Snow Cover Makes The Difference In The Strong -AO Pattern Ahead

Fig. 1-2: The Dec. 11-25 AO forecast is vigorously negative and well depicted by the GFS day 6-10 upper air forecast.

Discussion: A vigorous negative phase of the arctic oscillation (-AO) is ahead. In the Dec. 11-25 period, forecast models agree on a -2.2 signature of the AO (Fig. 1). The GFS identifies the attendant high-pressure block near the polar region (Fig. 2). Interestingly, the polar vortex split which results in 4 northern mid-latitude upper troughs are all located farther north than usual for a full-throttle -AO episode.

Due to the unusual farther north displacement of the polar vortex in North America the 6-10 day outlook is warmer-than-normal for the northern U.S. (Fig. 3). The cold climate typically produced by the -AO pattern is there but farther north in Canada. The lack of southern displacement of anomalous chill associated with -AO is likely due to the limited (or no) snow cover across the northern U.S. (Fig. 4).

The last time -AO episodes of stronger than -2.0 occurred in December was 2009 and 2010. Note weekly examples of the cold U.S. signature for each month biased toward the eastern half of the U.S. (Fig. 5-6).

Fig. 3-4: The day 6-10 temperature anomaly forecast is warmer-than-normal across the northern U.S. due to lack of snow cover. Normally a much wider aerial coverage of cold into the U.S. occurs with -AO regimes.

Fig. 5-6: U.S. temperature anomalies for weeks in December of 2009 and 2010 when strong -AO was present.

Midday gas population weight HDD forecast update: Despite the -AO pattern ahead for mid-to-late December (normally a vigorous cold signature for the U.S.) the gas population weight HDD forecast remains warmer than the 10-year normal (Table 1).

Table 1: The gas population weight HDD forecast from the midday GFS compared to 12 and 24 hours ago valid into later December.