A harsh period of winter weather is poised to strike the mountain region during the next 7-14 days. This has been well anticipated by large-scale changes amid the atmosphere across the Northern Hemisphere.
Temperatures above the North Pole at 10 MB have recently been at RECORD high ( warm ) levels for this time of year, having warmed dramatically from near record low ( cold ) values in late December and mid January.
Zonal Mean Temperatures in the 60-90 degree latitude range at 10 MB have also peaked at record high ( warm ) levels in recent days.
Record strong Wave 1 forcing has been responsible for the dramatically high stratospheric warming in upper levels.
Wave 1 forcing has been strong through the depth of the stratosphere ( near bottom level above ), but clearly has been strongest in upper levels of the stratosphere.
Wave 2 forcing has been weaker than average this winter.
A sense for what Wave 1 means can be gained by looking at the European Model’s 10 MB analysis ( above ) at 7 AM on February 7, with dramatic warming over Siberia associated with a single ridge and trough couplet ( High and Low at 10 MB in the Stratosphere ). Upward WAF ( Wave Activity Flux ) from extensive Siberian snow has helped to drive this event along with a favorable large-scale pattern that allows for co-location with the MEAN climatological standing wave pattern of the Northern Hemisphere winter-time, in addition to Torques driven by major mountain barriers.
For comparison, above, I have the 10 MB analysis at 7 AM on January 30. Note that the TEMP MAX is listed at -22 C verses the TEMP MAX of +18 C on the previous 10 MB analysis for February 7. That is a 72 degree F ( 40 C ) increase in temperature in 9 days.
Despite dramatic high stratospheric warming, this event has not been able to yet reverse winds at 60 degrees North and 10 MB to meet the technical criteria for a major SSW event ( Sudden Stratospheric Warming ).
There is considerable debate about whether this technical definition for a major SSW event needs to be defined in different terms and/or expanded to also include additional scenarios like this event which has clearly had an important impact upon the Polar Vortex and Northern Hemispheric weather pattern.
The European Model forecast is ever so close to meeting the technical definition, with easterly winds shown at 45 N and 10 MB as well as at 72-90 N and 10 MB but not at the defined 60 N and 10 MB required for the technical declaration of a MAJOR SSW event. So the debate continues.
The Arctic Oscillation ( AO ) which has recently been in a positive phase is forecast to trend back negative for a long period of time. Past climatology, following SSW events, show that -AO periods of 4-6 weeks are not uncommon.
Despite the +ENSO in the Pacific, temperatures this winter have been strongly correlated to the phase of the AO.
Note above how the AO was mostly in a positive phase from November to the start of January ( when we observed above average temperatures ), then shifted into a negative phase through most of January ( when it was cold ). Recently, the AO has been positive and temperatures have been mild. A very high temp correlation this winter to the AO phases.
The North Atlantic Oscillation ( NAO ) is also trending back toward a negative-neutral state on the GFS. The ECMWF Ensembles forecast a more negative NAO trend than the GFS ( I am not allowed to show the ECMWF due to rights established by the European Center For Medium Range Forecasting which require a very costly license ).
A positive Pacific North American ( +PNA ) oscillation phase is predicted to continue, and become very strong in the next few days as a big western North American ridge builds.
The Eastern Pacific Oscillation ( EPO ) is forecast to remain in a negative phase through most of this week, then become positive before dropping again later. Recall last winter that temperatures had a high correlation to the EPO phase.
*Temperatures in the eastern USA almost always tending to have correlation to the PNA phase of various degrees, with western USA ridging naturally correlating to downstream troughing ( and visa versa ), with variations in tilt and amplitude at any given time.
In basic summary, all teleconnection phases are coming into alignment for an arctic outbreak this week with a prolonged negative AO phase upcoming to suggest that colder than average conditions will continue.
**It is very important to again stress that teleconnections are not driving these changes, but instead are in response to large-scale changes and allow us to be able to more clearly predict what such changes may do in the near future to a given weather pattern.
The European Model 100 MB stratospheric forecast ( which I can show ) is supporting this via ridge building and blocking in the North Atlantic into Greenland, and a vortex split and orientation that is acting to drive cross-polar flow from Siberia through Canada into the eastern USA.
This vortex split being only visible up to around 50 MB at 10 day, with the big change in orientation of the Polar Vortex and the pronounced North Atlantic ridge near Greenland being most important and pronounced.
A pattern that is simply primed for delivery of bitterly cold air ( late season ) into the region during the next 7-10 days. Any snow on the ground will act to make this colder.
During the next 10 days the European Model snow forecast is predicting a general 6″ to 16″ to fall along the upslope side of the Appalachians ( with respect to W-NW flow ).
The problem, of course, this is based upon 10:1 snow densities and the upcoming cold will make snow density much lower. Amounts in such a setting could easily be 2-3 times as great. Climatology of past settings like this certainly supports such amounts, especially amid upper elevations in the High Knob Massif and other of the most favored sites such as Snowshoe Mountain in West Virginia, Mount LeConte in Tennessee & Mount Mitchell in North Carolina.
Snowfall will be over a prolonged period of time, and not associated with a single dump. With that noted, the El Nino enhanced subtropical jet is not dead by any means. Merely suppressed to the south. So danger remains that a phasing event between the sub-tropical and polar jets will again occur as happened during January. It is very difficult to say when that might occur, with the most likely time currently being around and just after mid-February when arctic air begins to relax ( allowing the sub-tropical stream a potential entrance into more northern latitudes ).
The Bottom Line…Winter 2015-16 has a great amount of “gas” left in the tank, with a period of harsh winter conditions likely to develop this week into this next weekend. The strong +ENSO event of this winter has not been typical and nothing like that of 1997-98.
Snowfall during the next 1-2+ weeks could be significant, with total amounts that fall likely being greater than depths at any given time due to melting, sublimation, and settlement of snow on the ground over time ( if you want an accurate snowfall reading, then measure and sweep the snow every 6 hours to obtain 4 amounts to add up for each 24-hour total ).
As always, there are likely to be “wrinkles” and changes in this pattern that are not currently seen. The +ENSO jet adds complexity, with fluctuations in the timing of all embedded disturbances in arctic, polar, sub-tropical streams acting to play a role in the type of weather experienced on any given day. This extended is only a general guide to be used for the synoptic ( i.e., large-scale ) pattern, with details of any given day having to be worked out.