Worst fuel mileage always 1st 2 weeks of Dec.
 

I live in what you would call a salted road portion of the Northern US. I have noticed that at this time of year, and EVERY year, my MPG drops (PREDICTABLY) like a leaf falling from a tree. Literally, when the leaves fall, so does my MPG.

It's to be expected, many factors play a role in this. Presumably you have a winter blend fuel there? Perhaps the switch to that makes you notice an instant drop.

I notice exactly the same here in Scotland. As soon as the temperature starts to drop and made worse by wet roads. You notice that where you descend a hill on a closed throttle, you have to open the throttle earlier in the winter than you do in the summer. The tarmac is much colder, and usually the tyres are colder too. I also find that from a cold morning start I have to drive further before the temperature gets up to the normal operating level (the Jazz fas a blue light that goes out at 55 degrees C.

Winters are already there having the fuel consumption and mileage problem will be common for overcoming it start your vehicle each morning for avoiding the starting problem you might get faced.