Wow!  After an incredible 79-degree afternoon Thursday (75 at PDX), we have flipped the seasonal switch, going from June to February in just three days. Today’s high of 46 at PDX was the coldest April day in 19 years…and we have more of the same for most of the upcoming week!

As I write this at 9pm Sunday, it’s 36 in Orchards with a steady light rain falling.  The air smells like winter and it just has the vibe of a chilly December night out there.

For several days, models have hinted at the possibility of extremely low snow levels in NW Oregon and SW Washington…with a chance of flakes or even accumulation in the metro area. The airport has never recorded measurable April snow in its 82-year history, so naturally I was skeptical at the idea of anything more than a few wet flakes in the air below 500 ft. elevation. But the recent models all seem to be converging on a historic April snow event for the Lower Columbia metro area, with sticking snow that MIGHT give several inches to many lowland locations.

Today, nearly all the models came into agreement on the “perfect” Portland snow scenario:  a strong and wet low-pressure center coming ashore southwest of us, somewhere between Tillamook and Newport, some time around midnight or just after.  That puts us in the northern sector of the system, where southerly flow aloft collides with cooler north or NE flow.  This creates a “deformation band” of heavy precip late tonight into tomorrow morning, and allows the lower airmass to cool a few degrees more than would otherwise be the case.  In the actual winter months, this would all but guarantee a major snowstorm for Portland, with cold east wind getting pulled through the Gorge.

But what about April, with the strong sun and warmer ground working against us?  While I doubt we get the same snow totals we would in January, I’m quite confident we at least see minor accumulations on grass/cars/roofs.

Several factors are coming together just perfectly here:

1.  Low is situated in the “ideal” spot, the central Oregon coast. Quite often these lows in winter will, at the last minute, veer further north or south than originally planned. But the event is basically on as we speak, and the models have only gotten more favorable for snow.

2. It’s already getting cold outside.  This isn’t a scenario where we’re in the 40s and then all of a sudden temps drop as a heavy shower passes, turning the rain to snow. Most of the area is already between 35 and 38, with a balmy 39 at PDX.  We are only a couple degrees away from the change point, and the heaviest precip hasn’t gotten started yet.

3.  Timing of this system is just about perfect for avoiding the “April sun angle problem.” Heaviest precip is from about midnight until late morning tomorrow. If this storm were coming ashore in the middle of the day, accumulation would be significantly tougher due to the daytime warming.

 

MAPS

I’m not going to elaborate, but here are the pressure maps for 11pm on the 18z GFS, 18z ECMWF, 12z GEM and 12z WRF.  (Image credit:  TropicalTidbits.com for the first three, and atmos.uwashington.edu for the WRF)

 

Looks like a solid agreement to me!  We will have light northerly/easterly wind at the surface throughout the night, locking in the cold air near the surface as precip picks up.

I don’t have a snow map for the free version of the Euro, but here are GFS, GEM & WRF snow maps for the period from today through 5pm tomorrow. First, the GFS:

Holy cow! Eight to ten inches for the city and well over a foot in the coast range!  That would be the kind of event we tell our grandkids about someday….IF it happened. Which I don’t think it will.

Now the GEM:

Not quite as crazy for Portland and Vancouver lowlands, but we still pick up 3-4″ while the Coast Range seems a massive 12-18″ dump.

Finally the WRF:

This is probably the most conservative of the three, but only slightly less than the GEM.  It has 2-4″ for the metro.

 

My gut instinct is as follows:

Rain turns to snow at lower elevations across the Metro between 11pm and 2am tonight.  It probably won’t stick right away, but by dawn I imagine it will have cooled to 33-34 degrees, which is enough to start sticking at least on non-road surfaces. The snow will continue until at least 10am tomorrow morning, possibly noon.  By then we could pick up anywhere from 1″ to 4″ in most lowland locations. Car roofs, tabletops and lawns may see somewhat more accumulation than roads in this scenario.

That means a better than even chance for at least some travel problems early tomorrow morning, before the daytime “warmth” begins to melt things back. The more snow we get, the longer the driving conditions will linger into late morning.

The Northern Oregon coast range is likely going to see a historic April snowstorm, with 8-12″ or more over the passes on highways 26 & 8. Not to mention the Cascades, which will get a major help to their languishing spring snowpack.

Places south of Portland will probably see less snow, if any at all.  I think the Gorge at least sees a few inches between Washougal and Hood River, too.

One final note: We will likely stay unseasonably chilly through at least Thursday or Friday this week. The Euro ensembles agree on the cold April trough lingering for the next 5-6 days:

If so, this would be an April cold wave to remember. PDX has never seen more than 4 consecutive sub-50 days in April before. We have a real chance at setting a new record!

Enjoy the weather watching tonight and tomorrow! -Karl