Midlevel guidance

Forecast clues are evident in reported data. Information gleaned from 500-mb weather charts gives valuable insight to coming fronts.

Surface precipitation forecast map that also shows the 1000-500 mb thickness (decameters). Note the 540 dm (5400 m) line extends into the Gulf of Mexico and Florida, indicating a possibility of mixed precipitation over the northern Gulf, and mostly snow in precipitation areas further north.

The second factor is curvature vorticity. When air is forced to flow in a curved path, eg, around a high or low, it has vorticity. Air flowing anticlockwise, such as around a low, will have positive vorticity, while air curving clockwise will have negative vorticity.

The magnitude of the positive or negative vorticity will rise with increasing curvature.
Shear vorticity is the last of the 3 factors. In the sense of 500-mb vorticity, this is horizontal shear, or the change in windspeed as viewed moving outward from the center of a trough or ridge.

Positive shear vorticity is encountered when winds are increasing away from the trough (or ridge) center. Negative shear vorticity is present when the winds decrease with distance from the center of the trough or ridge.

Overall vorticity at 500 mb is the sum of each of the vorticities—Earth, curvature and shear. So the more of these are positive, and the greater their positive magnitude, the more positive will be the overall vorticity.

The most positive values of vorticity are often indicated on 500-mb charts by a small "x." These values are normally found around the base of sharp troughs—generally slightly southeast of the trough center. The lowest (or least positive, in most cases, since the positive Earth vorticity dominates the equation) vorticity values normally occur in the crest of a ridge.

Vorticity and divergence

Positively tilted trough in the 500-mb flow over the eastern US. This deep trough exhibits positive vorticity just off the Atlantic coast of Florida, indicating strong divergence aloft and the likelihood of significant convective activity.

The LND (500-mb) vorticity provides an indication of the overall vorticity of the troposphere (which is least near the surface and greatest at jet stream levels). The greater the positive LND vorticity, the greater the overall divergence and the more dynamically unstable the troposphere becomes.

However, the greatest divergence does not occur directly above the maximum vorticity (vort max)—rather, it occurs downwind of it. On a 500-mb chart, draw a line along the trough axis—then draw a line perpendicular to that axis, through the middle of the vort max.

Your top right quadrant will be the region of greatest positive vorticity advection—the location where the positive vorticity will migrate, and thus the location of the greatest divergence aloft. Similarly, the upper left quadrant will be the region where the greatest negative vorticity advection will take place—and, correspondingly, the greatest convergence of air in the upper tropospheric levels.

Divergence aloft means that air will be lifting and cooling throughout the upper levels of the troposphere, raising pressure height and making the environment more unstable, since air being lofted from the surface will be better able to stay warmer as it cools than the air in the upper levels.

When compared with a surface map, a surface low that resides beneath that upper right quadrant stands the best chance for strengthening, as does a surface high that rests beneath the upper left quadrant of the LND vort max.

Surface pressure centers that do not reside beneath supportive upper air conditions are unlikely to be strength­ened, and are often likely to dissipate as conditions equalize in the lower levels.

But vorticity alone won't provide the entire picture about the potential for adverse or severe weather. Fortunately, other aspects of the 500-mb chart can help with this. For one, the tilt of a trough in the 500-mb flow pattern has significant implications for generating clouds, storms and precipitation.

When you draw a line along the axis of a trough, examine whether it is oriented from northeast to southwest (positively tilted), is straight north-south (neutrally tilted), or tilted from northwest to southeast (negatively tilted).


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