1) Trends in “Omega” blocking, AO, NAO
“High-latitude blocking” is typically characterized by persistent high pressure in northern latitude areas such as Greenland and northern Canada. Without this type of upper-air pattern in the atmosphere, it would be quite difficult to get sustained cold air masses in, for example, the Mid-Atlantic region during the winter season; especially, during El Nino (warm) events. Coastal storms in the I-95 corridor absent sustained cold air would be much more likely to generate rain or snow changing to rain in the big cities along I-95.
One way to assess the likelihood for “high-latitude blocking” in the upcoming winter season is to monitor patterns during the preceding summer and fall seasons. In this past summer and during the fall so far, there has been much in the way of “Omega” blocking in the upper atmosphere with numerous highs and lows around the globe. This kind of pattern unusually results in slow movement of surface highs and lows and can lead to extended stretches of good or bad weather conditions. In fact, this “omega” blocking pattern has resulted in some extreme rain events in recent months where, for example, Greece and Libya suffered through excessive rain events as they were underneath slow-moving upper-level troughs of low pressure for extended periods of time. The recent history of frequent “high-latitude blocking” aloft is a moderately favorable sign for additional blocking events to occur during this upcoming winter season which, in turn, favors cold air intrusions into the eastern states.
Another way to assess the likelihood for “high-latitude blocking” is to monitor trends in teleconnection indices known as the Arctic Oscillation (AO) and its closely-related cousin called the North Atlantic Oscillation (NAO). The Arctic Oscillation refers to opposing atmospheric pressure patterns in middle and high latitudes. When the AO is positive, for example, surface pressure is low in the polar region and this helps the mid-latitude jet stream to blow strongly and consistently from west-to-east keeping Arctic air locked up in the polar region. When the AO index is negative, there tends to be high pressure in the polar regions (i.e., “high-latitude blocking”), weaker zonal winds, and greater movement of polar air into the middle latitudes. While the AO and NAO indices are primarily used during by forecasters during the winter season, trends in summer and fall seasons can provide important clues about the ensuing winter season.
Evidence shows that when AO and NAO values are largely negative in summer and fall, the subsequent winter season will typically have frequent “negative” AO and NAO periods which are correlated with “high-latitude blocking” patterns. As it turned out, the AO signal through the summer and fall has been rather neutral whereas the NAO has been often on the negative side. As a result, I would consider the combination of these two signals to be slightly favorable for “high latitude blocking” events to take place this winter season.
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