So many have asked us over the years about the famous "Donut Hole" that appears nearly every time there's a snowfall forecast put out for our area. If you're new to the region, eventually you'll notice it too. It doesn't just show up on our maps, but in the computer models as well. Here are some examples...
...And when it comes time to snowing, sure enough the final snowfall tallies sort of resemble a donut-- a powdered donut:
With a bunch of people on the inside with not much snow looking at everyone else with either the day off of school or at least enough to play with.
(and those model images were just a couple of what has showed up even in the last year).
So what causes this to happen? If we're seeing it on the models then you know it's got to have some physical/mathematical basis. It does, but it's actually the combination of several factors that each war on our ability to receive snowfall. Since this is a good run-through on factors that affect snowfall that are not related to the storm itself, I thought it might be a good topic to tackle today. :-)
Let's start off with the simplest factor: Latitude. In the mind of math, because places farther north receive less heating from the Sun than those farther south, it stands to reason that the better potential for snow falls in the north rather than the south. If we were going to paint a canvas picture of snowfall potential before the first storm arrives, we would start here with a base coat that looks something like this:
Now let's talk about another factor: Mountains. This one is also fairly simple, as we've all been able to figure out over the years that higher elevations are colder, and therefore they can get more snow on that basis alone than the valleys. Here's what our area looks like by elevation:
Pretty standard stuff, with the Appalachian Mountains running east of Charleston on down south and west of there to southern Kentucky. I suppose now the picture already is getting a tad more into focus, as much of the River-Cities area is in that greener valley shade. If we only took elevation into account, our map would then look like this:
Next there's the specific situation that we find ourselves in by virtue of our location near to the Great Lakes: Lake Effect Snow. We also talk about the "Lake Effect" all the time on the air, and it's that enhancement of snowfall due to the moisture on the lakes being carried by the winds all the way down here and deposited in our area. It can enhance an existing storm, or by itself be the genesis of a snowfall event. Being close to the Great Lakes puts you in line to receive the lake effect, but you can also get it farther away provided you are up in elevation. The lifting mechanism when air is forced up hill can squeeze out lake borne moisture even when hundreds of miles away. This is why some of the West Virginia ski resorts can get more than 200" of snow per year while closer to the Ohio River some places struggle to get to 20". Now, remember that lake-effect snow only comes to the slopes that face the lakes. On the other side of the peaks that face away from the peaks don't get lake effect snow enhancement because the wind is going down the mountains at that point (not to mention the fact that the clouds spent all their moisture going uphill the first time). Combining the ingredients of elevation and lake-proximity, we get a snowfall map like this (again, if no other factors matter):
Now there are some smaller factors that come into play as well, but they are not nearly as important as the top three.
There's the effect that proximity to a large body of water has against snowfall when the warm water keeps the land next to it warmer than the air farther away from the water. The Ohio River is a good example of this, because it's not a big/wide enough body of water to generate significant water-based enhancement of snowfall, but it does offer a buffer against cold nights by spreading fog around to the nearby communities. The other smaller rivers we've got do this too, but not to the same degree. Temperatures have been in the single digits recently, but the river water temperature is still around 35. This impact is more noticeable at night, and would only occur when an approaching storm is marginal on its rain/snow line.
There's also the effect of "downsloping". It works similarly to the upsloping that brings extra snowfall to the mountain slopes that face the wind, but in reverse-- it brings less snowfall to places that are downwind from the mountain ridge. The time when we get windflow that is 'downsloping' just happens to be when a storm system is approaching. Winds flow counter-clockwise around an area of low pressure, which means when a storm advances on the region, initially we get winds coming out of the south. The result looks like this:
The downsloping scenario is often what is at work when we wonder why a storm hasn't 'started' yet. The air that is forced down the mountains dries out on its way, which makes it harder for advancing precipitation to reach the ground without itself evaporating into the air. It can also ruin snowfall prospects for very hopeful school students (and teachers).
Putting all of this together, we finally paint a snowfall picture that looks something like this:
...And voila! The makings of the "Donut Hole". You'd have to adjust this picture a little more to account for the river temperatures, and then remember to toss in the effects of downsloping that affect locations north of the mountains (which is again the Ohio River) when a storm approaches. (Oh yeah, and I'm not exactly Picasso here, so be kind when it comes to locations up I-79 that do get more snow than folks along parts of I-64) ;-)
So the next time the computer models advertize an approaching storm snowfall and puts a big 'donut hole' in the River Cities area, now you'll know a little bit more as to how that happens. :-)