One of the ways to monitor the potential for Arctic air outbreaks in the northern U.S. is to follow what is happening in the stratosphere over the polar region of the northern hemisphere. Sudden stratospheric warming (SSW) events in the region of the North Pole have been found to set off a chain of events in the atmosphere that ultimately lead to Arctic air outbreaks from central Canada into the central and eastern U.S. Indeed, there appears to be a significant stratospheric warming event in the offing over the next 10 days or so (above) centered over the North Pole that could prolong winter-like conditions across the central and eastern U.S. as we progress through March and into the month of April.
During the winter months in the lower polar stratosphere, temperatures on average are below minus 70 degrees Celsius. The cold temperatures are combined with strong westerly winds that form the southern boundary of the stratospheric polar vortex. The polar vortex plays a major role in determining how much Arctic air spills southward toward the mid-latitudes. This dominant structure is sometimes disrupted in some winters or even reversed. Under these circumstances, the temperatures in the lower stratosphere can rise by more than 50 degrees in just a few days. This sets off a reversal in the west-to-east winds and the collapse of the polar vortex. In recent SSW events, the polar vortex has split into two pieces and that opened the floodgates for Arctic air to move southward. In response to the stratospheric warming at the high latitudes, the troposphere in turn cools down dramatically and this cold air displacement is then transported from the tropospheric high latitudes to the tropospheric middle latitudes. The entire process from the initial warming of the stratospheric at high latitudes to the cooling in the troposphere at middle latitudes can take several weeks to unfold. This doesn’t mean that each and every day following an SSW event will be below normal as that will not be the case. However, it does suggest that, based on historical similarities, we could be looking at an overall below-normal temperature pattern in the central and eastern U.S. continuing well into the month of April. Indeed, the very latest NCEP Couple Forecast System (CFS) temperature anomaly forecast (below) for the month of April is colder-than-normal for much of the central and the eastern U.S.