3rd cold spell of winter arrives late next week; could we double the 0.2″ PDX seasonal snow total?

The headline, of course, includes a bit of snark. Yes, we haven’t even picked up 1/4″ of snow so far this winter in Portland. Some dustings here and there in the bulk of the metro area, but that’s about it.

Could we FINALLY see real snow next week? That remains to be seen, and I’m not too excited about it yet. At this point, there is a great enough chance snow could disrupt your life next Wednesday that we’re calling it a POSSIBLE First Alert Weather Day. This is exactly the reason we changed our branding; to draw attention to days that really matter.

A FEW KEY POINTS

1) It was 3 weeks ago that a cold modified arctic airmass plunged out of Canada, then surged westward through the Gorge and over the Cascades. We had a very windy (but sunny!) day with temperatures only in the 30s. That’s what I see again next Thursday and Friday, but wind doesn’t look as strong this time around.

2) Expect hard nighttime freezes for 1-2 nights late next week. But not as cold as what we’ve seen earlier in the winter. Lows in the lower 20s in Portland, upper teens outlying areas.

3) We do not yet know if the transition on Wednesday (from mild/wet to cold/dry) will include widespread snowfall west of the Cascades. That said, there are strong hints that this WON’T be a big snow producer. Yes, once again we may have spotty snow accumulations. But it’s still 4-5 days out in time.

4) Anyone, or web site, that tells you they/it knows exactly when it’ll snow, how much, or to what elevation, is just guessing. We have enough trouble getting this spotty shower stuff right just one day ahead of time, let alone 4-5 days out.

5) Cold spells in late February don’t last long, unless another wave of cold air comes south. Arctic airmasses modify quickly as the sun angle increases. Notice that Portland has only had ONE day where the high temperature stays at/below freezing beyond February 22nd. That’s in 83 years of weather records. It was a cold/snowy east wind day on March 3rd, 1960. That tells you that it’s ALMOST impossible to get a “Snowpocalypse” this late in the season. ALMOST impossible to shut down the entire metro area for a day. Several of the dates below were under sunny skies.

WHAT I’M SEEING FOR NEXT WEEK

We’ve got another cold upper-level trough sliding down the east side of the persistent eastern Pacific upper-level high. Here’s Sunday...lots of clouds streaming by but not much rain in this pattern. It’ll be a gray and just slightly drippy weekend.

But the ridge will be “backing up”; that allows cold modified arctic air to surge south through B.C. by Tuesday. At this point we’ve still got cool onshore flow = no snow to the lowest elevations Tuesday. But a good foot of fresh snow in the mountains. Two great Tuesdays in a row up there!

Then by Thursday the trough has dug all the way down into California

By the way, Northern/central California will see 850mb temps down around -6 or -7! With precipitation, that’s sticking snow on higher hills in the San Francisco Bay and Central Valley. Temps could go down into the -3 or -4 range over Los Angeles. Chilly for them.

One difference from earlier this week? We’ve got a cold east wind arriving at some point Wednesday and sticking around through Thursday as the arctic air arrives. It won’t be a raging east wind like we sometimes get, and it’ll be gone by Friday. But enough that wind chill will be an issue later Wednesday through Thursday.

So the snow forecast is tough of course. It’s not a surprise that the GFS model has been much snowier than others the past 24-48 hours; that’s not unusual. But if I had $1 for each time it does that...ugh. Just today I asked a meteorologist friend “when is the last time in the past 2-3 years that a snowy GFS has panned out?”. Crickets. In this case it’s because that model forms a 2nd surface low that slides down through the region, or just offshore, late Wednesday or early Thursday. The Euro and GEM models just continue pushing the cold air south and don’t form that 2nd low, or at least it’s much weaker. Basically if a surface low forms and SLOWLY moves by while the cold air is arriving...THAT is how we can get “sleddable” snow. So I’ll be watching that closely over the next 3 days. For now our 7 Day forecast assumes we don’t get a secondary burst of precipitation; we just go from showers Tuesday to widely scattered showers (mainly snow) Wednesday and then dry out late Wednesday.

Notice less than 20% of the ECMWF ensemble members (from midday run) produce real snow (2″+) in Portland.

Even drier at Salem...

Maybe we’ll see something like this...leading into the late February 2011 cold spell scattered clusters of snow showers developed. The next day, under the cold sunshine and east wind, you could see white ground in southern Clark county, Clackamas county, and southern end of the Willamette Valley. This is February 25, 2011. On this day PDX only made it to 34 degrees. It clouded up the next day under a calm wind and we only touched 33.

Of course I’ve got a file for that cold spell, I know, hardcore...Notice how quickly the cold spell ended on the 27th that year. A bit of south wind and the cold air was gone.

That year we had an 850mb temperature bottom out around -10 over Portland. Right now, ensemble averages are pointing to -10 or -11 later next week. I’m assuming they will warm slightly as we get closer; that happened in late January.

This is one more reason I think a high temperature close to freezing is unlikely UNLESS several inches of snow fall. You can add 2-5 degrees to these surface temperature forecasts from the Euro...