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 Height | 500 mb Height Anomaly |
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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.
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.
