Three different patterns, three different snow possibilities

[Mallow]

So our last "snow event" wasn't that great. Certainly worse than I expected, though I did get my first (and second) measurable snowfall of the winter. 1.0", followed by 0.1"... neither of which accumulated on the roads. But the accumulations on everything else, followed by bright sunshine, made for some pretty scenes, even so.

But the past is the past. We have an interesting several days ahead of us.

The GFS continues to trend colder and colder with each run, in general. The 18z is no exception. At face value, the 18z GFS gives us three separate snow events.

Of course, for all three of them, very small changes would mean no snow. And that’s more than likely what will happen. But it’s interesting to see it nonetheless.

What’s especially interesting/funny about this pattern is that each potential “snow event” is a different type. Here in Portland, we may have about 5 total possible ways of getting snow… and three of them are all lined up in a row.

1st event: Sunday morning (20% chance of accumulating snow)

Very cold, dry air is already in place. The high today in Portland will likely be at or below 35F, which is incredible for a sunny day this late in the season (unprecedented, even?). Another low dropping south along the Canadian coast, not dissimilar to the one that gave us our last bout of snow (though generally further north and with a longer overwater trajectory, hence a warmer maritime airmass) will send moisture into the Portland area late Saturday night and into Sunday. 850s are not that low (-3C, eventually dropping to -6C), but with the cold, dry air currently in place, southerlies won’t scour out the cold air immediately. Therefore, it’s possible we could see a snow event to start out, before onshore flow scours out the cold continental air and replaces it with “cool” maritime air.

What could go wrong

Precipitation. Though the operational models generally bring showers in starting Saturday night, the mesoscale models show what’s more likely to happen. There are two issues working against moisture in the Valley while the cold air is still in place. First of all, the airmass is very dry, so showers could be largely virga to start. Second, and more importantly, the trajectory of the showers heavily favors Coast Range shadowing, so it’s likely few showers will even make it to the Valley in the first place… at least until later Sunday, after the cold air is scoured out.

2nd event: Monday (30% chance of accumulating snow)

A maritime-polar airmass will have taken over the entire northwest by Monday, with 850mb temperatures around -4C to -6C by Monday. A heavy baroclinic band is forecast to move slowly from north to south overnight Sunday night and into Monday. Pressure gradients will gradually slacken as well, weakening onshore flow. The combination of a cool airmass, heavy, steady precipitation, and weaker onshore flow could mean a surprise heavy, wet snowfall for the region.

What could go wrong

Temperatures. The baroclinic band could be too weak to drop temperatures in the lower levels, or onshore flow could be too strong to allow temperatures at the lowest levels to drop off.

3rd event: Wednesday (30% chance of accumulating snow)

A stronger low with tropical connections will swing in early Wednesday morning, making landfall somewhere along the Pacific Northwest Coast (could be anywhere from Oregon to Vancouver Island). This low looks a lot like a “traditional” overrunning snowstorm. The airmass ahead of it is chilly, though not as cold as I’d like to see it, especially this late in the season. The low could either come in to our north (which most models have shown) and threaten with a snow-to-rain scenario, or it could come in to our south, as the 18z GFS now shows. In that case, it could be an all-snow scenario. There’s lots of moisture available, so that shouldn’t be an issue.

What could go wrong

Temperatures. First of all, 850s ahead of the low will likely be in the -3C to -6C range, even east of the Cascades. However, very cold air is bottled up just to the north in southwestern Canada, and gradients favor pulling that south at the lower levels (strong north-south gradient east of the Cascades, decent east-west gradient through the Gorge). Also, if the low comes in to our north, 850mb temperatures may be near or above 0C in the Portland area. This would likely mean a possibly quick transition to rain, with a slight chance of freezing rain (I don’t really think it’ll be cold enough east of the Cascades to support freezing rain in that case).

EDIT: After a little discussion with folks on Western, I have reconsidered the probabilities I gave... I'd probably adjust them to 25%, 20%, and 20%, respectively.

[Chicago WX]

Very nice and informative write up Mallow. Good luck with the possible snow out there.

[Mallow]

Thanks, Chicago WX!

00z GFS is as good or better.

The first two events are extremely borderline. The third (Wednesday's) wouldn't be if it came in as the 00z shows, but the track of the low is key. A shift of 50 miles north and it would be snow-to-rain or just rain.

The 00z GFS shows two more lows similar to Wednesday's before hr 180, for potentially two more snow events... but those are surface features beyond day 5 that likely will change significantly with each run.

[Mallow]

FWIW, the GFS has been generally trending in the wrong direction again today.

[Mallow]

Okay, after the 12z GFS, I'm out. Not gonna happen.

[Mallow]

First "event", as expected, didn't have enough moisture.

Second "event" is tomorrow morning. Most likely it'll be about 3F too warm, but you never know.

Third "event" is now a windstorm. Zero chance for snow.

Gonna be snowy on Mt Hood, though!