WRAL WeatherCenter Blog

I have a question yhat has puzzled me for years. Why is it in the winter snow is normally north and west of the triangle and in the summer severe weather is normally south and east of the triangle. Sounds strange but it turns out this way more often than not.

Posted March 25, 2008

MIKE MOSS SAYS:     Reggie,   Although there are many exceptions in the historical record, your observations are largely on target in a general sense, and the reasons have to do with our latitude and the geography that surrounds us. These are somewhat oversimplifications, of course, but here are some reasons for that distribution of wintry and severe weather.

During the winter, surface temperatures tend to be colder toward the north and west, and the depth of cold air likewise tends to become thicker toward the north and west, in part due to the mountains to our west that tend to serve as a block againstcold airmasses that flow southward over the eastern U.S., while at the same time heat stored in the ocean waters and especially supplied by the northward flowing Gulf Stream) tends to moderate the cold air to our south and east. This all comes together in many cases in such a way that the Triangle area winds up on the borderline between liwquid, mixed and frozen precipitation. Snow is more likely where the cold air is deepest, while mixed precipitation, sleet and freezing rain are more likely where very cold air is quite shallow, and rain tends to dominate where the cold air is very shallow or gives way to warmer air drifting in from the east.

Likewise with severe weather, there is a tendency in many severe weather situations that involve a sharp upper level trough to our west and often a front pushing in from the west, for greater amounts of low level moisture and instability to be concentrated closer to the warm ocean waters to our southwest. It is not unusual for a coastal warm front to develop in these situations and drift inand a bit. That boundary can then act as both a focus for low level convergence (increasing upward mothions that can triger thunderstorm cells) and also as a region of enhanced horizontal and vertical low-level wind shear, which can increase the chances of rotating storms capable of longer lives and greater intensity, and thus an enhanced potential for producing severe weather in the form of strong downburst winds, large hail and sometimes tornadoes.

Again, just to emphasize the point, these influences often lead to the distributions of snow and strong storms mentioned above, but there are certainly scenarios that lead to large departures involving severe storms over central and western parts of the North carolina, and likewise occasions when ice storms or heavy snow can occur all the way east to the coast.

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