The 500-millibar prog represents the level where about one half of the earth's atmospheric mass is below it and half the earth's atmospheric mass is above it. The number 1 item a forecaster looks for on this prog is advection vorticity. Vorticity is a spinning motion (or eddy) created by directional and/or speed changes in the wind field. This model prog is available on UNISYS weather at:
This prog shows colorized 500-millibar absolute vorticity, height contours, and wind vectors. Absolute vorticity is a combination of curvature, shear and Coriolis vorticity. Coriolis vorticity is always positive because the earth rotates counterclockwise when viewed from the North Pole. Curvature or Shear vorticity can be positive OR negative. Since the model shows absolute vorticity, the values of vorticity will be skewed toward positive values (because Coriolis vorticity is always positive). The model determines the value of vorticity for numerous locations across the model panel. These vorticity values are then colored using the scale below the panel.
The normal range of vorticity values are from near zero (or single digit negative) to sometimes more than +30 units. A unit of vorticity is equal to 1 times 10 to the negative 5th with units of seconds to the negative 1. The units of vorticity are derived from the change is wind speed over a horizontal distance. Since wind speed has units of m/s and distance has units of m, the units cancel to seconds to the negative 1. The highest vorticity regions on the panel (caused by the greatest combination of positive shear and positive curvature) are termed vorticity maximums. Positive curvature is a counterclockwise windflow (such as in a trough) and positive shear is a horizontal speed change of wind with distance that causes a counterclockwise rotation.
An eddy of vorticity spins like a low pressure in that it spins counterclockwise. This counterclockwise spin-up causes the air to diverge and rise downwind from the vorticity maximum when there is a wind flow through the vorticity maximum region. A rough guide to the intensity of a vort max is: less than 14: small vorticity; 14 to 20: moderate; 22+ large. The value of the vorticity maximum does not tell the whole story. For strong upward motion to result with a vort max, there must also be a strong wind flow (long wind vectors) flowing through the vort max. A strong gradient in vorticity values is also helpful in strengthening vorticity advection. Strong wind flowing through the vort max will create regions of NVA and PVA, where NVA stands for negative vorticity advection (wind flow region toward the vort max; advection of smaller values of vorticity) and PVA stands for positive vorticity advection (wind flow region away from the vort max; advection of higher values of vorticity). It is the downstream PVA region of a vort max that has upper level divergence and rising air (also called the downwind region of the vort max).
The 500-millibar chart is also important for forecasting the trough / ridge pattern propagation across the forecast area as well as the forecasting of "shortwaves". A shortwave is a lower height kink in the height contours that is about the size of a U.S. state or a little bigger. The kinking creates positive vorticity due to the counterclockwise curvature the kinking produces. The trough / ridge pattern will determine which regions are having above or below normal temperatures and if the flow is conducive to storms systems or quiet (zonal) weather. For further information on shortwaves click here.
For an examples of vorticity advection and further discussion click here.