Winter '11 / '12 - Sudden Stratospheric Warming Watch

A sudden stratospheric warming (SSW) event appears to be getting underway this week.

NWP output from the ECMWF continues to show an expectation for significant weakening of the stratosphere's polar vortex  (PV) by year's end.  The PV's west winds...having reached a maximum speed ~210 kts (110 m/s) a few days ago...are forecast to fall off a fookin'cliff and decrease to ~50 kts (25 m/s) by the end of the period.  Diminishing PV wind speeds can lead to a reversal in flow where east winds become dominant.  These are highly favorable conditions for a negative state of the Arctic Oscillation (AO) to develop if they persist and propagate into the troposphere.

Also note the increasing intensity of the sub-tropical jet (STJ) INVOF 30°N @200 mb.

The decay in the PV's intensity comes courtesy of warming in the atmosphere's highest level millibars between the pole and the equator.  The warming all but eliminates the strong temperature gradient which in turn causes the wind to diminish.

The American Meteorological Society's 'Glossary of Meteorology' defines stratospheric warming...also called sudden warming...as...

"(a) rise in temperature of the stratosphere in the polar region in late winter resulting from enhanced propagation of energy from the troposphere by planetary-scale waves."

From Wiki...

A major warming is defined as "...westerly winds at 60°N and 10 mb...become easterly [...].  A complete disruption of the polar vortex is observed and the vortex will either be split into daughter vortices, or displaced from its normal location over the pole."  

The D+10 ECMWF panel shown above depicts east winds ~20 kts in the layer just above 10 mb.

From Wiki...

A minor warming is defined as "...the westerly...winds are slowed; however... (they) do not reverse. [...] a breakdown of the vortex is never observed."

An alternate definition of a minor warming considers an observed temperature change of "...25°C...in a...week or less at any stratospheric level in any area of winter time hemisphere. The polar vortex is not broken down and the wind reversal from westerly...to easterly...is less extensive."

This is excellent news for anyone waiting for winter to get underway in the eastern CONUS.