Medium AND extended-range forecasts have trended colder across the East in the 6-10 day period possibly extending to late December. A polar vortex-style weather pattern is expected to develop by next weekend and in varying intensity exist for 2-3 weeks. The colder scenario has caused heating degree days forecasts in the East to increase. The colder East regime is somewhat balanced (nationally) by a very warm pattern in the West. Therefore the national heating degree day forecast for December in very close to the 30-year climatology BUT colder than the string of warm months this just-ended autumn season.
How cold will the mid-to-late December cold outbreaks be? Models settle on high temperatures of only 22-26 with lows in the 12-15 range for Chicago next Saturday. A potentially colder scenario occurs in Chicago one week earlier. In the East cold spot is Pittsburgh were mid-to-upper 20’s for high temperatures and mid teens for lows could occur Dec. 12-17.
The key to the forecast is Canadian snow cover spreading south to prevent too much moderation of the cold air mass once shifting south of snow cover. Right now snow cover is along and north of the U.S./Canada border. However, the cold polar vortex spinning over the relative warm Great Lakes for 1-2 weeks will cause widening snowfall and snow cover. Chance of a white Christmas are increasing for much of the Northeast U.S. despite lack of major storms.