WX MODEL 17: Shortwave


A shortwave is a small trough that can bring an increased chance for precipitation. The shortwave will have a sharp curvature in the flow pattern. This sharp curvature introduces a vorticity or spin to the air. This is associated with upper level divergence and rising air. Shortwaves can also develop due to flow over a mountain range, along a frontal boundary and with a developing surface low pressure system. These mechanisms produce rising air. Since rising air cools, heights will fall aloft and will fall the most where the rising air is maximized. This produces a kink in the height contours aloft that can lead to the generation of positive vorticity. Flow through the curved height contours will produce positive vorticity advection downwind from the vort max (vort max defined as the center of region of highest vorticity). Shortwaves have important forecast implications since they bring in an increase of cloud cover and increased precipitation chances.

The model image below shows a shortwave over Idaho. Notice the cyclonic kinking of the height contours over Idaho. This is aiding in the generation of vorticity (shown in yellow). Flow through this region of vorticity produces positive vorticity advection downwind from the trough axis. Positive vorticity advection is associated with upper level divergence and rising air. Regions downwind from the center of the vorticity region (such as southeast Idaho) have increased cloud and precipitation chances.