What we do: We are making sub-seasonal forecasts for time periods of high probability of cold air outbreaks in Eurasia and North America 30-40 days in advance. We issue such forecasts on a weekly basis and this website is updated around Thursday each week.
New Feature: Maps of probability of cold surface temperature anomalies over both North America and Eurasia continents in the vicinity of forecasted stratospheric PULSE events
New Forecast Made on February 4:
No new forecast is made this week.
Major stratospheric sudden warming (SSW) event
The stratospheric sudden warming event, which we forecasted on 01/07 as a minor SSW event and then upgraded as a major SSW event on 01/20, started on 01/28, just few days earlier than expected. The first peak of STRAT_F significantly decelerated the jet and weakened the polar vortex and finally the second peak broke the polar vortex and caused a major SSW event. The (total) zonal mean zonal wind averaged over 50-80 °N in the stratosphere has been easterly since January 28 and is expected to remain easterly for a few more days.
Therefore, we continue to forecast a long lasting cold period over vast large area in mid-latitudes until the end of STRAT_G (02/15), a total of 3 weeks (01/24 – 01/15).
Follow up on Forecast Made in the Previous Weeks:
STRAT_F (01/23 – 01/28), first forecasted on 12/10 with a 45-day lead, is taking place across the United States. STRAT_F looks to be a twin-peak event. The first peak has already taken place on 01/24, with a peak intensity of 0.6 trillion tons per day. The second as well as the main peak occurred on 01/28, transported about 0.9 trillion tons per day of air mass into the polar stratosphere. The STRAT_F was strong enough to eventually break the polar vortex and caused a major stratospheric sudden warming event on 01/28. The first peak of STRAT_F brought below normal temperature over most area of North American continent, especially west coast and mid-west regions, while most area of extratropical latitudes of Eurasia experienced a remarkable drop in surface temperature as well.
STRAT_F1 (02/02 – 02/06), first forecasted on 01/07, has taken place on 01/31, which is a few days earlier. STRAT_F1 is the first aftermath event after the major stratospheric sudden warming event caused by STRAT_F. Associated with STRAT_F1, another round of cold air outbreaks mainly took over the North American continent, bringing below normal temperature over 60% of the surface area.
STRAT_G (02/11 – 02/15), first forecasted on 01/07, is still expected to occur on time. We consider STRAT_G as the second “aftermath” event that to occur after the major stratospheric sudden warming event during the migration of the polar vortex back to the Arctic. We expect that associated with STRAT_G, mid-latitudes of both continents especially North America will have below-normal temperature.
STRAT_H (02/23 – 02/28), first forecasted on 01/21, is still expected to occur between 02/23 to 02/28 with a peak on 02/25. STRAT_H looks to be a moderate event with a peak net mass transport into the polar stratosphere about 0.7 trillion tons per day. Associated with STRAT_H, we expect modest cold air incursions mainly over the North American continent.
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