Thursday, October 30, 2008

The AO (Arctic Oscillation) is about to change the pattern near the middle of November to a suppressed Jet Stream and Colder air.

The AO (Arctic Oscillation) which is a key teleconnection for the CONUS, is likely to aid in a colder and more suppressed jet stream for the second half of November into December. Over the past month or so the AO index has been strongly positive, ranging from +2 to +4. When the AO is in this state during the fall and winter months, it allows very cold air to become locked up in the arctic and polar regions of the Northern Hemisphere.

This is why we've seen a massive expansion of ice and well ahead of schedule so far this year. Current indications are for a big switch in this signal and a very sharp drop over the next couple of weeks. As we head into later November the ensembles are showing a NEG AO headed for -2 to -3 Values. In many cases a NEG AO allows for colder air to shift south and SE into the CONUS from the Arctic and Canada. Depending on other teleconnections, such as the NAO and PNA, it can lead to a gradual suppression and feedback, or deep trough amplifications into the nation.

Left is a POS AO signal / Right is a NEG AO signal

At this time, I'm thinking this big swing/shift in the AO, will lead to a suppression in the Jet Stream's northern Branch, starting near the middle of November, and this continues into the later part of the month. This will help in developing a broad based trough across the central and eastern part of the country. I say this because it appears during this same time frame, the NAO and PNA signals will be Near Neutral. If the NAO were to be NEG and the PNA POS, this would lead to a sharp and amplified trough in the east and big west coast ridge. I don't see this happening just yet for November.

As we head into the later part of November and into early December, the NEG AO and suppressed jet stream implications, could lead to the NAO going NEG as Greenland and Atlantic blocking develop with this new pattern. Feedback with this change will be gradual as the index drop will be steep, and will take some time to catch up with the pattern change in response time of the atmosphere adjusting.

My thoughts for November in the Eastern US:

The beginning of the month we start out slightly above average and head to near and slightly below normal temps near Mid-month. The second half of the month, I expect temperatures to average Below normal, with increased storminess and above normal precipitation. With a Colder and Stormy pattern for the second half. So, my overall call for the Month of November 2008 is for Slightly Below Normal temps, and Above average precipitation for a good portion of the Eastern US.

It's likely this pattern will continue into December, with possible increased trough amplification and coastal storms due to the NAO going NEG. A good shot for a cold and stormy December and the snow geese a flying.

Take Care,

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