Summary: Forecast models continue to indicate a major stratospheric warming episode steadily strengthening over north/northeast Asia through the next 10 days (Fig. 1). The episode will trigger the coldest air mass of winter 2018-19 (so far) primarily targeting Russia, at least initially.
Fig. 1: NCEP GFS 10 MB temperature anomaly forecast in 10 days.
What causes a stratospheric warming event? Despite the name, stratospheric warming events initiate in the lower atmosphere (troposphere) where weather occurs. In this case a large mass of convection associated with the Madden Julian oscillation in the Indian Ocean and Maritime Continent produced an energy wave, known as a Rossby Wave. The Rossby Wave weakens turning pole ward out of the tropics and into the stratosphere where the warmth caused by deep convection near the equator is released into the pole ward upper atmosphere causing weakening of the polar vortex and a warmer stratosphere. The polar vortex has a tendency to split and shift toward the middle latitudes causing cold air outbreaks at the surface. The polar vortex split toward the middle latitudes is easily identified by negative phase of the arctic oscillation.
Fig. 2: A pole ward Rossby Wave is emitted by persistent deep convection in the Indian Ocean/Maritime Continent tropics causing the stratospheric warming event over north/northeast Asia.
What is the arctic oscillation forecast? In the short-term the AO responds properly to the stratospheric warming event generating over northern Asia. But! The AO shifts out of the negative phase in the medium-range forecast which implies, at least for the moment, the arctic air generation by the stratospheric warming event is confined to Russia in the short-term and less so anywhere else later this month. Why?
Fig. 3: A blocking pattern emerges the next week or so caused by the stratospheric warming episode over northeast Asia but –AO is less certain days 8-14.
Deep “polar vortex” modifies: By this weekend temperatures across Central Russia will plummet to -30F to -40F as stratospheric warming causes the polar vortex to shift south and intensify over Central Asia (Fig. 4). Note that in the Pacific Ocean a well-established convection pattern in the tropical West Pacific (MJO) emits warmth northeastward into the prevailing Pacific jet stream pattern that farther downstream warms the U.S.
In 10 days the stratospheric warming episode has blossomed over northeast Asia. The polar vortex over Asia caused by this event is progressive and shifts toward Japan (Fig. 5). Deep wintery chill is expected with this trough into Eastern Asia with this east-shifting polar vortex. Note that the polar vortex shift toward Japan coupled with the persistent MJO induced warming into the middle latitudes of the Pacific Ocean makes the jet stream mover “wavy” i.e. stronger trough/ridge short wave energy. Downstream a polar trough forms over the West U.S., not an arctic trough but a more moderated Pacific polar trough.
Finally, on New Year’s Eve the ECM ENS indicates the polar vortex shifts to the Aleutian Islands (Fig. 6). The maritime influence causes the polar vortex to broaden and weaken. The arctic air generation eases due to proximity of the polar vortex to the warming ocean. The MJO warming remains and makes the Pacific flow into the Northwest U.S. faster than normal and farther downstream Pacific maritime air keeps the U.S. mild.
Fig. 4: By Friday the emerging stratospheric warming event over north/northeast Asia spawns a harsh/frigid polar vortex over Central Russia.
Fig. 5: In 9 days the polar vortex shifts toward Japan.
Fig. 6: By New Year’s Eve the polar vortex broadens/weakens due to maritime influences as feature crosses the North Pacific.