I almost never use the term “freak” while describing weather events, because media can easily overuse the word. But one week ago we saw a freak mid-April snowstorm in far NW Oregon and SW Washington. I use the term because, just like the heatwave last June, the setup is unlikely to be repeated again in my career (or lifetime?). The reason for this post (a week later) is that I want a good record of the event. I was out of the area. In fact I was on the other side of the planet, somewhere between Greece and Montenegro on a large ship. With little/no internet. Typically April is a “safe month” for a chief meteorologist to take a vacation. I hit the wrong week this year.
April is generally considered a relatively mild month weather-wise in our region. Cold showers mixed with warm sunny spells is normal. Sometimes those cold showers can contain snow pellets, hail, or even snow mixed in. At 1,000′ in the hills it’s not even unusual to get snow in April. I’ve had brief morning snowfalls a few times at my home the past 18 years in that location. There was nothing really unusual about Tuesday and beyond last week – lots of cold showers with thunder and hail. We have had some very mild Aprils lately, in fact April has been turning warmer as our climate warms. Notice how often we used to have sub-50 degree highs in the past compared to now.
But in the past 8 days we have reverted to a chilly pattern similar to what we’ve have seen in the past. We hit 75 the day I flew out of PDX. But then things went downhill around the 10th
That snowstorm one week ago goes WAY beyond a normal chilly April weather setup though. It was the perfect “snow storm” pattern that would have given us 10-15″ snow in the metro area if it would have been December-early February. Maybe most interesting is that it was almost perfectly forecast by models, yet (just like June heat wave) meteorologists were doubtful that such an extreme event could occur so late in the season and downplayed snowfall totals. I would have gone for lighter snow just like local mets did that Sunday ahead of time.
500mb forecast charts show cold air dropping south as a cold upper-level low sweeps across the area Sunday night through Monday night. 1st image is Sunday afternoon, next is Monday 8am. All these charts are from the WRF-GFS model.
By Tuesday another cold trough was swinging through, then the last of the really cold stuff moved by Wednesday
What has happening down below? The approaching trough spun up a very deep surface low. Notice on Sunday it’s about 1014 millibars (mb)
It crosses the coastline Sunday night around Lincoln City, then is east of Madras by 5am Monday. This wasn’t about “cold east wind” like wintertime. It was about very heavy precipitation dragging the snow level down lower than it would typically be during an event like this in April. At this points a breezy northwest wind had picked up across the snow storm area.
So at this point it’s down to around 993 mb. That’s a 20 mb. drop in 24 hours – technically a bomb cyclone passed right across Oregon last Monday! It’s rare to see a low deepening AFTER it crosses the coastline. By 11am Monday it’s way over in NE Oregon and you can see the dynamic cooling is ending over NW Oregon and SW Washington. Most lower elevation spots are changing back to rain at this point.
Those are some serious atmospheric dynamics (lifting and atmospheric cooling) going on around such a quickly deepening low pressure area. And it’s well known that the area just north of a surface low can give some great precipitation and snow totals. Take a look at the precipitation forecast from Sunday evening through Monday evening. 1 to 1.5″ likely, a big April soaker!
Now look at the snowfall forecast from the WRF-GFS from Sunday evening. Just about nothing south of Wilsonville or Chehalem Mtn. A foot is possible in foothill locations (and West Hills), 5-8″ in parts of Clark County and hills of Clackamas/Multomah counties. And 2″ in lowest elevations along Columbia River in middle of metro area.
To make a forecast of 2″ snow in Portland in mid-April, when that hasn’t happened anytime after first few days of March? I think I would have said Trace-1″ and left it at that. Quite a good performance by models!
The final snow totals…
- This was a historic event for our area. I’d say it’s similar to getting a couple of 90 degree days in mid-October. Or maybe similar to a 3″ snowfall in the city for Halloween? A freak snow storm for sure!
- 1.6″ fell at NWS Portland, and 2.0″ downtown, both the latest on record. 0.3″ fell the next day (officially), but that was mainly graupel and temperature didn’t fall to freezing. We will see if that number remains.
- The weight of extremely wet snow (falling at 33-34 degrees) approaches that of an ice storm. Lots of trees either snapped off or fell over under the weight. I lost all or part of two ornamental trees, yet they’ve been fine through several 1″ ice storms. No leaves on those trees either.
- It’s unlikely this will happen again in our lifetimes, or at least in the next 20 years. Everything had to work out PERFECTLY to get the heavy/wet snow down to sea level. Timing + precipitation intensity. Similar, but not as extreme, events occurred in April 1963 and April 1936.
- If it had occurred 6-10 hours later, it’s likely no snow would have stuck in the lowest elevations. Notice that Portland officially picked up 1.5-2.0″ snow, out of 1.40″ precipitation that day. It just barely worked out in the lowlands. The high later that day was 51 degrees! If we had been 25-30 degrees with this setup, a solid 10-18″ could have fallen in the entire metro area. The track of the low was similar to the January 1998 snow storm.
- Out of all that drama, we didn’t get a freezing temperature in Portland. Latest frost is still March 10th.
ABOUT THAT FORK
Each February, I have people asking me if winter is “over” in February. And each year they ask if it’s okay to turn on outside water spigots, take off the chains, or if we’re done with the chance for an all-day snow event. So about 15 years ago I started “forking winter”. Twice during that time I’ve been burned. In late March 2012 a heavy/wet snowfall of 1-8″ occurred in parts of the Willamette Valley. Then this year we had a real snowstorm 6 weeks after I pulled out the fork. To be clear, March and the first week of April was very mild this year and we are definitely in spring. Yes, it can snow in spring every 10-30 years in the lowlands. Just like it can get hot in fall or spring too, but that doesn’t mean it’s summer. I’m guessing the same people thinking I should drop the fork thing will be asking next February 20th… “Is winter over?” We will see!
Chief Meteorologist Mark Nelsen