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 January 21:
STRAT_H (02/23 – 02/28)
We are forecasting a new stratospheric PULSE event this week, named as STRAT_H. It is 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.
Major stratospheric sudden warming (SSW) event (02/03 – 02/08)
The strongest PULSE in this winter so far, STRAT_E, decelerated the jet and weakened the polar vortex significantly. A cluster of back-to-back PULSE events composed of STRAT_F and STRAT_F1 is coming soon. STRAT_F and STRAT_F1 are expected to be driven by strengthened wanenumber-1 and wavenumber-2 waves, respectively. There is a high probability that the polar vortex would break up completely. Therefore, we this week upgrade the intensity of the SSW following STRAT_F1 forecasted on 01/07 from a minor event to a major event. The major SSW event is expected to occur a few days earlier, but still around STRAT_F1.
We forecast a long lasting cold period over vast large area in mid-latitudes from STRAT_F (01/23) to STRAT_G (02/15), a total of 2-3 weeks.
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. STRAT_F looks to be a twin-peak event. The first peak is very small, which is going to take place on 01/23 with its peak intensity at 0.6 trillion tons per day. The forecast of the first peak is very tricky because of the back and forth changes in the peak time and intensity around 01/23. We captured the twin-peak structure since 12/30, and revised it to a 1-peak event last week. The second as well as the main peak looks to be successfully forecasted since 12/10, and the cold air outbreaks are mainly associated with this strong second peak. It is going to occur on 01/28, transporting about 1.2 trillion tons per day of air mass into the polar stratosphere. Highly consistent with our forecasts since 12/10, the 1-week GFS forecasts of surface air temperature anomalies show that most area of extratropical latitudes of Eurasia is expected to experience below-normal temperatures, while North America will experience a remarkable drop in surface temperature over North American continent, especially the western coast, during the last week of January.
STRAT_F1 (02/02 – 02/06), first forecasted on 01/07, is still expected to occur on time right after the STRAT_F between 02/02 and 02/06 with a peak on 02/03. STRAT_F1, together with the STRAT_F event, forms a cluster of PULSE events, which is expected to break up the stratospheric polar vortex. Associated with STRAT_F1 is another round of cold air outbreaks mainly over the North American continent.
STRAT_G (02/11 – 02/15), first forecasted on 01/07, is still expected to occur on time. We consider STRAT_G as an “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.
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