The cold January experienced is likely to be repeated during February ( perhaps March ) as the upcoming thaw period is merely a reloading interval for more wintry conditions ahead.
A stratospheric warming event is beginning and looks to become an important player in weather conditions for the remainder of Winter 2015-16 ( whether or not the warming becomes a major stratospheric event is yet to be seen ).
Observe how the change in phase of the AO from positive through December changed to negative at the beginning of January ( when colder weather overspread the eastern USA ).
Temperatures this winter have had a strong positive correlation to the phase of the Arctic Oscillation ( AO ) which is currently expected to trend positive to close January and open the month of February.
Given the previous trend in the AO it is not surprising to see a shift into above average temperature to close January and open up February. However, this positive trend in the AO is different from that observed during autumn-December and is now much more strongly linked to coupling between the stratosphere-troposphere than earlier in the season.
The latest 6-10 and 8-14 day outlooks are already reflecting the flip back to cold conditions as the AO trends negative.
Note how temperatures above the North Pole have been very cold, below climatology ( below ), but that the forecast now shows a sudden increase in temperatures over the North Pole at 10 MB heading into February. Note warming to near climatological mean levels occurred in early January but it was nothing compared to the magnitude of this predicted warming. Still even this warming is not yet shown to generate a technically Major SSW.
An upward flux of wave activity above extensive snowpack across Siberia initiated changes in the Polar Vortex which acted to drive the AO phase shift observed at the beginning of January.
Models predict new surges of WAF ( Wave Activity Flux ) during the next few weeks to greatly perturb the polar vortex & drive a Sudden Stratospheric Warming ( SSW ).
The European Model is predicting around 40 degrees F of warming at 10 MB in the stratosphere, over Siberia, by the end of the first week in February ( above ).
It is well understood that a Major SSW event does not technically occur until winds reverse direction at 60 degrees North & 10 MB.
There is some debate, perhaps well justified, that this type of strict definition needs to be modified.
That is why I term the above a stratospheric warming without reference to it being a Major SSW. That may or may not happen. Any technically Major SSW would tend to have large impacts on the Polar Vortex, but warming periods as observed earlier this month and which appear now do impact the vortex.
Very strong Wave 1 forcing is predicted to continue through this coming week, with new surges in WAF likely during the second week of February ( to further perturb the Polar Vortex ).
Observe that Wave 1 forcing ( below ) is currently near record levels with near surges in Wave 1 forcing ( above ) predicted during the next 10 days.
This initially tends to push the AO positive ( note above that a +AO spike occurred in December before the sharp negative phase developed into January ). The SSW event upcoming is expected to drive the AO negative again ( this time it will be long-lived in nature if past climatology is any indicator ) in its wake, or during a prolonged SSW period.
The polar vortex is like a great blob or amoeba and is constantly changing shape. It is forecast to change both shape and orientation during the next 10 days.
Current indications suggest building heights over the northwestern portion of North America and Alaska will combine with circulation around the PV to again develop cross-polar flow into central-eastern portions of North America beyond this upcoming thaw period.
A big western North American ridge and eastern USA trough pattern is looking likely by later next week into the week of February 7-13. Along with that may come the next MAJOR Winter Storm threat for the southern Appalachians.
The Bottom Line…The potential for more arctic air mass surges into the eastern USA is increasing for February ( even March ) as a sudden stratospheric warming ( perhaps a major event ) weakens the Polar Vortex and allows for development of cross-polar flow from Siberia through Canada into the USA ( +PNA ).
A continued active sub-tropical jet enhanced by the +ENSO is likely to generate more storms, with the next potentially major winter storm event for the eastern USA and southern Appalachians timed for the week of February 7-13 ( timing subject to change ).
February 3 Update – Odds of an important fall of snow is increasing for the February 8-9 period, especially along the upslope side of the mountains.