Highlight: Emerging -PNA/+NAO indicates West warmth shifts eastward. Can sudden colder changes return? Yes! Stratosphere looks warm 2nd week of December.
Fig. 1: Lead climate signals heading into December are emerging positive North Atlantic oscillation/negative Pacific North America pattern which supports warmth shifting into the East.
Discussion: Most prominent in today’s medium-range forecast are the reversal in the North Atlantic oscillation/Pacific North America pattern(s) beginning later this week and indicating warmth in the West fades while emerging across the eastern half of the nation (Fig. 1). The -PNA pattern should also lead to an increase in rain and snow in California in the extended-range. The 12Z GFS is generally representative of the -PNA/+NAO shift. In the 6-10-day period anomalous warmth is swept eastward and across the southern 70% of the U.S. mostly to the south of northern U.S. snow cover (Fig. 2). In the 11-15-day period the +NAO transition is completed as the Northeast Corridor turns very warm (Fig. 3). In the medium-range precipitation outlooks there is a snowy forecast along the U.S./Canada border while wet events affect 4-Corners and the Mid-south States in the 6-10-day period (Fig. 4). In the 11-15-day period a wetter regime appears in California due to the -PNA pattern (Fig. 5). The U.S. gas population weight HDD forecast is much warmer than 48 hours ago due to the more confident warm influence of a +NAO pattern (Fig. 6).
Is the warmer forecast absolutely confident? No. The stratospheric warming concern is trending stronger. However, the stratospheric warming is not centered over the North Pole but rather Canada which implies questionable influence on ground level (Fig. 7-8). Nevertheless, warm stratospheric conditions in the medium-range require close monitoring for a possible sudden colder change. Also, the stratosphere is cold over Eurasia supporting lack of cold arctic air masses for that region in early-to-middle December.
Fig. 2-3: The U.S. 12Z GFS day 6-10/day 11-15 temperature anomaly forecast.
Fig. 4-5: The U.S. 12Z GFS day 6-10/day 11-15 precipitation anomaly forecast.
Fig. 6: Using all models, the U.S. gas population weight HDD forecast for the U.S. through mid-December identifying the consensus forecast, outlook from 48 hours ago and the 30-year/10-year normal.
Fig. 7-8: The 12Z GFS continues to indicate robust warming of the stratosphere across Alaska/Canada in 10 and 15 days.
For the current week, forecast highlights are an “atmospheric river” Pacific storm track into the Canadian West Coast extending south to Washington where Flash Flood Watch areas remain in effect. The NOAA/WPC 7-day precipitation outlook identifies the excessive precipitation for Washington State and northward while elsewhere storms are minimal and feature mostly snow Great Lakes to the Interior Northeast (Fig. 9).
Fig. 9: The 12Z GFS continues to indicate robust warming of the stratosphere across Alaska/Canada in 10 and 15 days.