I’ve been pondering all the forecasts that I’ve seen for this coming winter. Without going into painful detail I will state this. Put all together they point to a warmer and drier winter than normal. As you have probably noticed the leaves are tardy to show color this year. That is because not only August, but both September and October were the warmest ever recorded in Portland.
There seems to be no real change in this regime until at least January. One thing to think of is that last year was abnormally cold- we had two winter blasts- how could we forget? The cold wave last December was a once in a decade event- in Eugene where it was an astonishing -10ºF at the airport it was a 40 year event.
We typically do not see two cold winters in a row. That is just a fact and beyond that it has no real basis but if you look at the climate record for the last 60 years that is one pattern that holds true. A cold winter is characterized by more than 2 days below 20ºF. Remember that our thirty year average lowest temperature at the airport is 20.3ºF (Zone 8b). Two cold winters in a row has happened in the past but there is a whopping 80% chance based on records from 1940 to the present that supports at least a year of mild/normal winter weather between massive arctic intrusions.
One of the best ways to think about weather is to divide it into averages that gives a better overall picture. For instance, in a 10 year period we typically average 6 winters that fail to drop below 20ºF, (Zone 9) 3 years that drop between 10º and 20ºF (Zone 8) and one year below 10ºF (Zone 7). That last figure is becoming more and more rare. The Portland Airport has not seen a temperature below 10ºF since it registered 9ºF back in 1990. Thats almost a quarter century. (It was 11ºF in 1998 and 12ºF in 2009 as well as 12ºF last December. )
Things are quite a bit different in the suburbs. Hillsboro dropped to 7ºF last December, 9ºF in 2009. The average annual low at the Hillsboro Airport is 14ºF (Zone 8a). Its important for gardeners away from the urban core to account for lower temperatures when they choose plants. This is true for Clark County as well.
There is a weak to almost neutral ENSO (El Nino). Whether or not it gains strength remains to be seen. In a word the weather during an El Nino winter is Bland, Bland, Bland. The northern jet stream weakens and weak troughs are split before then enter the PNW. The weather is warmer than normal (but there can still be an arctic outbreak- its just much, much, lower chances.) Split storms give us mild, dry weather. We have more than one reason to cheer on El Nino- it strengthens the sub-tropical jet aimed into California providing them with prodigious moisture. I know that everyone wants this to happen to have at least some effect on the historic drought.
Als0- the NOAA predicts a colder than normal winter in the eastern U. S. This also brings us warmer weather as a ridge remains over the west and a trough over the east ushering arctic Canadian Air far to the south.
So, if you are a gardener expect a milder than average winter- there is always the chance- even if it is much less for an arctic event (and as we know it only takes one arctic night to do all the damage). If I see anything like that coming I will post it here.
So, in the mean time. The leaves have yet to change color and the next several months do not look to be below climate normals.