New Forecasts Made on February 18：
In this week’s forecast, we are forecasting a new STRAT PULSE event, named as STRAT_N, to take place during the period 03/14-03/18 with its peak around 03/16. STRAT_N is going to be a relatively weak since it will take place right after the second stratospheric polar vortex breakup event in 2015-16 winter. As an aftermath event of the polar vortex breakup similar to STRAT_K1, the associated equatorward mass transport of polar cold air near surface can still be anomalously strong, despite the stratospheric PULSE is weak. As a result, we still expect that there will be cold air surges over both North American and Eurasian continents during STRAT_N event.
4th polar stratospheric warming event is going to take place after STRAT_M (03/8-03/12), which will cause the 2nd complete breakup of the stratospheric polar vortex:
After the first stratospheric polar vortex breakup event (02/08), the stratospheric polar vortex has been gradually recovering its strength. According to the NCEP CPC’s stratosphere-troposphere monitoring website (http://www.cpc.ncep.noaa.gov/products/stratosphere/strat-trop/gif_files/time_pres_UGRD_MEAN_JFM_NH_2016.png), polar jet already reached the strength of 40 m/s (averaged over 50-80N) at 10 hPa today. Since there will be two back-to-back relatively strong PULSE events (STRAT_L and STRAT_M) starting in the next few days, the polar vortex would have little chance to reach the condition in the end of January (65 m/s) recorded before the 1st breakup of stratospheric polar vortex. These two back-to-back PULSE events together are expected to last up to 18 days with peak intensity of 0.9-1 trillion tons per day. Despite neither of the two events is expected to be as strong as STRAT_J, the accumulative warm air mass to be transported into the polar stratosphere is expected to be considerably large due to the extra-long duration. Therefore, it is highly possible that the stratospheric polar vortex will be weakened significantly during or after STRAT_M, or even the polar jet can be reversed for the second time in this winter, after STRAT_M. Associated with the 2nd stratospheric polar vortex breakup event, there will be massive cold air outbreaks over mid-latitude continents. Several days after polar vortex breakup event, another round of cold air surges will be delivered to the mid-latitudes by an aftermath stratospheric PULSE
Follow up on Forecasts Made in Previous Weeks:
STRAT_K1 (02/12-02/15), first forecasted on January 22, has been materialized. As we forecasted, STRAT_K1 was associated with cold air outbreaks over North America (Eurasia) in the mid-latitudes with more than 50% (60%) of areas occupied by below-normal temperatures in the period 02/12-02/17.
STRAT_L (02/19-02/26), first forecasted on January 22, is going to take place soon as we forecasted. STRAT_L is indeed going to be an extra long-lasting event. STRAT_L has multiple peaks with one of the major peaks on 02/19 and the other one on 26 with peak intensity up to 1 trillion tons per day, consistent with the forecasts made two weeks ago. The first peak is driven by stronger wave activities of both wavenumber 1 wavenumber 2, particularly wavenumber 2, which will lead to massive cold air outbreaks occur over both continents in the meantime. 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-1and week-2 forecasts support our forecasts that there is a quick round of cold air surge over eastern regions of North America during 02/18-02/19, and then another round of strong cold air outbreaks is going to take place over vast areas of North America, especially the mid-eastern part around 02/26. For Eurasian continent, a massive cold air outbreak event is expected to take place mainly over East Asia, during the same time period of STRAT_L.
STRAT_M (03/03-03/08), first forecasted on February 4, is still expected to take place on time with its peak around 03/05. We are continuing to forecast that cold air surges are going to take place over both continents during the forecasted time period.