New Forecasts Made on September 25:
As the winter approaches, we are shifting the focus of the sub-seasonal forecasts of cold air outbreaks back to the Northern Hemisphere since September 25. This week, we are forecasting three strong stratospheric circulation events:
There appears to be another relatively small peak of the strong mass transport into the polar stratosphere in period 10/21-10/28, which we name it as STRAT_A. Considering the intensity of STRAT_A is weak, it may bring cold surges over limited regions in the mid-latitudes. Therefore, the Halloween weekend is expected to be mildly cooler. We will keep an eye on the intensity changes of STRAT_A and set alarms if it strengthens.
We also attempt to forecast STRAT_B, which is expected to occur between 11/5 and 11/12. STRAT_B is a week-long event which may be able to cool the surface over the large portions of North America and Eurasia.
Note that during the next two weeks, when the forecast skills of temperature itself are trustworthy, there will be a cold event with below-normal temperatures over up to 60% of the Eurasian continent in the period from the last week of September to the first week of October. This event is found to be accompanied with a stronger stratospheric circulation event occurring between the dates of 9/24-9/29, which actually is the first strong stratospheric circulation event since our forecasts have been launched. A few weeks later, we are forecasting to see a strong stratospheric circulation event occurring between the dates of 10/05-10/10. This event is expected to bring cold surge processes mainly over North America. Since the forecast lead times for these two events is within 2 weeks, we do not name them in this sub-seasonal forecasts.