Record cold expected.
The Al Gore effect as well as the testimony on Capitol Hill yesterday has caused a massive chunk of Arctic air to descend from Western Canada into the United States. How do temperatures 6o-70F degrees below average sound to folks along the Canadian border? During the next 10-days (or more!), a hemispheric realignment of the large-scale atmospheric circulation will cause a significant decrease in temperatures over North America and Europe. If this regime reinforces itself during the next few weeks, and a negative Arctic Oscillation phase strengthens, the brutally cold temperatures will provide a dramatic reminder that winter is cold regardless of “global warming”. I anticipate plenty of fossil fuel use coming up.
Weather weenie discussion after the break w/maps
With strong anti-cyclones developing over the North Pacific and North Atlantic at the same time, a highly amplified pattern is developing over the Northern Hemisphere. At the permeable barrier separating the troposphere and stratosphere lies the tropopause. On the tropopause, which is often identified by a constant potential vorticity (PV) surface of 2-PV units, one can plot the potential temperature (Θ) to identify upper-level synoptic phenomena such as the jet stream, Rossby-wave breaking, anticyclones, and cut-off lows. From these so-called “dynamic-tropopause” maps, it is relatively easy to see what’s going on.
The warmer potential temperatures ( Θ ) on the tropopause mean it is higher in the atmosphere since Θ increases with height, unlike regular temperature which decreases through the troposphere (before increasing in stratosphere due to ozone absorption of UV). Thus, tropical and subtropical origin air has much higher dynamic tropopause Θ than the Arctic vortex air which is signified by the gray and purple colors on the map above. Often the difference is well over 100-120 degrees Kelvin.
The sharp gradients of dynamic tropopause Θ are typically where the planet’s jet streams are located. During the 180-hour forecast period, the subtropical jet stream is easily identified over the central United States in conjunction with a powerful storm system. The counter-clockwise turning of the Θ represents cyclonic circulations while the opposite, clockwise circulations are associated with anti-cyclonic circulations or ridges.
The flow for the next week comes straight out of the Arctic over North America. Temperatures will easily sink below zero in many places and a hard freeze is likely for a lot of the US. Europe does not escape the bitter onslaught, either. I provide a couple maps below from my FSU website that show the temperatures for the next 180-hours, and the deviations from climatology.
All of this discussion is on the synoptic scale or days to a week time scale. Forecasts longer than this are not particularly skillful, so it is not clear how the La Nina will evolve over the next month. Furthermore, no two La Ninas act the same so averaging together previous events is not particularly skillful either. Main point: the atmosphere rarely matches the “mean state”. Perhaps Heidi Cullen can explain it better than I — after all she was on the Weather Channel.