68 NC counties and 1 VA county are under alert, including Wake, Cumberland, Durham, Johnston, and Orange counties. Details
Published: 2007-07-28 12:05:00
Updated: 2007-07-28 12:05:00
Posted July 28, 2007
MIKE MOSS SAYS: Hunt, You were seeing a good visual reminder of the fact that winds at various altitudes above the ground are not necessarily from the same direction. In fact, it is quite common for there to be significant "directional shear," meaning a change in direction of the winds, in addition to "speed shear," meaning a change in wind speed with height. Many times when thunderstorms are active across the region, especially the stronger and better organized groups, clusters and lines of storms, we find ourselves with west to southwesterly winds above about 5-10,000 feet, while winds in the lowest few thousand feet of the atmosphere are south to southeasterly. In these cases, the storms tend to be advected along in more or less the direction of the deeper winds aloft.
Even in cases where this large scale wind pattern does not exist, you can have significant localized variations in winds with height in the vicinity of thunderstorms, because of the low-level inflow that feeds warm, moist air into the storm in support of (and in part due to) the strong buoyant updrafts that are occuring within the storm itself. Essentially, in this scenario the storm itself helps create the winds that are blowing in the opposite direction of its motion. In some cases, even when winds aloft are light and variable, a storm may appear to move in the opposite direction of its low level inflow winds. This can occur because the storm is advancing by "propagation" rather than advection, meaning that the original storm cell is actually replaced by new cloud mass or a new cell that builds up on the side facing the incoming "fuel" (low level warm, humid air) to replace the original and now diminishing cell.
The bottom line is, just because winds at the surface are blowing toward a storm in the distance, or toward a looming storm nearby, that does not mean the storm will necessarily move away from your location!