In my game it is Fall, 2 days before winter, 48 degrees Fahrenheit...and the ponds are frozen solid with frost on the ground.

Is there a tunable XML or some other way to change the temperature at which this weather happens? I found an XML called Seasons Terrain Manager or something like that, and it has a tunable for changing the temp at which extreme cold weather effects start. But the default value for that tunable is 32 degrees. So the effect I want to change must be governed by some other tunable, if at all. I think.

Actually, the XML is SeasonsManager+TemperatureManager, and the tunable is kExtremeColdThreshold.

It only has to drop below 32 once for the game to start making everything freeze over so if the temperature drops below 32 at night the freezing effects will last around 2 days.
That tunable you're referring to is the one the game uses to start the winter effects when the temperature stays close to or below that.
What you want is in the Seasons XML minimum temperature (morning min, evening min and night min and set it to just above 32 like 33 which is the value I use as the coldest for Spring and Fall.
Look for Spring and Fall just becareful how high you have your minimum temperature set you don't want to accidentally set it higher than the max.

Although sometimes it does snow in early Spring and late Fall and it's kinda funny to watch my sim tend his garden with snow on the ground when I have it set a 33 cause plants with only freeze over when the temperature is at or below 32.
But no matter how high you have your max and min temps trees will always hibernate at around the same time every year as they use some other variable that I've never been able to find.

That is really good information, sittingbear.

I use Tempest to govern the minimum and maximum temperatures for each season, not the Seasons XML. This is because I set three profiles for each season, which the Seasons XML doesn't allow me to do. I'll check my Tempest settings for late Fall and post back.

Do you use Tempest? Because Tempest allows you to change the temperature range for snow. So, if you wanted, you could make it so that it only snows at or below 32. If you're looking for realistic weather like I am, that wouldn't be too far off because in reality, snow doesn't actually occur above 33 F or so. In my game, it really only snows in the Winter, so I haven't had the experience of Sims gardening in the snow.

I checked and in my game, the lowest allowed temperature right now is 42 degrees. So the temperature shouldn't have recently dipped low enough for frost to develop. Don't know what's going on.

I do use Tempest but I also use a tuning mod to hard code the lowest temperature I use. Being a script there is small delay before Tempest kicks in that delay is long enough sometimes for the game to realize it should be colder and send the signal to start freezing things over. And according to the Seasons XML the maximum high temperature that snow will fall is 40.

I had to double check where I live we have large inland lake which can make it snow when temperatures are as high as 45 according to the town records. The town I live in looks kinda like Midnight Hollow just not as dark.

Well, I learned something new. I didn't know it could snow at that high of a temp. I looked it up and it sounds like that is pretty rare, except in some areas where the conditions are right. If your town looks like Midnight Hollow then it sounds like you live at a relatively high elevation, which would explain why you can get snow at temperatures like that. I've never seen snow in the 40s but then again I've never lived far from sea level.

I guess I'll just have to mess around with the Seasons XML to bring the settings closer to my in-game Tempest settings.

You could say that but you'll never find it on map as most don't even know there is lake or let alone small town in that area as the town filled with social misfits, outcasts or in other words just plain old weirdos. About 7000 feet above sea level nuzzled in the Rockies. It was an old coal mining town originally founded in 1830 but abandoned in 1860 when the mines dried up. Re-founded in 1881 by 3 families of well misfits. With the exception of a few things like internet where pretty much off the grid only 1500 people live there.