Brief history of the Atlantic basin in December since 1950
Storms "named" during
the month of December Since 1950-
TS #14 1953
STS #2 1975
Hurricane Lili 1984
TS Odette 2003
TS Peter 2003
TS Zeta 2005
STS Olga 2007
Attempting to make heads or tails of a trend here without the benefit of widespread satellite technology is certainly a little challenging. And I guess one has to ask themselves if they believe ship and air reports since 1950 have been good enough not to have missed any that would have formed between roughly 1953 and 1975. But since the 70's anyway, satellites have been in widespread use, and it might just be very reasonable to conclude that beginning this decade a marked upturn in the number of nameable December sub and/or tropical cyclones has been underway.
Next, if one accepts that during this decade a pronounced increase in the number of off-season named storms has been underway, the follow up many ask is, "why?" Has there been a liberalizing of standards at the NHC, for example? Certainly former NHC Director (1974-1987) Dr. Neil Frank, believes so. He is squarely on record as suggesting that new technologies have lead to speedier trigger-firing of names out of the hurricane center since his departure. On the other hand, prior to the available improved modern analysis techniques, many argue that several systems back then would not get classified nowadays, as they may not have had a closed surface circulation, or were even really just extra-tropical cyclones misinterpreted as subtropical or tropical.
Has there possibly been a climate connection, related to climate change? Some logic exists to back such a supposition up here, as well. It has recently been shown that the tropics are extending up to hundreds of miles outward, arguably in response to global warming; and certainly waters staying warmer, longer, and the atmosphere becoming a bit more tropical, progressively poleward, suggests that tropical weather, and thus tropical cyclones, could be forming in regions and during months previously considered far too chilly, dry, windy, or otherwise hostile. However, once again and on the other hand, a direct link between global warming and tropical cyclones is far from established.
One interesting feature about Olga (2007) pointed out by noted tropical cyclone researcher Clark Evans, is that Olga appears to be our first December named storm since 1953's TS #14 that developed from a tropical wave... with the named system continuing to head west. Excepting 1953 and Olga, in the instances I've listed above, the names were given to features that generally developed from cold core to warm core transitioning extra-tropical cyclones (the most typical way subtropical storms are formed - such as Epsilon, and more recently Andrea, just to name two that became media darlings), and the others were born as more classic depressions from waves of low pressure along decaying fronts, but headed eastward.
Just a bit of tropical trivia, perhaps, and I am certainly not sure of what it means, other than to highlight how rare an event of the type Olga seems to be, actually is.