(Stock flood image courtesy of CorvallisOregon.gov)

The past week in NW Oregon has seen a fairly active winter pattern pick up. Several wet systems and cool enough airmasses for real Cascade snow last weekend. There was even a little wet snow in the Gorge, under low-level east wind, on Sunday.

All the rain, snow and gray skies have been bad news for Pacific Northwest astronomers, who haven’t been able to view the conjunction of Jupiter and Saturn in the early evening sky.  The conjunction is happening on the night of Monday, December 21 – the winter solstice- when the two planets will be only 0.1 degrees, or one-fifth the moon’s diameter, apart.  This is the closest conjunction that’s visible in the night sky, since the year 1227! ( Read more about this rare event here)

We will probably get an opportunity to see the planets, but it may be a couple days later than ideal.  That’s because we have three different weather systems moving in between tomorrow and Tuesday.  The system on Saturday night and Sunday looks extremely wet for Portland and neighboring counties.  There is a risk of flooding as a result.

Our first system is the most innocuous of the three.  It moves in late tomorrow (Friday) afternoon and evening. The heaviest rain will be from the Olympic Peninsula north to Vancouver Island; it will just be a “normal” damp night for our locale though.

Things calm down briefly during the day Saturday. Then the second system begins in the late afternoon:

But this system is much juicier for NW Oregon and SW Washington. Rainfall should ramp up quickly in the evening hours, and look like THIS by about 4am:

Wow…that’s an atmospheric river if ever there was one!  A narrow band of moisture over the ocean points to the “target” of deluge.  In this case it runs from about Aberdeen to Newport along the coast.

The UW’s WRF model shows rain totals more clearly. Here is the total for 4pm Sat. – 4pm Sun.  Notice that parts of the Coast Range & Cascades receive over 5″ of rain in a 24-hour period!

It’s hard to see how we don’t at least get minor flooding if this comes to fruition.  Luckily the winter thus far hasn’t been exceptionally wet, so our soils and rivers aren’t as vulnerable as they might be if it was February after a soggy December and January.

Another system pushes in Monday afternoon, as colder air begins to push in from the NNW and the atmospheric river buckles.  Here is 4pm Monday:

The WRF-GFS shows the colder air clearly moving in that night.  925mb temps of -1 to -2 C means a freezing level below 2,000 feet!

Beyond that, we go into a drier and clearer pattern as high pressure sets up along the West Coast, the latter half of next week.  By Wednesday afternoon (Christmas Eve Eve), lookie there! – a cool high pressure centered in the Columbia Basin.

This would likely mean clear skies by Wednesday evening, two days after the peak of the conjunction. Jupiter and Saturn will still be less than one moon apart…grab those binoculars or low-power telescope and take a look!

Long-range GFS maps for the upper atmosphere (500mb level) show ridging centered slightly to our west as we go into Christmas, after a quick-moving cold trough Monday night:

So it appears that Christmas Eve and Christmas Day might be clear and dry this year. However, at this time of year any chilly air that gets into the Intermountain West, lingers in the valleys and basins even as the air overhead gets warm again.

But for the next few days, the story is WET.  We don’t know exactly how much, or exactly where it will be centered; atmospheric rivers can be unusually hard to pinpoint the exact bulls-eye of the rain.  Right now Saturday night and Sunday look the wettest for Portland. Maybe Monday night for Seattle. This is the final projected rain total for the next 6 days, by the way:

Snow levels will be high over the weekend in the north Oregon Cascades: probably 6,000 feet or higher with heavy rain on top of the snow on lower mountain slopes.  Not good weather for skiing, sadly….

Prepare for a wet & wild weekend! -Karl