July Frost

A very chilly air mass originating over the Arctic Ocean has produced frost in many of the usual cold spots of the central and eastern interior in the past few days.  Here are a few of the readings I noticed:

Fairbanks Area:
UAF Smith Lake   27°F
Ester 5NE Coop   28°F
Goldstream Creek Coop   29°F

Elsewhere:
Tok 70 SE CRN   26°F
Chicken Coop   27°F

Here's the 7-day temperature trace from UAF's Smith Lake 2m sensor:

According to the balloon sounding from Fairbanks airport, the 850mb temperature fell to -0.5°C on Tuesday afternoon, which is the first time since 2009 that the 850mb temperature has been below freezing between mid-June and the end of July.  The last such cold spell in 2009 (June 25) was also the last time that Chicken fell to 27°F in the same high-summer window.

The 26°F at the Tok CRN is the coldest in the 9-year history of the site for this time of year; it's the first instance of 26°F between June 14 and August 21.  Data goes back to autumn 2011.

Here's the reason for the cold: according to a back-trajectory estimate from NOAA, the lower tropospheric air that was over Fairbanks on Tuesday afternoon had traveled from near the North Pole in the previous 10 days.