MIKE MOSS SAYS: Grayson, Supercell thunderstorms are those thunderstorms that feature a rotating updraft column and a self-perpetuating organization that fosters long life and potential for severe weather including strong downdraft winds, large hail, heavy rain and sometimes tornadoes. Supercell storms are less common than other storm types, and depend on some large scale weather features to produce an environment favorable for their formation.
Some of these features include moist air near the surface, moderately moist air in the mid-levels of the atmosphere (8-15,000 feet or so), unstable air (meaning a difference between warm low level temperatures and much colder mid-level temperatures that is sufficient to cause air that is lifted to become significantly buoyant compared to it's surroundings and to then rise rapidly on its own), a source of lift such as the appropriate sector of a passing jet stream segment or jet streak, frontal boundary, mid-level short wave trough, or surface low pressure center, and finally significant vertical wind shear (this can be in the form of speeds increasing with height, wind direction changing rapidly with height, especially through the lowest few thousand feet, or a combination of the two).
It turns out that climatologically, the geographic region of the U.S. stretching from about southeast Texas and Louisiana up through Oklahoma and Nebraska and over into Iowa and Illinois (very roughly speaking) is favored to develop this combination of factors, due in part to the tendency for low level warm, moist air to flow into this region from the Gulf of Mexico at times when strong upper level troughs are crossing the western U.S. and frontal boundaries are pushing southward into the plains and Mississippi Valley. As it happens, the Rocky Mountains often give a boost to the intensification of surface low pressure systems in the lee of the mountain range (therefore over the western plains) and this just enhances the transport of warm, moist gulf air northward on the east side of the system. It also increases low level winds having a southerly direction, while the upper level trough swinging in from the west produces more of a west or southwest wind direction over the same region and is often quite strong (the location of a jet stream or jet streak) due to the contrasting temperatures at lower altitudes. From late winter until late spring, features of this sort regularly develop over the western U.S., intensify with assistance from the Rockies over the central U.S. and then often are diverted toward the northeast and weakened by the presence of a persistent high pressure ridge in the southeast and along the Atlantic coast.
This last point is probably the one that really helps us out in terms of likelihood of supercell storms. While we get our share of them here in NC, the number of instances in which a strong upper trough makes it to our part of the country at a relatively low latitude, while a strong flow of low level warm, most air develops over the area and the other factors involved become relatively intense, is limited in comparison to the area between the Rockies and the Appalachians.
Again, it's important to stress that we are not by any means immune from the formation of supercells here. A number of them develop over our state every year and lead to severe weather up to and including significant tornadoes. But the geography of North America conspires with meteorological patterns at our latitude to make them notably less frequent, and in many cases somewhat less intense here, than in some areas farther west.
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