2:50 PM | *Winter just keeps hanging on in parts of the Northern Hemisphere*

Teleconnection indices of AO (top) and NAO (bottom) are firmaly in negative territory which often favors cold air outbreaks into the middle latitudes - even in the month of May; courtesy NOAA

Overview

It snowed on Sunday in the Swiss capital of Bern and it certainly wasn’t enough to disrupt travel or most outdoor activities as it only amounted to about an inch and a half.  It was, however, the latest snowfall ever recorded in Bern with the previous latest date for snow on May 1st in 1945. The unusual cold snap that led to this snowfall in Europe looks like it may continue for another ten days or so before more normal temperatures return to the central part of the continent.  The unusually late cold and snow hasn’t been confined to that side of the Atlantic Ocean as parts of the interior western US, for example, continue to get accumulating snowfall even as we slide well into the month of May.  In fact, there is a strong likelihood that Denver, Colorado receives another couple inches of snow later this week and new accumulations are possible in other parts of the Rocky Mountains.

Much of the European continent will be colder-than-normal in days 1-5 (left) and days 6-10 (right); courtesy ECMWF, tropicaltidbits.com

Discussion

There probably are many frustrated snow lovers in the central and eastern US who closely monitor teleconnection indices such as the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO).  The reason is it just so happens these two indices are in sharply negative territory right now which is exactly what snow lovers would love to see in the month of January or February as they typically favor cold and snow during the winter season.  Even in the month of May, negative AO and NAO index values can result in cold air outbreaks for the middle latitudes of the Northern Hemisphere and that is exactly what has taken place in recent weeks.

Much of Canada and the US will average out colder-than-normal in the days 6-10 time period (May 12-May 17); courtesy ECMWF, truewx.com

In Europe, all indications are that colder-than-normal conditions will continue for the next ten days or so and this could result in more snow for the Alps right in the heart of the continent.  In the US, more cold air outbreaks are destined over the next ten days or so and this will continue an overall wet pattern for much of the nation and also continue the threat for severe weather including tornado outbreaks.  There has been and will continue to be quite a divide from the cold air across the northern and western US and the warmth in the Southeast US and this has set up a “battle zone” in the central Plains and Ohio/Mississippi Valleys. Tornado watches are, in fact, up this afternoon in portions of Texas and Oklahoma as cold air to the north and west clashes with warm, humid air to the south and east.

Negative AO (left) and negative NAO (right) in the month of May typically result in colder-than-normal weather conditions for the western US; courtesy NOAA, MAD teleconnections

Low pressure will produce more snow across the Rockies in coming days and Denver, CO could see two or three inches from late Wednesday night into Thursday.  Even the Sierra Mountains of California can get additional snow accumulations in coming days to add to the massive amounts that piled up out there during the past winter season.

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com