Posted on November 23, 2015 by the SCO Winter Outlook Team
This year's Winter Outlook series concludes with the official outlook from our office.
Over the past two weeks, we've laid out the puzzle pieces for this year's Winter Outlook. Now we're ready to put them together and outline our predictions for the coming winter. First, let's review the background from our Outlook posts so far:
Current Pacific sea surface temperature anomalies (Image from NOAA CPC)
The current El Niño event is on track to be the second-strongest since 1950. The warm sea surface temperatures in the central equatorial Pacific should reach their peak strength by mid-winter. The atmospheric connections of this oceanic event should bring winter-long impacts -- namely, an enhanced and active storm track over the Gulf of Mexico and Atlantic coast.
Warm water off the west coast of North America -- characteristic of a positive-phase Pacific Decadal Oscillation -- should help enhance the effects of El Niño. In addition, this should support ridging over the Western US and troughing in the East, which would help funnel colder air down the east coast.
Cold air can also become available when the polar vortex weakens, the polar jet stream relaxes, and patterns such as the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) enter a favorable configuration for Arctic air to surge southward.
Typical impacts during a negative NAO phase. (Figure by NWS Huntsville, AL)
These patterns can change quickly and are often tough to predict far in advance, but researchers have found three key atmospheric precursors of a weakening wintertime polar vortex.
Two of those three ingredients are in place this fall. The strong El Niño should help build a persistent North Pacific low, and above-normal Siberian snowfall in October should help establish an Eastern European high. Both features can help weaken the polar vortex, which can eventually force it into a weaker, wavier configuration that favors that cold air diving south.
The key word there is eventually. These features don't always have an immediate impact, so even in years with favorable patterns, December is rarely cold and wintry in North Carolina. For that reason, we're expecting that our major wintry weather chances may not begin until January.
To see the possibilities for this winter, we identified five past years with moderate to strong El Niño events and varying polar conditions. Several common themes emerged from these years.
Average snowfall in analog years to 2015-16.
Four of the five had a warm December followed by a cool January and February. Four of them also had near- to above-normal precipitation.
Each winter had at least two major accumulating snow events somewhere in the state. Not every year was snowier than normal -- the 1997-98 winter had little snow outside the mountains -- but on average, they produced above-normal snowfall across the state.
More than 70% of the wintry events in these years came from late December to late January, further suggesting that's the time to watch for wintry weather chances to emerge this year.
Even though meteorological winter begins December 1st, don't expect an immediate shift to a wintry pattern then. We expect near- to above-normal temperatures this December with few to no wintry events, thanks in part to a strong polar vortex and more northerly storm track for most of the month.
Several indicators suggest that January will see a transition to a colder and more wintry pattern. As the atmospheric impacts of El Niño and Siberian snow accumulations kick in, polar conditions should become more favorable to supplying cold air across the Southeast US.
We anticipate the favorable wintry regime will continue into February, with ongoing potential for cold air to interact with moisture-laden coastal storms.
As the current El Niño event weakens heading into March, the active storm track and cold air availability is also expected to wane, likely signaling an end to wintry potential. This could happen as early as mid-to-late February or possibly early March.