BLOG: Mountains vs Fronts - | WBTV Charlotte

BLOG: Mountains vs Fronts


I received a good question from William Rodgers on my Facebook page recently. He noted how we meteorologists occasionally refer to cold fronts “breaking up” as they cross our North Carolina mountains and he wondered why that happens.

First of all, not all fronts fail to make it across the mountains, if so, we’d be in a very arid climate here. But their angle of attack can make a big difference in how effective the mountains are in weakening the front’s impact.

Clouds and their associated rain and storms are formed by rising air. That is why summertime thunderstorms are so common as the low-level hot air rises up into the cooler atmosphere. But rising hot air is not the only way to push air upward. Any horizontal wind which encounters a mountain chain will obviously be deflected upward. And it is no surprise mountains on average get more rain than surrounding regions. The mountains in Hawaii average 400” of rain every year; our average around Charlotte is about 40”.

See the accompanying picture. As the front approaches the mountains, the air associated with it starts to rise which enhances the rain formation on the western slopes. But notice on the lee side of the mountains the air starts to sink. That motion is counterproductive to clouds and rain as sinking air tends to warm and dry. And that is also why deserts tend to form on the lee side of mountains.  

If the air mass is approaching from the northwest (Ohio Valley), the mountains are very effective at wringing out the moisture. But we also get systems approaching from the Gulf of Mexico and also from the Atlantic. With systems coming from those directions, the mountains have little if any effect.

Finally, as winter approaches and storms head up the east coast, you’ll hear us refer to ‘wrap-around moisture’ which describes the circulation around the backside of these storms blowing moist air back into the mountains from Tennessee. In this case, the rising air can produce additional snowfall in the high country while everyone outside the mountains doesn’t see a thing.

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