Freezing At Last

Fairbanks airport finally had its first freeze of the season yesterday morning, marking the latest date on record for reaching 32°F.  It's not, however, the latest date for dropping below 32°F; that didn't occur until October 5 in 2017.  Here's a 2017 post talking about the remarkable warmth back then:

https://ak-wx.blogspot.com/2017/10/hard-freeze-yet-to-occur.html

In much the same way as in 2017, abundant cloud cover and high humidity are significantly responsible for the absence of a freeze until now in Fairbanks.  Reader Gary commented about this last week: the dull and humid weather has been notable.

Here's a chart to show the evolution of daily mean humidity and cloud cover throughout the warm season this year; the smooth black lines indicate the 1991-2020 normal values.

The sudden arrival of clear skies is what has finally allowed the overnight temperatures to drop off to more seasonable levels, and even a bit colder than normal this morning (below 20°F in the coldest spots, including Eielson AFB).

Click to enlarge the map below for this morning's low temperatures.  11°F at the Salcha RAWS (bottom right) seems notable, but it's not really: sub-zero isn't unheard of at the end of the month at that cold spot.

As we would expect from the abundant cloud cover, daily high temperatures have been quite suppressed in Fairbanks lately:

The average difference between the daily high and low temperatures has been the smallest since 1985 for the month of September.  And yet total precipitation hasn't been much higher than normal.

Here's the mid-atmosphere circulation anomaly responsible for the dreary conditions: a trough extending eastward from Russia.

We also see strong ridging to the south of the Aleutians, and this has been a persistent feature since July; it's related to the persistent and quite extreme warmth extending east from Japan to the waters south of the Aleutians.  The enhanced north-south pressure gradient on the north side of this warm ridge has produced a strong jet stream, and from that perspective I think the ridge bears some of the blame for the persistently damp weather pattern across large parts of Alaska in recent months.