This time stratospheric warming is REAL; Arctic air risk to the U.S. in early February is increasing.
Discussion: As a weakening Madden Julian oscillation episode in the tropical Pacific (Fig. 1) shifts back to home base (Indian Ocean/Indonesia) in about 10 days the heat release pole ward caused by this transitions splits the polar vortex (Fig. 2). In-between the two centers a stratospheric warming episode occurs (Fig. 3) which causes arctic air to develop across northwest North America and plummet toward the U.S.
Fig. 1-2: The 15-day MJO forecast indicates a shift from the tropical Pacific back to the Indian Ocean. The heat release pole ward by the tropical convection pole ward during this transition causes the polar vortex to split.
Models vary on how far into the U.S. this air mass will travel but certainly any snow covered areas (Fig. 4) will feel the brunt. Forecast models are also unsure how long the arctic air will last. Models have been consistently warm in recent days for the medium-range holding cold risk to the North-central U.S. (and possibly the Interior West). However, the arctic air episode described looks a bit more ferocious than previously anticipated as the stratospheric warming episode is very supportive.
Fig. 3-4: The cold temperatures associated with the polar vortex in the upper atmosphere shift toward northern Europe in 8-10 days while the stratosphere warms over northwest North America leading to arctic air risk at ground level. Snow cover has advanced into Central and Interior Northeast U.S.
The midday GFS indicates what may happen…waves of arctic air each with increased chance of making a larger impact on the U.S. Mid-next week a potent arctic air mass moves across Central and into East Canada (Fig. 5). Enough cold air shifts into the U.S. to increase snow in the prevailing storm track. The second wave of arctic air for February 9th causes widespread 25-30F below normal temperatures across the U.S. (Fig. 6).
The GFS has a tendency to be spurious however the stratospheric warming is agreed upon by all models and given the time of deep winter has a better chance to generate arctic air.
Fig. 5-6: The 12Z GFS depiction of an arctic air mass mid-next week and second more widespread arctic air mass February 9th.