Please explain "backing winds" and their significance in tornadogenesis and chase planning stratagies. I thought veering winds were more important but read a lot about backing winds in some TA discussions. Thanks
Backing Winds: Winds which shift in a counterclockwise direction with time at a given location (e.g. from southerly to southeasterly), or change direction in a counterclockwise sense with height (example westerly at the surface but becoming more southerly aloft). The opposite of veering winds. In storm spotting, a backing wind usually refers to the turning of a south or southwest surface wind with time to a more east or southeasterly direction. Backing of the surface wind can increase the potential for tornado development by increasing the directional shear at low levels.
Source: National Weather Service
backing windâ€”In the Northern Hemisphere, a wind that rotates in the counterclockwise direction with increasing height.
Opposite of veering wind.
This is technically correct, of course, but chasers commonly (if incorrectly) refer to, well, let's say "moisture advection", as "backing winds".
When used in that context, backing winds are generally from the S, SE, or E, and what chasers really mean is that moisture and CAPE are rising, or have risen, to unstable, convective levels, and that directional shear is favorable for the development of rotating convection... whether or not there has actually been a change of direction in the surface winds.
Fact is, when we see advection of moisture from the Gulf of Mexico into The Alley (and with it warm temps...=theta-e), we call it backing winds, correct or incorrect.
Mike is correct -- winds that are backing with TIME are winds that are turning counterclockwise with time. Winds that are backing with HEIGHT are winds that turn counterclockwise with height (e.g. west at the sfc, southwest at 850mb, south at 700mb -- a backed wind profile).
Of course, since "backing" (and veering) are relative terms, there really isn't an objective definition of "backed". Most chasers will call winds with an easterly component (SE, E, etc) to be "backed" for most of the southwest-flow-aloft cases. In the plains, as Bob mentioned, this type of wind (S/SE) aids in moisture advection as Gulf moisture is transported northward. I'm not sure I've heard the term "backed" referring to moisture advection as Bob mentioned, but rather that" backed" winds usually ALLOW for moisture advection (given the location of the plains relative to the Gulf), especially for the western/high plains states.