Wednesday, February 4, 2015

Anatomy of a Cold Spell

Regular reader Eric asked about the upper-level flow pattern associated with the late January cold spell and how it contrasted to the earlier persistently warm pattern.  We'll start with a few maps (see below) showing the 500 mb height pattern and anomalies in November (top row) and December (middle row), and for January 1-22 (bottom row).  The basic flow pattern was very persistent, with a strong ridge over western Canada and eastern Alaska creating a zone of southerly flow to its west that transported warm air up to Alaska.  The 500 mb heights were particularly anomalous in November and early January.

500mb Height500 mb Height Anomaly

We can contrast this flow pattern with the events that unfolded in late January.  The charts below show the daily mean 500 mb height and sea-level pressure beginning with January 22, which was two days before the temperature dropped below -20 °F for the first time in Fairbanks with this cold snap (but Huslia reached -51 °F on Jan 23).  Fairbanks saw minimum temperatures of -40 °F or lower January 26-30.

500mb HeightMSLP

Obviously the upper-level trough was primarily responsible for bringing cold air into interior Alaska, but as so often happens the surface temperatures did not really drop off until cloud cover dissipated in response to rising pressure and subsidence aloft.  The chart below shows the evolution of temperatures at 850 mb and at the surface, as well as the hourly cloud cover observations.  Interestingly the 850 mb temperature bottomed out right when the sky cleared and the surface temperature dropped late on the 25th; by the time the surface temperature reached -40 °F, the temperature aloft was already on the rise.  This provides a very nice example of the inverse correlation between cloud cover and inversion strength.


  1. Thanks, Richard.

    So I gather that cold air was only a few hundred miles and a skip away from Alaska all through November, December, and most of January. Then the ridge moved a little southeast and a trough moved over us and strengthen a little to give us freezing temps. All this with an extremely warm northern Pacific.

    This leads to the question why the winter ridge was so strong and persistent and then why is moved out of the way. Did some high pressure build half way around the world, moving the jet stream, and forcing the ridge to move? Is it possible to model these large dynamics on even a course scale? I feel a little chaos theory being invoked.

    1. Eric, certainly these things have a good degree of randomness in them, although perhaps less than we might guess as I'm certain there are many drivers yet to be discovered. At a basic level, the PDO was responsible for the ridge, and we've discussed that the PDO effect is weaker in January than in surrounding months.

      Medium-range forecasts suggest the ridge is getting ready to strengthen again, perhaps in dramatic fashion, so this cold should be out of the way in 7-10 days.


    Here's one factor Eric but I can't address the ultimate drivers. Build a 30 day animated loop of Northern Hemispheric 300 mb flow from late January to the present. Note the cold advection over Alaska later in the period.

    Another factor is the cold mass of near-surface air that resides over Canada. When it bleeds W/SW we get cold.


  3. And here we are @ 11:00 pm 2/6 and -40F. No ice fog which is unusual. From the latest cams there's enough NE wind to dissipate any major ice fog in Fairbanks. GN.