...This is the best time of the year for finding deep old coins!
The best you could hope for is when the soil is soft and saturated from snow, melt, rain, snow, melt, and then you get a couple inches of snow on top, just enough insulate the ground and keep the frost out during the next cold snap. Under those conditions, targets like rusty square nails don't false as bad as usual and the deep coins glow in the ground between them.
The other thing is you can see every foot-print and every swing of the coil in the snow. It's a real eye opener to look back at an area you've just gone over and see just how little of it you've actually really covered.
Even when the frost finally gets into the grassy areas, you can still dig in the bush for quite awhile as the fallen leaves and litter help insulate the ground a little longer. A good time to wade through brush when the leaves are all down and the tall weeds are dead and dry and snap off when you move through. You'd never get though some of those areas, certainly not as easily in the summer. No ticks or mosquitos either. You're usually good to go in the bush right into the winter until the snow gets too deep to wade through.
When everything else freezes up or the snow gets too deep, if you can find a river that is low you can still generally dig through the stones and in the wet gravel along the banks.
There's a lot of days that the wind is just too bitter cold, and you gotta be content cleaning and sorting the finds from last season, doing research and dreaming over old maps. Still in all, in the past several years, I've been able to get out detecting at least a couple times in every month of the year. This January has been mild, mild so it hasn't been a problem at all so far, but for sure it's gonna get worse before it gets better.
Last February, I dug an 1856 US Quarter dollar out of the banks of the river and that got me through until the thaw in March.
Never had a problem with the machine (garrett 1250/2500) in the cold other than the cold saps the batteries pretty fast and they are good for one or two times out tops (get a couple sets of rechargeables). The environmental cover is plenty of protection against rain and snow. Also consider putting a weedeater strap on your machine, you can pick up a cheap nylon one for a couple of bucks at the hardware store, and that way the machine always hangs at your side and you never set it in the mud and snow when you kneel to retrieve a target. A couple knee pads are nice in the mud and snow as well. Long-johns too!
Other than that, if it isn't blistering miserable snot freezing cold, and you can find soft ground anywhere, dig it!
Steve,
Ontario, Canada