There is going to be a strong stratospheric PULSE event, named as STRAT_I, in the third week of December. The stronger poleward mass transport into the polar stratosphere associated with STRAT_I is contributed from stronger wave activities of both wave number-1 and wave number-2. According to our latest research findings, the variability of the net mass transported into the polar stratosphere due to wave number-1 is more related to cold air outbreaks over one continent, particularly North America, while wave number-2 tends to favor simultaneous cold air outbreaks over both continents. For this reason, we expect this round of cold air outbreak events will be felt strongly over both North American and Eurasian continents.
Continue to forecast the first breakup of the 2015/16 stratospheric polar vortex to occur right after STRAT_H (01/06-01/12)
We continue to forecast the 1st breakup of the 2015/16 stratospheric polar vortex to occur right after STRAT_H, as we originally forecasted on December 3. It appears that the first breakup is a minor and short-lasting since it is followed immediately by another new event, STRAT_I. In this sense, the first breakup is more like a wobbling of stratospheric polar vortex, which will be restored back to Arctic very quickly.
The timing of the first breakup could be delayed after the STRAT_I event, because there is evidence suggesting that STRAT_G could become weaker or even disappear. In that case, the accumulative warming effects on the stratospheric polar vortex from STRAT_F, G, and H may not be strong enough to break up the polar vortex and another stratospheric PULSE event, namely STRAT_I, is needed to break the polar vortex. We will keep an eye on the evolution of these events in the next few weeks and update our forecasts of the timing of the 1st breakup of the 2015/16 stratospheric polar vortex accordingly.
Follow up on Forecasts Made in Previous Weeks:
STRAT_E (12/11-12/16), first forecasted on November 12, have multiple peaks, consistent with our previous forecasts. The first main peak already took place on 12/09, 2 days before the forecasted time window, but this peak did not lead to noticeable changes in cold area over both continents in mid-latitudes. The second and third main peaks are going to occur on 12/12 and 12/15 respectively, which are within the forecasted time window. These two peaks of STRAT_E are associated with stronger cold air mass transported out of the polar region near surface, thus responsible for a 10% (20%) increase in the percentage of area occupied by below-normal temperature over North America (Eurasia) in mid-latitudes. Note that the North America is quite warm with the percentage of cold area shrinking to 8-9% on 12/10 due to lack of large stratospheric PULSE events in the period 11/29-12/10 and the general warmness associated with the 2015/16 El Niño. Because of the general warmness prior to STRAT_E, we will not feel that much cold in most regions of North America associated with STRAT_E.
STRAT_F (12/19-12/24), which was first forecasted on November 12, is expected to be perfectly on time. As last week’s forecasts, STRAT_F is going to be a long-lasting stratospheric PULSE event, transporting more than 0.8 trillion tons of warm air mass into the polar stratosphere. Right after STRAT_E, STRAT_F is expected to lead to a rapid temperature dropping, manifested by a further 20-30% increase in the cold areas over both North American and Eurasian continents. GFS 2-week forecasts also show that a much colder surge of air arrives by Friday of the next week, December 18. Therefore, cold surges associated with STRAT_E and STRAT_F could affect the Christmas holiday travelers.
STRAT_G (12/25-12/29) is highly possible to become weaker, based on the updated forecast data. But it is still capable of bringing another round of mild cold air surges mainly to North America in the week after the Christmas.
STRAT_H (01/02-01/08) is still expected to take place as we forecasted. STRAT_H is going to have twin peaks with one on 01/02 and the other on 01/08. Therefore, colder air is expected to return in the first week of 2016.