Cold Outbreak

The first serious cold spell of the winter is now under way for Alaska, as strong northerly flow has brought cold air plunging down from the Arctic in the past day or so.  Temperatures have dropped below -40° at the usual cold spots, and Fairbanks airport is sitting around -30°F this morning for the first time since last January.

The immediate cause of the cold outbreak is a powerful high pressure ridge that ballooned up over the Bering Sea and far eastern Russia in the past couple of days, driving a sharp change to northerly flow over Alaska.  The shift in the flow pattern has been very dramatic: check out the 500mb flow change over Alaska from Thursday morning to this morning:

The north-south pressure gradient across Alaska is extreme today, with MSLP near 1040mb on the Beaufort Sea coast and near 980mb on the south coast of the Kenai Peninsula.  As a result, strong winds are producing ground blizzard conditions and nasty wind chill in exposed locations.

Remarkably, today's near-60mb north-south pressure difference is close to (or may be) an all-time record; here's a chart showing the annual maximum and minimum MSLP difference between Deadhorse and Seward, based on daily mean data.

The ERA5 record for the daily mean pressure difference is 55.8 mb on December 8, 1955.  I need to rerun the analysis using hourly data and compare it to today's conditions.  Note that very strong pressure differences like this are found in winter, when the jet stream is strongest and flow anomalies are greatest.

Looking ahead, there are strong signs that the Bering Ridge setup could have some serious staying power, judging from model forecasts.  For example, here's a diagram of 500mb heights averaged over the 50-80°N latitude band, with the time progression from top to bottom.  Look at the yellow/orange colors at the left and right sides in the forecast section (below the dashed line): the ECMWF model expects a ridge to persist near the Date Line all the way into mid-January.

This would mean persistent, significantly below-normal temperatures for southeastern Alaska and western Canada.  The CPC forecast is locked into this signal for their 6-10 day, 8-14 day, and week 3-4 forecasts: