30
Day

As the next arctic airmass of the extended January series surged into the north-central U.S. late this past weekend models shifted markedly colder throughout the entire eastern half of the country to the Gulf and East Coasts in forecasts valid the 1^st half of this week, ensuring yet another several day period of 15°-20° below average temperatures anomalies at the climatological peak of winter. Little time will be available for moderation before an even more intense arctic airmass (coldest of winter) dives between the northern Great Basin and northern Rockies this weekend (Jan 18-19).  This is the 1^st arctic air outbreak of January to enter the U.S. west of the Rockies which ensures fast reversal of above average warmth observed along the West Coast during the 1^st half of winter. Westward displaced entry of arctic air typically sets stage for progressive modification (weakening) as this cold air spreads east of the Rockies to the Gulf and East Coasts during the 3^rd full week of the month (Jan 19-25). However, in this case models significantly limit temperature moderation with ECMWF 11-15 day forecasts coldest across the East, especially the Great Lakes, Ohio Valley, mid Atlantic region, and Northeast where additional snow is likely.  Longer range forecasts of the 30-day MWA ensemble continue to interpret this as the final multiday period of below average temperatures across the central and Eastern U.S. extending into the start of the final week of January, before arctic air steadily contracts into western Canada late month consistent with recent development of La Niña adding confidence to a relatively mild start to February.