A brief weather update and review of the surprisingly sharp freeze in late November. This freeze was memorable for several reasons. It didn’t break any records by a long shot but it was prolonged and in the realm of global warming sufficient to be of note. Portland recorded 6 and as of this writing 6 days below freezing in a row. After the warmest October on record this is a departure. Portland dropped to 24º on two non-consecutive nights but the real story was in the outlying areas. The NOAA hinted ONCE that temperatures in the most wind sheltered areas MIGHT see upper teens. That was more than true in several locations. Highlighting this period of weather was a rex block and N. N-easterly flow. The offshore flow that dominated out of the Gorge was not especially strong but it did have an effect on the ultimate lows. When the wind stopped the temperatures dropped.
Those chilly hinterlands
Remember that cold air sinks. That is counter intuitive to what you hear about elevation. But in a stable, cold, dry regime the lower you are the lower you go. So the wind sheltered valley areas with the least wind had the lowest temperatures. Eugene was the clear winner with four days in a row with minimums in the upper teens. (19º, 18º, 17º, 18ºF). Thats impressive even for the coldest time of the year. What was striking was that on all but the final day of the cold snap the daytime temperatures recovered to the low 40’s. This is unusual for an inversion. Portland recovered to 50º+ on two days- unusual. Other calm low elevation lows were 19ºF at Hillsboro, Corvallis, and Vancouver (which often records exaggerated lows so that recording station is suspect), and 20º on two nights in Aurora and Salem. Those are all official reporting stations. You might have noticed much different values in your own back yard. I’ll get to that.
Wind is a mixer
Portland was especially prone to mixing from east winds. These winds when they surfaced kept the ultimate lows from plunging. Many neighborhoods where the east wind prevailed failed to record a hard freeze while others dropped precipitously. Location, location, location. Troutdale where the wind was steady 20-30mph gusting to 35-40mph for 5 days straight had only two nights below freezing. Match that to PDX just 8 miles to the west with 6 nights. Wind mixes things up. Also in this episode the calm conditions really allowed cold air to settle thats why reporting stations in the bottom of the valley recorded such cold temperatures. In the end a weak low from the SW intruded with a dying front on the final day and gave weak spats of sleet and freezing rain in spots.
Compare this to the past- No comparison
The last great November freeze was unarguably 1985. For nearly two weeks arctic air held sway with lows ultimately dipping to 13ºF at PDX and throughout the Willamette Valley. High temperatures were below freezing for 9 days and snow fall occurred early and then spectacularly late in the freeze. This freeze was more representative of what you would find in December-February. And it wasn’t unprecedented. It happened in 1952 and 1955, 1978. But there has been nothing approaching it since. (Global warming?). All four of these freezes were of note not just for their record breaking early occurrence but were ruinous to agriculture (and gardens). Even more surprising was that arctic air never really re-0ccurred in two of these winters. 1978 was an outlier- arctic air came back later in the year and 1985 was a cold year but after November nothing of arctic note really impinged. And its worth noting that in many years when the lowest temperature happens in November it isn’t exceeded later in the year. Thats it. Thats winter. Will it be this way this El Nino winter? Time will tell.
The next several weeks show a zonal flow with an active jet stream and mild and wet conditions. Not necessarily a classic El Nino pattern but we haven’t even reached winter yet so we have to wait and see. I’ll update this blog as weather and my ideas for blogs hit me. Until then, clean up the weeping snot of your bananas and kiss those annuals good bye. heh.
Have a great Holiday season.