Days and days
If someone had told me that the first 17 days of January in Portland would be below freezing I would have laughed- recoiled in horror but laughed. Well, woo wee we just did that and some of the lows were pretty far down the slope as well. In most years the repeated arctic episodes we experienced in December would qualify as a “real winter”. But it just kept coming. I had a feeling we would have a real winter, but what we have been through so far exceeds my wildest presumptions. In the past we’ve had what I call Columbia Gorge Winters – winters dominated by easterly flow with arctic air entrenched east of the mountains- our source of frigidity. The colossal snow and ice winters of 1992/1993, 1979/1980, 1968/1969, 1949/ 1950 are examples of Columbia Gorge winters. That is there were unbroken repeated intrusions of air from the east and repeated snow and ice events. This winter will go down in history (and its but exactly half over) as one of those winters. Notice that there are about 20 years separating those events. So we are looking at a winter we can expect once in 20 years. And it has been a LONG time since we last experienced one. Earlier in the 20th Century they were more frequent which is interesting to note.
December to remember
In a normal year the events of December would classify as a real winter. Nineteen days below freezing, two snowfalls, two ice events and an average low of 32.2º. Temperatures way below normal following what was our mildest November, it was something of a shock. And for those of us in N and NE Portland there was a very damaging ice storm. I’m in north Portland and the ice was most damaging there and to the east in town. Other parts of the metro area were spared the 3/4″ of ice that caused so much localized destruction. We can expect at least one Columbia Gorge event each year in Portland, and a truly arctic event every three to four years. If I was a betting man I would have said that December was the sum total of winter 2016/17.
Over achiever- January 2017
From the first through the 17th we had lows below freezing and our warmest high was a meager 41ºF. And a surprising six days had lows in the teens at the airport. Over a thirty year period we average 2 days below 20ºF per year- and that usually happens with regularity every three to four years. We doubled that so far this January. The snowfall of January 10/11 was the piece de resistance and never have I seen a low pressure target the metro area with such an intense amount of snow in such a short period of time. I even had thundersnow at my house- I saw a flash and then heard the muted rumbling. I dismissed it as a blown transformer at first but when the rumble kept going I was really shocked. Its a snowfall that exceeded snowmageddon of 2008 because it was just one condensed event and 2008 was a series of three storms. If you throw out 2008 it was the most snow to fall in the city since 1980. A long time. In most parts of the city it was the most snow in one event since January 1943. Whoa.
Snow and cold- a reinforcing pair
Arctic air that flowed into the Columbia basin/eastern Oregon had a continuous reinforcing source with a jet stream from the north. Subzero temperatures there provided a source of the cold immensely dry air that has fueled the weather this month. Remember that low pressure to the south and west of Portland and high pressure east of the mountains provides a gradient that sucks air through the Gorge and it enters the the Portland Metro area and then due to the coriolis effect curves to the left and fills the Willamette Valley – usually as far south as Eugene. This year it was not only the gorge as the primary conduit of air- early in the month a large push of arctic air crossed the mountains all the way into Southern Oregon- there Medford recorded its most snow in one event (8.1″) that hadn’t occurred since 1919 (1919!!!). Substantial snow accumulated on the southern Oregon Coast in locations which hadn’t seen it since 1990. Truly remarkable. And this set the stage for the arctic events that were to come. Record breaking snowfall in the Columbia basin and Central Oregon meant that temperatures were not modified between each reinforcing wave of arctic air. Snow covered land refrigerates- and it enhances the evacuation of long wave radiation – read: it gets really cold. The ice box was primed and ready. This was to happen in Portland following our mini-blizzard- (stay tuned I’ll get to that). So, the stage was set with a source of cold air and regime that kept the east wind blowing.
The atmosphere for really cold temperatures.
Following the mini-blizzard the city was set up to experience some very frigid nights. Cold, arctic high pressure (not historically cold either) set up and gave us day after day of brilliant sunny weather and clear nights. The east wind even gained in intensity forcing cold air into the metro area over the snow covered surface. It could have led to crippling cold for almost all of us but this did not happen. The strongest winds coming through the Gorge were restricted to the west end – Multnomah Falls to Troutdale and then much diminished through the metro area. Its important to reiterate that wind actually keeps the temperature up. The atmosphere naturally inverts, cold air sinks to the lowest points while warm air escapes via long wave radiation to the sky. Wind keeps the lower levels of the atmosphere mixed and interrupts the evacuation of heat. The coldest conditions occur on perfectly calm nights over snow covered terrain. So, though the wind feels horribly cold due to windchill, and dry arctic wind can damage plants its calm conditions when temperatures really plunge.
Most of us dodged a bullet
On the 13th as high pressure built over Portland and deep snow cover had not begun to melt I was convinced we would dip to devastating lows. Luckily, that night and early morning there was just enough of a drifting east wind to prevent a plunge. At the airport the temperature which had been hovering at 20ºF all night took a perilous dip to 11ºF at 7am. The wind had stopped. This shows how volatile the airmass was. Luckily, the wind returned in the next hour and the temperature rebounded. All over the city the difference between Antarctica and relatively mild 20ºF lows was the capriciousness of a 5-10mph breeze. At Greg’s house in NE his low was just 21ºF- the same time 11ºF was recorded at PDX one and a half miles away. My low was 16ºF. But many areas repeated the temps that Greg recorded. One location that was not as lucky and which is often the coldest in the metro area was the Hillsboro Airport. Again, farther away from the winds of the Gorge it is a low and calm place and coupled with 8″ of snow it allowed the mercury to plunge that night to 3ºF. Probably very damaging to gardens there. And the coldest temperatures at an official reporting station in Western Oregon in four years.
Wait and see
So the real news of this winter so far has been the duration of the cold and as well as snow cover. The overall intensity of the cold has not been historic (Jan.13, 11ºF) broke the record of 14ºF for the date. In my own garden I’ve noticed very little cold damage as it begins to thaw. Some of of my intentional indicator plants show that damage will not even exceed the last arctic year of December 2013. This gives me hope. What I am really astounded by is the amount of damage that 13″ of heavy wet snow can do to trees and shrubs. All over town trees suffered horribly- possibly the most damage from a cold event since the great Ice Storm of Dec. 26, 1996. And even very surprising species succumbed. Downtown huge Ginkgo trees surrounding the First Presbyterian Church lost immense limbs- a tree known for its ability to endure winters wrath. A friend has a great theory that its the extraordinary time and benign intervals between events that sets all of our trees up for damage when it happens. This is most likely true. If you are anticipating cold damage it is most prudent to wait at least two weeks before true damage is apparent. It should show by then. Then, wait until the first day in spring above 70ºF- this is the most telling and you have even given it time to break dormancy and grow. Patience often pays off. For plants damaged by snow or those that look bent beyond hope. There is hope. Most trees and shrubs have great elasticity and over the next few months will correct the misshapen habit they have now. Give them time before you go hog wild with the pruners.
The second half of winter.
It will be fascinating to see if we return to what has been a stubborn weather regime for the past month and a half. At the moment we are enjoying a much more normal and deserved thaw. Pay close attention to long range weather forecasts. The NOAA Portland has done a fantastic job in its prognostications so far this winter. Remember that Presidents Day/Valentines Day is normally the end to when we can still have arctic air. Which is why it is often recommended as the time for pruning many garden plants. Our Shop will be opening on February 9th, barring any more arctic intrusions. Greg and I will be organizing a symposium for gardeners at the shop to discuss what failed and what sailed through this winter. If not for just some moral support. Thanks.
January 18, 2017