DISCUSSION: As shown in the upper-most graphic, over the last 24 to 48 hours there was a fairly strong low/mdi-level low pressure system slowly moving across much of far southern Europe. This particular extra-tropical cyclone focused the majority of its higher-magnitude impacts across a good portion of central and southern Greece. As shown in the video (attached just above) you can clearly see during the course of this particular eyewitness footage (courtesy of Agis Poulkou), there were several different occurrences of thundersnow and they evidently occurred in a number of different parts of southern Greece including (but not necessary limited to) the capital city of Athens based on the different scenery which appeared throughout the different segments of this particular snowstorm footage compilation video. The environment became favorable for convection in Greece over the last couple of days due to a significant increase in regional warm/moist air advection running into the cold air presiding over much of central and eastern Europe over the past several days and weeks for that matter. This evolving low/mid-level convective environment promoted a more favorable Wintertime set-up that ultimately promoted the developement of cumulonimbus clouds within the core of this strong winter storm's precipitation shield. As noted above, here is some of the great first-hand footage of the snowy conditions and thundersnow which occurred in and around the greater Athens metropolitan region on the 29th of December 2016.
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©2016 Meteorologist Jordan Rabinowitz