I had other plans for this coming week.

I was hoping to get some serious research done on the history of supernovas, and discuss the current situation with the red giant star Betelgeuse in Orion.  (For those who don’t know, Betelgeuse has dimmed dramatically over the past few months, and is now a full magnitude fainter than it “normally” appears.)  The dimming has provoked awe-inspired speculations that the star might be about to explode – and if it did, it would be one of the most spectacular events in the history of human astronomy.

Unfortunately for that, real-world weather has intervened.

We have a serious turn toward colder weather in the Pacific Northwest on tap for the next 7-10 days, possibly longer.  Depending on which model map you have the most faith in, it could be the coldest region-wide airmass since at least December 2013.  For the past few days, the weather geek community has gone through its “pre-event rituals”:  half of us are busily salivating at the long-range maps showing cold and snow, while the other half are cautious not to engage in too much ‘wishcasting’.  But now it appears the forecast is coming into clearer vision:

 

WHAT WE KNOW:

The Pacific Northwest is in for a period of colder-than-normal weather.  Technically the shift to cold has already begun, but the “really” cold air won’t arrive until early next week.

 WHAT WE DON’T KNOW YET:

How long does the cold air stick around?  Just how cold does it get?  How many snowstorms & ice storms will happen west of the Cascades, and where will they hit?

 

THE BIG PICTURE:

December and early January have been dominated by upper-level ridging along the West Coast.  We’ve also had a couple atmospheric river events brush by to the north, bring warm & cloudy conditions to Western Oregon.  Snow pack in the mountains was doing terrible, at least until cooler systems began moving in a few days ago.

Now the upper pattern is shifting.  Here are the 500mb maps for last Tuesday.  Notice the warmer ‘anomalies’ over the Western U.S.  We were still well above average two days ago, with widespread upper 50s in the Portland area.

Now here’s what it looks like as of 4am this morning Thursday.  A chilly upper-level trough has formed over the entire Western U.S., bring cooler temps and much lower snow levels to our region.  Cooler storms are able to produce a big pile-up of snow in the mountains, something we haven’t seen yet this season.  Meanwhile, a warm upper-level ridge is forming over the Eastern Pacific, which is almost always a sign of colder weather in our region:

By next Tuesday morning however, the cold heights over the Northwestern U.S. and Western Canada have deepened.  Meanwhile, Alaska is looking warm and ridgy.  This isn’t a “picture perfect” image of the amplified jet stream, but you get the basic idea:  The jet stream will swing north over the Pacific and southern Alaska, then dive southward along the West Coast.  This pattern is what Oregon and Washington want to see, if we’re craving cold and snowy weather.

Closer to home and lower in the atmosphere, we can track the arrival of the cold air with the help of the 925mb maps from the UW’s nifty WRF-GFS model system.  First, this morning:  We already have cool air in place, with snow levels down to 2,000 feet or lower.  The 925mb elevation is slightly below 2,500′, so any area in the white or blue is below freezing at that level.

The real cold air starts to move into the region Sunday afternoon, though.  This low pressure center is the leading edge of an arctic airmass diving down from Canada.  Some of that cold air is expected to slide into the Columbia Basin, but some of it will spill out over the eastern Pacific.  This “split” in the arctic airflow can be key to generating snowstorms in Western Oregon & Washington; more on that in a moment.

By Tuesday morning, the frigid arctic air is entrenched in the Columbia Basin and Gorge.  Meanwhile, the cold air spilling out over the ocean tends to spin up low-pressure storms, which can interact with the cold east wind at the surface, causing snowstorms.  One of these “secondary boundary lows” is being shown for Monday night and early Tuesday; if this map holds then Portland and most of NW Oregon would get a significant snowstorm then:

 

 

It’s important to note that 850mb temperature maps don’t always tell the whole story when it comes to conditions in the lowlands.  They do an especially lousy job for Portland when an east wind is blowing, as the colder air is being pulled in closer to the surface, often well below the 850mb elevation.

Two days after that….a “primary” storm from the central Pacific may try to come in, according to the GFS model.  Right now it appears that a 2nd snowstorm is in the works…but bear in mind that this is still a full week away.  Seven days is way too far in advance, to track the details on an incoming storm with cold air in place.

(image credit of TropicalTidbits.com)

Snow totals over the next 7-8 days should be taken with plenty of grains of salt.  However, it is interesting to note that the entire NW quadrant of Oregon looks snowy at the end of the period.  Not only Portland and the Willamette Valley, but the coast too.  And of course, the high Cascades will get dumped on – easily 3 to 5 feet over the next week!

What happens beyond the middle of next week is still a bit uncertain.  Right now it appears the storm track will stay to our south a bit, which increases the likelihood of a prolonged cold episode with more snow & ice further down the road.  Again, however, the long range is pure speculation.  We can’t tell the exact track of storms even 3 or 4 days in advance; and in this pattern, the track of those storms can dictate how soon we break out of the cold.

One thing that sheds some light on the long range are the “spaghetti maps” (model ensembles).  Today’s chart for Portland clearly shows the initial plunge of cold air late Monday and Tuesday.  There is less certainty as to how long it stays cold, though – notice how all the squiggly lines start to spread out more beginning around the 15th-16th.  This indicates that we don’t know just how persistent the low-level cold airmass will be:

To sum everything up neatly:

1. We definitely have a turn to colder weather on the way. The coldest air of the season is expected to begin moving in late Sunday, and by Monday night a bitter east wind should be blowing through the Gorge.

 

2. High temps next Monday-Wednesday appear to be in the 30s for the Portland metro area – not historic, but cold enough to see snow. Models were originally going bat-crazy with the cold and showing highs in the 20s, maybe even teens for a day.  It’s a good thing forecast mets didn’t take that bait, because it probably won’t be quite that cold.  That said, we could still see one or maybe two days only in the 20s, if we get a solid snowstorm on top of strong east wind.

 

3. The Columbia River Gorge definitely gets snowstorms (plural) out of this turn in the weather, and probably some ice before it’s all over with. We don’t know whether it will be 8″ or 24″ along I-84.  But it appears that Monday could be the beginning of a long stretch of difficult driving conditions out there.  Temps will also be locked into the teens and 20s most of next week, maybe even single digits at some point in a few spots.

 

4. The Cascades will, obviously, be buried in snow. Expect anywhere from 3 to 5 feet between now and next Thursday.  Great news for winter sport lovers!

 

5. We STILL don’t know the details as to where, when, and how much snow or ice Western Oregon and the Portland-Vancouver area will see. But it WILL be cold enough for wintry precip from late Monday through at least Wednesday, probably longer.  It’s just a question of moisture!

 

6. Anything beyond next Thursday is still a big wild card. It’s important to note that low-level cold can often persist for some time after the middle levels of the atmosphere have moderated.  Stubborn cold air in the valleys, basins and Gorge can also increase the chances of ice or freezing rain further down the pike.

 

Whew…this was a long one!  I usually don’t go much over 1,000 words on these blogs.  But there’s a lot to discuss about the overall pattern shift this time.  Please note that I don’t intend to post any more lengthy write-ups next week, at least not about cold and snow.  You can follow me as things unfold on my Facebook page; it’s my go-to standard for quick updates during a developing event.

In any case, enjoy the real taste of winter coming next week!  -Karl