Good chance for increasingly cold and potentially wintry conditions throughout the eastern U.S. including NC for mid to late December.

Over the past week there have been increasing indications that there will be a shift in the upper level flow pattern over North America beginning later next week… possibly lasting through December. Those indices are pointing toward increasing cold for the eastern United States. This may be more important this year due to our current active subtropical jet and increased precipitation events across the southern U.S.

During the past week the cold air masses have been delivered into the Rockies and Plains… reaching all the way into Deep SouthTexas… and even northern Mexico. This combined with an active subtropical jet brought measurable snow to El Paso and even Houston. Snow fell Saturday over parts of the NC/VA Mountains into the DC area. The cold air mass was blocked by the Appalachians mainly due to the thrust of the initial surge of cold was driven into TX and Mexico… leaving little push to reach over the Appalachians. If the indications below prove accurate… the pushes of cold air will come down the eastern seaboard into our region beginning late next week or during the week of December 14-21.

The leading indicators include: the latest forecast Arctic Oscillation (AO) index (which is forecast to become strongly negative next week and remain there for several weeks)... the North Atlantic Oscillation (NAO) - which is forecast to become moderate to strongly negative during this same period (indicating a strong Greenland/Iceland blocking upper ridge supporting east coast United States troughing)... and the Pacific North American index (PNA)… which is forecast to become positive (which typically teleconnects to a Rockies ridge and Eastern U.S. upper level trough). When the PNA becomes positive combined with a strongly negative NAO… and strongly negative AO… we can get strong outbreaks of cold air into our region... especially in December, January, and February.

Attached are the latest AO… NAO… and PNA forecast indices from the ClimatePredictionCenter. In addition… attached is the December outlook for Temperature and Precipitation… along with the 6-10 day outlooks. It appears that the coldest air will arrive after day 10.

Also of interest from the CPC include the Analog years that are showing up with similar December upper air patterns and indices. They include: December 12-22, 2005, December 3-13, 2002, December 7-31, 1995, December 19-31, 1985, late December 1977, and late December 1963.

Central NC had several bouts of very cold weather and winter weather events during these aforementioned periods. There was a significant ice and snow storm in our region on December 3-4, 2002, and significant icing in a good portion of our region on December 9 and 15, 2005.

Please see the Winter Event Summaries from NWS Raleigh from December 2005 and December 2002 located at the links below:

There were more nuisance ice and cold events in December 1995 for our area. But some near miss winter storms to our NW over VA.

However… significant cold, ice, and at times snow affected the Greensboro and Raleigh areas in December 1985 (December 20 and December 31). Lows fell below 10 degrees.

In December 1977… very cold temps affected GSO and RDU with lows of 10-15 at times… and snow in Greensboro December 30, 1977.

In December 1963… Greensboro and Raleigh were hit with significant ice storms and cold December 22-24 and December 30-31, 1963.

Bottom line… there are strong signals for a turn to much colder weather across our region by mid month… lasting a few weeks. A moderate El Nino event should continue to lead to a very active subtropical jet which may be able to interact with the Polar Jet initiating winter weather or winter storm events along the east coast including NC.

Some analog years corresponding to the best fit to the forecast upper air pattern mid to late December 2009 suggest several past December weeks in the years of 2005, 2002, 1995, 1985, 1977, and 1963 as best fits. Research into the years does indicate bouts of cold, winter storms, and winter weather events across NC.

Stay tuned!