If you have watched any local news in the past 12 -18 hours you probably heard about the possibility of snow in our forecast. If you haven’t well here you go – it might snow tonight. Winter precipitation of any kind is notoriously difficult to predict in our part of the world because so many factors have to come together juuuust right to make it possible. Forecasters are usually especially hesitant to forecast snow outright, as the set up for it is even more fragile than other types of frozen precipitation, and because it garners the strongest reaction from people especially if it doesn’t happen. Seeing snowflakes down here requires a number of ingredients,that must come together at the same time, but rarely do. Here is a breakdown of the players involved tonight.
Cold air – Our usual winter precipitation set up involves a thin layer of cold air at the surface with a warmer layer above it higher in the atmosphere. Precipitation falls as rain through that higher warm layer and then those raindrops either freeze in the cold layer hugging the surface – turning into sleet- or remain liquid and freeze on contact with the surface as freezing rain. For snow you have to generally have to have the air be below freezing from the surface all the way up through the clouds. Getting that thicker layer of cold air here is difficult, but it is possible tonight as a second wave of cold air spills into the region.
Distance – Tonight temperatures will fall into the mid 30s, at ground level. Although this is warmer than the magic number of 32, snow may still be possible. The air just above the ground will be below freezing, and the altitude of that freezing air will be low enough that any snow falling from the clouds may not have enough time to melt before it makes it to the ground. But the location of the freezing level in the atmosphere is notoriously fickle and difficult to forecast once you get this far south. A change of a couple hundred feet can make all the difference between snowflakes and a lot of sad faces. However since temperatures will be above freezing on the ground, any snowflakes that do make it down will likely be mixed with rain as some of that snow will melt on the way down.
Moisture – There has to be enough moisture in the atmosphere to even produce precipitation. An issue with snow here is that the cold air necessary for it is also typically very dry – and the surge of colder air coming in tonight above will indeed be significantly dryer. If that push of dry air arrives before the freezing level makes it close enough to the ground our precipitation will shut off before it is cold enough for snow. It is going to be a very close call for a lot of the area. Oddly enough it looks like those further south and closer to the coast have a better shot at snowflakes since they will hold on to the moisture a bit longer.
Tonight it will be a race against time for each of these factors to come together. Since the temperature at the surface will remain above freezing we wont see any freezing rain, very little if any sleet, and any snow that does fall will melt on contact with the ground.
Right now it appears that the window for this is between about 9 PM tonight and 3 AM tomorrow morning and anything that does fall will be light. I am not saying it is definitely going to snow at your house, but I’d keep an eye out for a couple of flakes tonight as this is the best chance we have had for it in while. I’ll be back later with an update as more forecast models come in this afternoon.
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