Tornadoes in the City of Roses.

 Thanksgiving-like chill in September.

 Two historic snowfalls across Montana and even parts of Eastern Oregon & Washington.

It’s been a very interesting first half of Fall this year.  After a summer with mild days but stuffy warm nights, the rain faucet turned on big-time in September.  The airport got 3.80″ of rain, making it the second wettest September in two decades.  But there was also a twist toward cold by the end of the month:  two early-season arctic airmasses dropped into the Pacific Northwest and Northern Rockies.  The first came down on September 28-29, and the second on October 8-9.

Basically, we had two very cold Canadian troughs that dipped into the Western U.S., just like the pattern which gives us our arctic blasts in the middle of winter.  This has led to two unusually early snowfalls in Spokane, a historic late September blizzard in Montana, and more recently an October blizzard across both Montana and the Dakotas.

The September cold snap produced a record cold day in Portland on the 29th; just 52 degrees for a high temperature at PDX airport and upper 40s in some neighborhoods.  There was a record cold night going into the morning of October 10th at 33 degrees; most locations outside the immediate “heat island” saw freezing temps as well.  As for my former town of The Dalles, it hit 27 degrees on the morning of the 10th.  This was the coldest early October night in (much of) the Pacific Northwest since 1985! 

 Here is a data set that shows the first fall occurrence of a temperature of 35 degrees or colder, for each year since 2009, plus the earliest ever occurrence in September 1965:

Temps of 35F or Colder At PDX:

2019 – Oct 10 (33)

2018 – Nov 9 (32)

2017 – Oct 31 (35)

2016- Dec 4 (34)

2015- Nov 21 (32)

2014- Nov 11 (35)

2013- Oct 29 (32)

2012- Nov 22 (35)

2011- Nov 1 (35)

2010 – Nov 11 (35)

2009 – Nov 3 (35)

 

Earliest on record:  1965 – Sep 17 (34)

 In the modern heat island, we don’t usually drop to 35 until sometime in November…let alone 32.  Of course, several decades ago, things were a little different in Portland in the fall!

As of the 15th, temps for October 2019 are running nearly 5 degrees below normal at PDX.  I’m pretty sure this is the coldest first half of October in at least a decade or more, though I don’t have the data for half-months readily available to check.  There is an interesting “weather paradox” at play in October:  even though it is our fastest-changing month in terms of temperature, it actually tends to have smaller temperature departures from one October to the next, compared to other months.  (Read my blog from two years ago on “seasonal heteroscedasticity,” to find out the possible reasons.)

Do early intrusions of cold autumn air have any bearing on the coming winter?  I will say that the last two early Octobers with a sharp cold snap – 1985 and 2009 – both had significant arctic events in the first half of winter.  In 1985 it came in late November, in 2009 it was early December.  The chilly first half of October 1968 also was followed by a major mid-winter arctic blast.  All of this said, I think it would be unwise to hedge our bets on it.

My gut feeling is still with the ENSO (El Niño/La Niña) model on this one.  There is a clear correlation for less arctic air in Niño years, and more of it in Niña years.  (The cold winter of 1968-69 was a rare exception to the El Niño rule.)  Trouble is, we’re likely heading into a neutral phase – and those seasons tend to be major wild cards.  We might luck out and get caught on the cold side of an active storm pattern, and see a major snowstorm.  Or we could just get lots of mild rain storms coming in from the west, possibly including strong wind.  Or there might be extended periods of quiet weather, with plenty of fog and inversions.

There’s also the question of ‘The Blob’ out in the North Pacific.  The warmer our ocean stays, the less likely we are to see snow or ice in and around Portland (all other things equal).

Later this fall (probably my next blog post!)  I will give my humble thoughts about the upcoming winter season.

In the meantime….enjoy the switch from dry chill to wet chill!  A series of damp storms are expected to parade into the Pacific Northwest, beginning today Wednesday and continuing into early next week.  This is standard business for mid to late October.  The first storm came in this morning, another wave of rain is expected tomorrow evening.  Then another pulse Friday night.  Then again Sunday.  Get the picture?  None of these storms are super wet or windy, but it will be a steady barrage of moderately soaking rain.

Furthermore, it’s going to be a cool series of rainstorms, with snow in the High Cascades at times over the next 5 days.  Our GFS spaghetti plot (courtesy of WeatherTogether) shows snow levels around 4-5,000 feet at times the next few days; i.e. 850mb temps very near 0 Celsius:

In the 3rd week of October, cool rainstorms like this mean daytime temps in the 50s for the lowlands.  Not terribly unusual for this time of year, but we’ve gotten used to a number of recent Octobers where the upper 60s & 70s have run well into the 2nd half of the month.

Things may turn dry again as we move into the last week of the month.  However, with the weak sun angle and lots of ground moisture, it could be a recipe for inversions and fog as opposed to warm sunshine.  We will have to wait and see how that turns out.  In the meantime….enjoy the soaking fall rains!  My attitude is that if it can’t be warm at this time of year, we might as well start getting rain and mountain snow instead.  -Karl