(01/01/2015 − 01/10/2015)
· The strongest STRAT event recorded this winter season so far (STRAT_E event: the main peak on 01/01/2015 at a rate of 1.8 trillion tons per day and the secondary peak on 01/07/2015 at a rate of 0.6 trillion tons per day) resulted in a complete collapsing of the stratospheric polar vortex (zonal wind around the polar circle changed to easterly from westerly). The two peaks of the STRAT_E event were accompanied with two strongest peaks of the cold air mass transport out of the polar circle recorded this winter so far, 17 trillion tons per day on 12/29/2015 and 15 trillion tons per day on 01/07/2015. These two strongest mass circulation peaks caused a series of major cold air outbreaks over both continents, particularly over North America where 80% of the area experienced below-normal temperatures in the two weeks after Christmas
· The collapsing of the polar vortex was in a form of relocation of the vortex center from the Arctic to the eastern hemisphere side of the Arctic in addition to its weakening, forming the large-amplitude of Rossby wave number one. Because of the wobbling of the off-center, the lobbed polar vortex and the subsequent dispersive development of the Rossby wave #1, there will be a series of STRAT events. In the troposphere, there will be a series of cut-off lows that spin off from the off-arctic and lobbed vortex and blocking highs resulting from the continuous poleward movement of warm air into the polar region. The associated cold air outbreaks will take place over both continents in the second and third weeks of January. These cold air outbreaks will be more transient (fast pace) with sub-continental scales.
· The STRAT_F is the first round of such series of subsequent events. The STRAT_F is expected to peak around 01/15/2015, very close to the range that we first forecasted on 12/4 (around 1/15-1/20). The associated cold air outbreaks will not be continental scale events (i.e., no below-normal temperatures over vast area), but are still capable of causing sub-continent scale cold air surges as the synoptic waves that spin off from the off-arctic lobbed vortex pass through the mid-latitudes in the second week of January.
· The STRAT_G1 is the second round of the subsequent events after the vortex weakened and sided away from the pole. STRAT_G1, which was first forecasted on 12/17 to occur around 01/24-27, is expected to take place between 01/21 and 01/25 with multiple weak peaks. Although STRAT_G1 is the second round of the subsequent events of STRAT_E, it is still capable of producing wintery storms because the associated cold air surges lie on top of the cold ground which has been pounded by the series of cold air associated with STRAT E and F events.
· While the polar vortex that was squeezed out of the Arctic by warm air dissipates over vast area of the extratropics via dispersion of baroclinically Rossby waves, responsible for STRAT_E, F, and G1, the new polar vortex begins to form resulting from strong radiative cooling over Arctic. It appears that the new polar vortex is going to be rebuilt with the remnants of the demised vortex. Therefore, the new polar vortex is going to form soon after STRAT_G1 event. This makes the STRAT_G2 be on time, occurring in the first week of February, as we first forecasted on 12/17. STRAT_G2 is the first STRAT event of the new polar vortex. Cold air surges are expected to occur in the late half of the first week and the first half of the second week of February.
Forecasts Made on January 10, 2015
Follow up on Forecast Made in Previous Weeks:
· The STRAT_F event is expected to peak around 1/15 and to be much weaker in magnitude compared to STRAT_E. This cold air outbreak expected to keep temperatures below normal, but will not make a large impact on a continental scale simultaneously. But it may impact East Asia, Europe, and North America simultaneously because of these synoptic scale features.
· STRAT_G1, being the second round of the waves generated by the aftermath of the collapsing of polar vortex, will occur 2-3 days earlier than we first forecasted on 12/17, taking place between 01/21 and 01/25. Another round of sub-continental scale cold air surges are expected to take place in this period.
· STRAT_G2 will be the first STRAT event of the newly rebuilt polar vortex after the STRAT_G1 event. The STRAT_G2 event will be more or less on time with peak days around 02/02-02/06 as we first forecasted on 12/17.
New Forecasts Made on January 10, 2015