Special weather blog for PDX gardeners

A watch for gardeners

There is a very interesting forecast for the Thanksgiving week going into and through the Thanksgiving weekend. A sharp cold front will slide down the coast from the Gulf of Alaska ushering in much colder drier air. There will be precipitation with the front Tuesday evening but it appears that it will dry up after the front goes through. This is typical of our climate where modified arctic air follows the front by a bit. It reduces our chances of snow. And if you don’t want truly arctic air you DON’T want snow cover. That would cause low temperatures to plunge.  So it appears we may dodge a bullet. I have read that this is not very El Nino-like. Well, I remember plenty of El Nino years with modified arctic air- remember that does not preclude us from having cold weather. El Nino affects the WHOLE winter not just the weather week to week. That is an important distinction to make.

A few freezes then a plunge. Is this El Normal?

The good news is that in our current regime we’ve now had two freezes/frosts in the city and considerably colder in the out lying areas. That means that plants have received an early warning and will have hardened off a bit prior to this next wave of much colder air. This is good. By the way, early freezes/November events have happened in the past and they can be pretty devastating- I don’t expect that. (For more information see November 1955 as well as 1985- when each entire month was frigid and temperatures plunged to the teens and lower-horrible for plants.) So, this looks like a fairly typical cold snap. What is interesting about cold Novembers is that the rest of the winter is mild/normal 80% of the time. Its entirely possible- and it has happened many times in the past that this cold weather is our coldest of the winter. That is by no means a forecast but I can say that I’ve seen that many times in the past. Don’t think that this is what the rest of winter will entail- it may or it may not.

Normal weather progression with a buckle

Ridges and troughs that move along the jet stream give us weather in 2-3 week segments and following the wet part of this month it is completely within the realm to have it followed by a dry period. What is different about this switch is that there is a high pressure system in the North Pacific that has moved to the west (this is called retrograde because it is a reversal of the normal movement from west to east). This buckle in the jet stream reflects a conveyer belt moving north to south. Its bringing air directly down the coast. By the way, modified arctic air is just that. Air from the arctic that has moved over relatively warmer Pacific waters. The layer of warm/wet air at the surface mitigates the pure cold air and it is modified- it loses its pure arctic punch.  If the air were arriving from the northeast/east that air would not be modified by the waters of the Pacific. That would be worse. But stay tuned for #2.

Cold air pool- air pools east of the Cascades ROARS through the Gorge

A large area of cold high pressure sets up following the front Wednesday and beyond.  This will increase  the pressure gradient from east to west as the cold dense air pools in the Columbia basin east of the mountains. North/Easterly winds will roar through the gorge- this is called an outflow (or a Bora wind)  and reinforces the cold air. Starting Wednesday night these frigid winds will spread across the metro area. It will depend on the amount of snowpack east of the Cascades (for instance the Pendleton area) that modifies the air to be colder on how cold it will ultimately get in Portland. I generally worry when the temperature in Pendleton is below 15ºF- that is a really cold air source and it is somewhat of a benchmark for how cold we will be. Right now Pendleton is forecast to have a low in the low teens.

Our predicted temperatures:

As of now temperature look like in the northern Willamette Valley they will definitely be in the 20’s with some spots in the high teens. Luckily, highs will be above freezing in the upper 30’s to low 40’s and thawing should occur to some extent each day.  Be aware that wind is a double edged sword. Wind not only transports cold air it also stirs the atmosphere from stratifying cold- it produces mixing. That means that wind keeps the temperature warmer- but it will still be below freezing and that causes damage. Therefore, it may only be 28ºF at the airport but a 35mph wind can cause a lot of damage to plants. I estimate that situation would be the equivalent of 18ºF in still air.  That is how zone 8b (15º to 20ºF) plants can be damaged at warmer temperatures. Away from the Gorge temperatures will ultimately be lower without proximity to wind (wind sheltered sites) and actual lows in the upper teens will cause the same amount of concern as our (relatively warmer) temperature. Ya’ got it?

Dew point- watch the dew point carefully

Arctic air is cold, dry, and dense. So much so that it almost hugs the ground like a liquid and flows thusly. An outflow wind is a product of this dense property. Dew point is a measure of the moisture content of a parcel of air in the atmosphere. The lower the dew point the drier the air (By the way relative humidity is determined by the dew point- it is relative to the amount of moisture at any given temperature).  Arctic air has very low dew points- its freakin’ dry air. That dry air in concert with low temperatures causes damage to plants and wind exacerbates this. Think of arctic air + wind as a torch in the opposite. The immensely cold dry air sucks the moisture out of leaf surfaces and in combination with freezing causes damage to such things as broad leaved evergreens that don’t go as completely dormant as say, deciduous plants. Dew points below 12ºF and subfreezing temperatures spell danger for many borderline plants.

What kind of plants will be damaged?

Tender plants (zone 9) will be very challenged and most likely die. Plants that are still in active growth will lose that new growth to the freeze. Hardened off older foliage should be fine. Zone 8b plants (15ºF) will be damaged but probably not die- those in wind sheltered spots will fare better. This will likely NOT kill or marginally injure Phormiums. That should give you context for past cold waves/years. One slight amount of luck that can be good is that we’ve had a very warm summer which hardens off the wood of semi-tender plants. One bad thing is that we had the mildest October of all time. See how confusing this can be? Protect containerized plants if they are not hardy below 15ºF. Water containerized plants that are hardier than that. It appears that the real wind/cold goes from Thursday morning to Saturday. Then slow modification. If I may be allowed to geek often the coldest night of Portland cold snaps is the last night. That is because winds have slackened off and radiational cooling in the calm stratified air takes hold. So- the wind is good and bad.  But be wary of calm air as well.  There is still time to apply mulch and protective covering but make sure it is in place by Wednesday evening.

**** Wait and see***

The NOAA and other sources are very much on board with this forecast. I’ve noticed that their forecast ultimate lows have been very conservative. This is a good sign. However, I’ve seen this weather regime before and thats why I’ve prepared this post.   I’ll update it as things progress.

HAVE A HAPPY THANKSGIVING

-Paul

 

 

 

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Special weather blog for PDX gardeners

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