Chilly October = Pass Elevation Snow & First Frost In Lowlands

October 10, 2021

9pm Sunday…

We’ve all noticed the change. After 6 months of warmer than normal temperatures, October is running well below normal temperature-wise. For the first 10 days. During the last 10 days of September we saw 3 days in the 80s in Portland; it’s hard to believe that less than three weeks ago we saw highs around 90 in the metro area! Of course it’s normal to see temperatures drop off in October; no other month changes as dramatically in the Portland area. But going from above normal temps to below normal has accentuated that change this year. You can see that we are a bit alone in the Pacific Northwest, the rest of the country has seen a very warm start to fall.

The coldest airmass of the season so far has arrived this evening behind last night’s cold front. For the first time since spring, it’s snowing hard and sticking at Government Camp

Sticking snow should fall about 1,000′ lower than this elevation, down to around 3,000′ by sunrise. I expect 1-2″ at Govy and 3-5″ higher up at Timberline (6,000′). We’re in a showery airmass that dries out by sunrise, thus not expecting a ton of snow.

The reason it’s been cool? A weather pattern we saw this past spring; not especially wet, but cool. Take a look at the current 500mb heights and the anomaly (blue colors). Well below normal for October 10th as a cool trough or “dip” in the jet stream is passing overhead

A warm/strong upper-level ridge is out in the eastern Pacific and COULD move over us the next few days. But instead another cool trough flattens that ridge, and drops right into the Pacific Northwest late Tuesday and Wednesday. That’s another round of (light) rain and (light) mountain snow. Models say the following few troughs coming eastward on the jet stream will dig farther west; putting us in a milder southwesterly flow late this week and beyond. Wednesday the 20th…

So temperatures may turn a bit warmer the end of this week and beyond. As for rain, I don’t see a tremendously wet pattern over the next week, just showery at times. The same ECMWF ensembles show the near to below normal precipitation the next 7 days (brown = drier than average)

Then significantly wetter NEXT week; northern/central California may get its first soaking since late last winter

By the way, October 2019 was very cool and a bit dry. So there’s no reason to think that a cool October leads to a cool winter. Winter 2019-20 was mild & boring (no snow/ice/windstorms) with only a dusting of snow in mid-March in the lowlands.

Tomorrow night we get lower relative humidity, clear sky, calm wind, & a leftover cool airmass. That combo SHOULD give us the first frost in some outlying areas. No, not a big “pipe-busting” overnight freeze, just cold enough to end the growing season in spots. I’m going 36 in the city, which means some upper 20s in the coldest outlying areas. The NWS has just issued a Freeze Watch for tomorrow night for this reason for many areas west of the Cascades.

That’s it for now, I won’t be posting as regularly since I’ve been busier and we’re short two weather people. I’ll try to get it done once per week and more often if something dramatic happens.

Within the next two weeks I’ll get my winter thoughts together as well. We know it’s La Niña and a cold PDO this season. Hmmm…we will see.

Chief Meteorologist Mark Nelsen