[Increase in October snowpack across Siberia (white=snowcover, yellow=ice); courtesy US National Ice Center/NOAA]
One of the key reasons provided in the mid-month release of Vencore Weather’s “Winter Outlook” (http://vencoreweather.com/2015/10/14/400-pm-2015-2016-winter-outlook-for-the-mid-atlantic-region/) for the expectation of a snowy winter in DC, Philly and NYC had to do with the fact that there were numerous favorable signs for “high-latitude blocking” this upcoming winter. “High latitude blocking” is an atmospheric phenomenon that can be tracked by meteorologists through an index called the Arctic Oscillation (AO) and it often leads to cold air outbreaks in the Northeast US (usually with a negative index value). Studies have shown that an increase in snowpack across Siberia during the month of October – specifically in areas south of 60°N – is pretty well correlated with persistent negative AO index values in subsequent winter months and “high latitude blocking” events (http://web.mit.edu/~jlcohen/www/papers/Gong_JC03.pdf). This in turn is often correlated with cold air outbreaks in the Northeast US which, of course, is a necessary requirement around here for snow. Indeed, the snowpack across Siberia has increased dramatically from the beginning of October to now – even in those areas south of 60°N. In fact, there are reports that in just the last three days the Siberian snowpack has increased by over 2 million square kilometers (source WxRisk.com).
[Current Northern Hemisphere snowpack (brown=snowcover); courtesy Rutgers Snow Lab]
The top figure compares the snow cover (white region) across Siberia from the end of September to today’s level. There has been a steady increase in snowpack in Siberia over the past few weeks from the mainly snow-free grounds at the beginning of the month. In addition to northern Canada and Greenland, Siberia can be a crucial cold air source for the northeastern states during any given winter season. It is not uncommon for an air mass to build up over a several day period in the wintertime over a snow-packed Siberia and then have it make a move across the North Pole into northern Canada and then eventually into the northern US. The second figure above displays a current view of the snow pack across the Northern Hemisphere and - in addition to Siberia - northern Canada and Greenland are now well covered by snow (brown region; data courtesy Rutgers Snow Lab). (One final note, there is an unofficial report of the lowest October temperature ever recorded in Greenland on 10/24 at -67.72°F).