The ongoing negative arctic oscillation (-AO) is in-the-face of any significant U.S. warm-up. Overnight, forecasts are trending colder in the 11-15-day period, particularly GFS, as -AO inspires cross-polar flow of arctic chill into the U.S. Consequently, the February 6-12 HDD forecast increases substantially, 23 HDD from 24 hours ago.

The 30-day AO index (Jan. 21-Feb. 19) is projected -3.6 based on recent observations and projections for the 30-day period. The AO index of -3.6 is exceeded only twice as a monthly average in the 1950-2026 climatology: -4.3 (FEB-2010) and -3.8 (JAN-1977). Ongoing -AO (and strengthening -NAO) suggest the current extreme cold may break temporarily and then return.

The projected AO for February is about -2.6 which is historically negative. Similarity was observed in 1985, 1998, 2010, and 2011 when the following February maintained the cold U.S. trough. The modern day -AO analog (2010 and 2021) was slightly less effectively cold. The analogs suggest the East U.S. cold expands westward with time during February.

Leading into the 15+ day period negative phase arctic oscillation continues and negative North Atlantic oscillation strengthens. Coupled with snow cover, the East U.S. cold continues and eventually re-widens. Cross polar arctic air flow fuels another cold blast week-4 ahead for the Central U.S. Additionally, cold week-4 forecasts by AI were correct (from early January) for the current pattern. Finally, strong -AO in January has historical precedent to continue in February.