Fig: 1: 12Z ECM ENS 6-10-day temperature anomaly forecast for the U.S.
Discussion: The just-received 12Z ECM ENS is the preferred model run to project the U.S. medium-range temperature anomalies. In the 6-10-day period, the model identifies developing arctic air caused mostly by the strength of the polar vortex across the Northern Rockies to west/northwest Great Plains (Fig. 1). In the Gulf States and along the East Coast, temperatures are mild ahead of the frontal systems carrying the cold air slowly south and east. The warm/cold clash generates storms and in the cold air snow cover begins to pile and cover much of the Great Plains in 15 days, the entire northwest quadrant of the U.S. but showing some reluctancy in the Northeast States (Fig. 2). Note the snowfall across Virginia! In the 11-15-day period the ECM ENS responds to widening snow cover by projecting widespread anomalous chill although limited arctic air (Fig. 3).
Fig: 2: GFS projection of U.S. snow cover in 15 days.
Fig: 3: 12Z ECM ENS 11-15-day temperature anomaly forecast for the U.S.