Discussion: During mid-to-late autumn snow cover has expanded southward across both North America and Asia (Fig. 1). As of November 1st the snow cover in North America was the second-most expansive in the 1966-2018 climatology (Fig. 2). The rapidly expanding snow cover was caused by a much stronger than normal polar vortex creating cold wind across open water just south of the polar ice cap to produce plentiful snows initially in northern latitudes gradually spreading south.
Fig. 1-2: The northern hemisphere snow cover (left) and snow cover anomalies (right) provided by the Rutgers Snow Laboratory.
The strong polar vortex in the farther north latitudes is representative of the positive phase of the arctic oscillation (+AO). In 2018 the strong polar vortex has been most persistent across or near Greenland to northeast Canada (Fig. 3). This part of the AO expanse is represented by the North Atlantic oscillation (NAO). The strong polar vortex in this region is not unique to autumn but most all of 2018. Positive phase of NAO representing this climate pattern is the strongest on record easily more powerful than runners up 1989 and 1967 (Fig. 4).
Fig. 3: The strongest annual polar vortex on record over northeast Canada to Greenland.
Fig. 4: The strongest +NAO on record in 2018 compared to the 2nd and 3rd strongest annual +NAO regimes since 1950.
However, the persistent +AO and especially +NAO pattern is forecast to change abruptly and with intensity. The 14-day NAO forecast indicates an emerging strong negative phase (Fig. 5) as the Greenland polar vortex is wiped out and replaced by a blocking high pressure ridge. The high latitude/high pressure ridge over the northern Atlantic Ocean may be emerging due to the warming of the atmosphere over relative warm ocean water of the Norwegian Sea while surrounding continents (North America and Asia) are mostly snow covered with cooling atmosphere across the snow cover.
Fig. 5: The NCDC/PSD 14-day NAO forecast indicates an emerging strong negative phase. –NAO means the Greenland polar vortex is replaced by a blocking high pressure system.
The –NAO pattern will force prevailing storm tracks farther south – away from the polar region and cause colder mid-latitude climate for North America, Europe and Asia. Snow cover expanse into the middle latitudes will continue (Fig. 6-7).
Widening snow cover into the middle latitudes as meteorological winter arrives (December 1st) can be very difficult to dislodge as sun angle lowers and solar radiation to heat the ground becomes very weak. Normally, a mild maritime influence is required once meteorological winter arrives to melt middle latitude snow cover. Otherwise a colder pattern tries to lock-in with reluctance to ease.
Fig. 6-7: The ECMWF 10-day snowfall forecast for the U.S. and Europe indicates expansion in the U.S. and bare ground replaced by patchy snows in Europe.
Fig. 8: The strong polar vortex in 2018 occurs during very low solar energy as the solar cycle has entered the minimum phase. The strong +NAO/polar vortex patterns of 1967 and 1989 occurred when solar activity was increasing or at peak suggesting the solar cycle has limited influence on the extremeness of the polar vortex in the +NAO extreme years.