New Forecasts Made on February 25：
No new forecast is made in this week.
The 2nd complete breakup event of polar vortex is going to take place in the period 02/27-03/03, right after STRAT_L, which appears to be the final stratospheric warming event in the 2015-16 winter:
The first peak of STRAT_L, took place on 02/19, with the intensity of 0.9 trillion tons per day of warm air mass transport into the polar vortex in the stratosphere. It decelerated the polar jet (averaged over 50-80°N, at 10 hPa) from 40 m/s on 02/15 to 30 m/s today, as shown in the NCEP CPC’s stratosphere-troposphere monitoring website. The second peak of STRAT_L is expected to be at least 1.6 trillion tons per day, about twice as strong as we have been forecasting since January 22. This will abruptly bring a tremendous amount of warm air into the polar vortex, which will lead to a complete breakup of polar vortex and reverse of polar jet. Another evidence suggesting the timing of the breakup event is that STRAT_M, expected to occur in the period 03/03-03/08, becomes much weaker in this week’s forecasts. This indicates that the polar vortex will indeed be break up by the STRAT_L, and STRAT_M, and STRAT_N are a cluster of aftermath events of the breakup of stratospheric polar vortex as the remnant of the stratospheric polar vortex dispersed into the mid-latitudes in form of Rossby waves without returning to the Arctic.
For those reasons, we are shifting the timing of the 2nd complete breakup event of stratospheric polar vortex forecasted last week to earlier dates, namely right after the second peak of STRAT_L. Moreover, based on the lack of PULSE events and strong wave activities after STRAT_N in forecasts, we are forecasting that there is little chance for the stratospheric polar vortex to recover after the 2nd complete breakup of the stratospheric polar vortex in the winter of 2015-16. As a result, we are predicting that the stratospheric warming due to STRAT_L is going to be the final stratospheric warming event for the winter of 2015-16. After this, we will no longer issue any new forecasts because of the absence of PULSE events afterwards.
Follow up on Forecasts Made in Previous Weeks:
STRAT_L (02/19-02/26), was first forecasted on January 22 as an extra long-lasting and strong event. The first peak has already taken place with the intensity of 0.9 trillion tons per day. As we forecasted, this peak was mainly driven by stronger wave activities of wavenumber 2, thus corresponded to cold air surges over both continents. The intensity of the first peak and the associated cold air surges turned out to be slightly weaker than expected, but the timing and location were well forecasted at a lead time of 27 days. The second peak of STRAT_L is going to take place soon on 02/27. This peak is expected to be super strong, transporting 1.8 trillion tons of warm air mass into the polar stratosphere per day, corresponding to the strongest PULSE event in this winter. As we forecasted last week, the second peak is due to stronger wave activities of wavenumber 1, which is expected to cause cold air outbreak events mainly over North America. GFS week-1 forecasts confirm that there is a new round of strong cold air outbreaks over the mid-eastern regions of North America around 02/27 and this strong cold air outbreak event will last for several days. For Eurasian continent in the meanwhile, below-normal temperature is only expected to take place over northeastern regions of Asia.
We are continuing to forecast the STRAT_M (03/03-03/08) to occur during the period 03/03-03/08 with its peak around 03/05, as first forecasted on February 4. This stratospheric PULSE event right after the breakup event of stratospheric polar vortex is more likely the dynamical response to the stronger equatorward air branch of mass circulation in the troposphere. Therefore, though the intensity of STRAT_M is not going to be large, but it will be accompanied with massive cold air outbreaks over both continents during the forecasted time period, consistent with the GFS week-2 forecasts of surface air temperature anomalies.
STRAT_N (03/14-03/18), first forecasted on 02/18, is going to take place on time.
Its peak is still expected to occur around 03/16, as last week’s forecasts. As we mentioned hereinabove, STRAT_N is going to be the second aftermath event of the polar vortex breakup. As a result, we expect another round of cold air surges over both North American and Eurasian continents during STRAT_N event.