Back to Cool

[Update September 3: this morning brought a deeper frost for the eastern interior, and Fairbanks dropped to 30°F - the first freeze, and earlier than normal.  The usual cold spots were colder: 22°F at Goldstream Creek, 20°F at the Salcha RAWS, and the Chicken co-op observer reported a remarkable 11°F.  Data in Chicken only goes back to 1997, but this is the earliest on record for such cold; last year reached 14°F on August 30, but 11°F had not previously been measured before September 11 (2012).]

After a late summer climax of warmth in mid-August, temperatures have been on a steep downward slide for most of Alaska in the past two weeks.  The seasonal temperature change always accelerates into autumn for the interior in particular, but the transition from "summer" to "autumn" was especially sudden this year.

For now, temperatures are back into the "significantly below normal" category, and at this time of year that means widespread frost: many locations across the state dropped into the 20s Fahrenheit within the past few nights.  The coldest so far at Fairbanks airport is 34°F; the first freeze typically occurs in the second or third week of September.

Courtesy of NWS Fairbanks, here's a lovely pair of webcam photos from Eielson Visitor Center in Denali NP:

The chilly air mass of the last few days has reached the region from the northwest.  My previous post highlighted the west-southwesterly flow around the north side of the Aleutian ridge, but the flow orientation has shifted subtly to add a cooler northerly component over Alaska:

The forecast expects the ridge to shift east in the coming days, bringing a much warmer southwesterly flow pattern:

 

It will be interesting to see if high-pressure ridging remains a recurring feature near the Aleutians in the coming weeks and months.  The ongoing extremely unusual warmth in the northwestern Pacific may tend to favor a ridge near the Aleutians (or more precisely, weaker low pressure than normal), and that would normally be expected to keep Alaska on the cool side.

However, as winter approaches the influence of the very strong El Niño will probably have the opposite effect, favoring deeper low pressure near the Aleutians and bringing more warmth to Alaska, especially in the east.  Which will win out?  To my mind, this is more or less a tug-of-war between El Niño and the negative PDO phase; it's going to be very interesting to watch.