2:00 PM | *Interesting signals for the end of the month with significant storm potential*

12Z GFS predicts strong high-latitude blocking at the end of the month along with a deep upper-level trough over the eastern US; courtesy NOAA, tropicaltidbits.com

Overview

Numerous cold air outbreaks are likely to drop southeastward into the eastern US over the next couple of weeks and there are some interesting signals suggesting a possible strong storm could form near the east coast as October winds down. Long-range computer forecast maps strongly hint at “high-latitude blocking” to form late this month and teleconnection indices known as the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) are predicted to tank sharply into negative territory which supports this idea. Of course, this is still in the “speculation” phase, but late October has a way of ending with some interesting storms along the east coast of the US including Hurricane Sandy in 2012 and the “perfect” storm around Halloween Day in 1991. While these two are extreme examples, there is some reason to believe an east coast storm could form as the month winds down and there will be plenty of cold air around.

High-latitude blocking and the AO/NAO indices

High-latitude blocking is generally characterized by tenacious high pressure in northern latitude areas such as Greenland, northern Canada and Iceland. Without this type of pattern, it would is difficult to get sustained cold air masses in the Mid-Atlantic region which is, of course, quite critical for accumulating snow events. High-latitude blocking is tracked by meteorologists through indices known as the Arctic Oscillation (AO) and its closely-related cousin called the North Atlantic Oscillation (NAO).

AO (top) and NAO (bottom) indices tank sharply by the end of the month (observed in black, forecast in red); courtesy NOAA

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 such as the Mid-Atlantic region.

While the AO and NAO indices are primarily used by forecasters during the winter season, trends in the fall season can provide important clues about the ensuing winter season. Specifically, negative AO index values in October typically translate to negative values during the following winter season and the trend this October has been on the negative side. The latest forecasts of the AO and NAO indices for the next couple of weeks suggest they will drop sharply in coming days and into negative territory as October winds down and there will be plenty of cold air around.

Stay tuned…it could get pretty interesting along the east coast in the 10/25-10/30 time period.

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com