Fig. 1-3: GFS ENS days 1-15 temperature anomaly forecast for the U.S. and Canada and the last two very warm Central/East months of October.
Discussion: The October 2021 temperature anomaly outlook is ferocious warm Central and into the East. The GFS ENS is super warm east of the Continental Divide for most of the next 15 days (Fig. 1) and the remainder of October is likely to feature very little change. The West will cool-down and impressively so during the medium-range. However, the eastern 2/3 of the U.S. may receive a record warm mid-autumn. The last two very warm mid-autumn U.S. thermal patterns for the Central and East U.S. (Fig. 2-3) occurred in 2016 (3rd warmest October since 1895) and 2007 (8th warmest October since 1895). October 2021 has a good chance to eclipse both previous years of anomalous warm mid-autumn.
The front end of the cold season for the U.S. heating season is November. Although before meteorological winter, strong cold in November can produce a high heating demand. Conversely, a warm November pattern can keep anticipated increasing heating demand very low. From a climate perspective, occurrence of an extreme should be compared to past similar years as an analog to project what (historically) follows.
Following the 3rd warmest October on record in 2016, the month of November 2016 was 2nd warmest on record (Fig. 4). The OCT/NOV 2016 warm combination is the strongest for this time of the year on record. Following October 2007, the month of November was 6th warmest on record in the Southwest but chilly air was present and the high heating demand of the Northeast Corridor was 39th coldest of 113 years (Fig. 5).
Fig. 4-5: November 2016 and 2007 temperature anomalies for the U.S.
The catalyst to the warm October 2016 pattern was very similar to October 2021, a strong negative phase of the Pacific North America (-PNA) index regime which promotes chilly Northwest/West U.S. climate but warmth everywhere else. Although slightly different character, the October 2007 climate pattern across North America also featured -PNA. The following November, the warm oceans off each coast cooled off dramatically rendering upper trough(s) and in-between a warm upper ridge dominated the U.S. climate in 2016. The oceans were not as anomalous warm during mid-to-late autumn 2007. Instead, a blocking high-pressure system formed south of Greenland forcing a cold trough farther upstream over Eastern Canada which cooled New England.
The anomalous warmth of the mid-latitude oceans during autumn 2016 have similarities to 2021 but not 2007. From an analog perspective, November has a better chance of maintain a warm pattern vs. a colder reversal from October. Diagnostically, the warm October prevents early season snow cover from generating across Canada. Consequently, lack of snow heading into November in the northern latitudes lowers the risk of developing cold air masses which can be delivered to the U.S.
So…following snow cover becomes very important as to a leading diagnostic for cold weather risk on the front end of the U.S. heating season. Right now, the weather forecasts support below normal snow cover heading toward late autumn which favors a warm start to the core of the U.S. heating season.