get nearly 2″ by Monday evening.

Rainfall should arrive at the northern Oregon coast by late evening (8-9pm) and Portland sometime around midnight.  This is going to be a quick, heavy dump of rain; notice that by 5am tomorrow morning there’s already more than an inch in some parts of the metro area!

Saturday should just feature some scattered showers across the region.  A second, slightly weaker push of solid rain moves through late Saturday into early Sunday.  Then more showers during the day Sunday.  That’s how we get to the 2″ total.

Of course the Cascades will see 3″ or more in many places, and possibly even a dusting of snow on the high slopes by Sunday as the colder air moves in!  The 850mb temps over the Cascades by Sunday morning are around +4C.  That probably means snow levels around 7,000 feet, which is still higher than Timberline Lodge.

Hopefully if you have gutters in your home, you cleaned them already!  Also be cautious tomorrow on the roads because we have three months worth of ‘summer funk’ (oil, auto fluids, and similar) on our streets.  That will make things a bit more slippery than they would be after a normal winter rainstorm.

I’m going to try and make it to the basketball court for a few hoops before the rain moves in…enjoy it when it comes!

Karl