Over-running Precipitation: Nature's Trojan Horse
By Steve MacLaughlin on November 28, 2012, 9:00pm Last modified: November 30, 2012, 10:00am
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As we prepare for a long, Northeast winter here in Connecticut and the harsh reality that we are very unlikely to get off as easy as we did last year, we examine different ways winter weather can strike.
The most known type of storm in the winter is a good old "nor'easter," an area of low pressure that moves up the east coast and whose northeast winds (thus the name) can bring in cold and moist air at the same time...a recipe for big-time snow.
Another storm type is the "Alberta Clipper." This is a quick moving storm that comes from Canada. Sometimes it can move in from the northwest...sometimes it passes to our north. Either way, it usually does not lead to any big-time problems, but can still give kids a day off or an early dismissal if there's enough moisture.
"Lake-Effect Snow" happens when cold air rushes over the un-frozen Great Lakes. It is most prevelant in late November, December and into early January. Once the Lakes freeze, the snow-machine shuts off. The first significant round of Lake-Effect this season actually happened the second half of Thanksgiving weekend. While it can pound areas in the snowbelt southeast of the Lakes like Buffalo and Cleveland among other cities with feet upon feet of snow, the flakes very rarely make it to Connecticut. But every now and then, if the northwest winds are strong enough, we can see some snow here...even last weekend we saw a few snow showers...those showers came all the way from the Great Lakes.
Cold fronts can also bring snow, but since cold fronts come through pretty quickly, it's usually not much. Plus, since cold air is moving in and cold air is drier, the snow usually happens along the front and on the warm, moist side of the front and pushes out as soon as the cold air moves in.
But another type of set-up, while much less flashy or renowned and very under-rated, is responsible for many ugly days in an average winter. This type of event is more frequent than a nor'easter. In fact, it can actually be part of nor'easter. It's called over-running precipitation, and while it happens throughout the entire year, in the winter, the proximity of warm and cold air can make a mess on the ground and drive us weather people crazy.
Ove-running occurs with warm fronts. Warm air is lighter and less dense than cold air and therefore it rises. Warm air from the south gently rides up and over the colder air down on the ground where we live.
Cold fronts, on the other hand, are much easier to forecast. The cold air from the north rapidly pushes the warm air out of the way. The weather tends to be quick but can be also be much heavier and even severe when we are in the warmer season. It's easier to predict because once a cold front starts moving, it usually keeps moving until it's out of Connecticut.
But warm fronts are different. The precipitation may not be violent, but more steady and long-lasting. Warm front have a lot of juice and since they move very slowly or even become nearly-stationary, can cause major problems in the winter. Warm fronts can also cause all four types of precipitation at nearly the same time and nearly the same place: snow, sleet, freezing rain and rain. Figuring out what type of precip will happen when and exactly where is maddening.
Nor'easters can be tough, but once you figure out the path of the low, you can figure out the rain/snow line. And because nor'easters move up the east coast, we can look to Richmond, Washington, Baltimore, Philly and New York to see how intense the storm is and what we can expect in Connecticut. But with slow-moving warm fronts, we usually see rain moving from west to east from PA and NY. When that rain hits a cold pocket of air we begin to see snow or sleet or freezing rain, but finding that spot can be next to impossible until we see the snow or ice actually begin to fall.
Last year when we were dealing with one of our rare snow storms in late February, we expected snow and ice with some light accumulation. But our computer models showed temperatures just warm enough to give us a wintry mix. Our models thought the warm front was in one place when it was really in another. Instead of a wintry mix we got a few hours of heavy, wet snow. Our orinigal forecast for 1-2" with ice inland turned into 3-5" across the entire state.
And a month earlier in January of 2012, overrunning once again nailed us. We knew we would see a good snowstorm and 6"+ in some spots was certainly not out of the question. But there was no way to know exactly where that little band of heavy precip would decide to set up. We were at the mercy of the warm front and its precise, exact location and the location of the most intense areas of snow. As it ends up, everyone got some snow, but from Northern Fairfield County all the way east to Northern New London County we got a strip of 10"+. Again, the strip was literally unpredictable until the snow began to fall because when we are dealing with a warm front....sound familiar? Parts of this same strip just north of I-95 got over a foot of snow a couple of weeks ago while other area saw little or no snow at all.
A slight shift in the warm front, only by a few miles can be the difference between rain or snow; between snow and freezing rain; between a few inches and a foot. So...beware of warm fronts in the winter. They sound harmless and who doesn't like warmer temps? But remember, it's warm air that causes snow...not cold air. Sure, it's needs to be cold enough for freezing. But when it's right around 32 degrees and warm temperatures try to move in from the south, that's when we can get some nasty snow storms and get caught off guard with some incredible numbers.
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