Fairbanks briefly experienced above-freezing temperatures at the airport on Friday afternoon for the first time since November 14, and even warmer conditions were observed in favored locations in the southern interior of Alaska. An interesting aspect of the event in Fairbanks is that the valley-level wind was out of the north or northeast throughout the warming. Of course, the warm air mass was coming from the south, as winds were out of the south less than 1500 m above ground, but the wind aloft was not particularly strong and did not mix down to the surface. In this sense, I'm not sure whether it's right to call this a "chinook" event in Fairbanks.
The chart below shows the hourly observations of temperature and wind speed and direction at the airport, along with the twice-daily measurements of temperature at 850 mb. Temperatures rose in tandem with the low-level northerly wind speed, but as soon as the wind died away, the temperature dropped back to its former level.
Surface observations from across Alaska at the peak of the warmth in Fairbanks show an impressive temperature gradient both across the entire state and in the central interior, as temperatures remained much cooler to the north and west of Fairbanks.
The dichotomy of wind flow (upper S, lower N>NE) happens in Fairbanks. With an upper level southerly flow over the Alaska Range and through mountain passes to the south, winds often provoke the Tanana Jet south of town = speedy SE to E flow through the valley.
ReplyDeleteUnless the flow eventually mixes to the ground and moves northerly (often a March onward condx), we can get N>NE surface flow in town. I believe that's a response from downstream flow through the Chena River valley to our NE combined with lower air pressure south of town from the Jet (Bernoulli's principle) .
Regardless of the physics, it's nice to have the brief relief and increasing daylight.
Gary
Gary, Thanks for your insight as always. Undoubtedly the surface pressure pattern was the cause of the low-level flow. The map of surface observations gives a rather clear indication of a low pressure center south of Fairbanks with pronounced convergence of the winds.
DeleteIt would be interesting to look at the climatology of warming events to see what wind patterns are most typical, e.g. how often does the southerly flow aloft make it to the valley floor? I'd also be interested to hear if the term "chinook" is reserved for certain types of events in local lore and expertise, or if it is applied rather indiscriminately to breezy and warmer conditions in winter.
I can't speak to local lore or for others, but for me it typically includes both visual and sensory changes associated with strong southerly winds over and through the Alaska Range in mid-winter. If arriving more from the SW the effects vary.
DeleteVisually it can include the formation of lenticular clouds over the southerly mountains, often a cloud-free drying zone above high Alaska Range terrain and under any such clouds, and depending on residual moisture, a north moving gray cloud deck over Fairbanks. That in turn depends on whether the system causing the Chinook approaches over the Range, or from the flatter lower terrain to the SW.
Typical sensory changes include a falling barometer (sometimes rapidly), the formation of the E>W Tanana Jet and associated wind noise depending upon location, N>NE winds in town followed by a shift to the incoming wind direction if they reach the valley floor, and of course eventual rapid warming followed by possible surface precip. Wind direction away from town can follow the Jet's main valley flow, which can often vary from SE>E>NE exiting CCW to the SW towards McGrath.
It's important to separate the unique 'chinook' effect in mid-winter from the periodic wind and warming events we experience at other times of the year. Without insolation, the winter events take lots of energy to scour the surface cold. At other times, as in March and September, any pressure differential involving a H to the north and a L to the south can create wind and some warming.
I suppose daily soundings and pilot reports, reported atmospheric pressure aloft and at the surface, and relative wind and temp data would offer a clue to the vertical extent of the chinook's events.
Gary
I might add that modern IR satellite images visually portray the gray warming flow against a colder white surface background if an upper cloud deck is absent.
DeleteGary
"...Visually it can include the formation of lenticular clouds over the southerly mountains, often a cloud-free drying zone above high Alaska Range terrain and under any such clouds, and depending on residual moisture, a north moving gray cloud deck over Fairbanks..."
ReplyDeleteThis effect can be seen midday today here: http://co.fairbanks.ak.us/airquality/CRCurrentPhoto.jpg
Unfortunately they don't appear to archive their webcam's .jpg photos.
Gary
I've thought about this some...a brief winter warmup is a Chinook.
ReplyDeleteA longer event, like now with several waves of WX, is a Chinooooooooook.
A little humor for y'all.
Check the above webcam link mid-day for what it look like to the south from Fairbanks.
Gary
Thanks Gary, your characterization of the event is very informative. The webcam link is great - I have it bookmarked now.
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