One of the prime WSR-88D Storm Relative Motion (SRM) velocity product is for detecting shear regions, such as mesocyclones, TVS, and upper level divergence. But another SRM application may include tracking boundaries, like cold fronts, marine pushes, thermal troughs, etc. If base velocity data does not provide conclusive evidence on boundary locations, the SRM may be utilized for this purpose. A surface trough moving across eastern Washington on May 22nd, 1996, provides an example for another use of SRM.

During the early morning of May 22nd, a surface trough was moving eastward across the region. The exact location of the trough was not evident from surface aviation observations or other types of data. The closest observations west of the Spokane airport (GEG), besides Fairchild Air Force Base (SKA) 5 miles west, are Moses Lake (MWH) and Ephrata (EPH) at 85 and 90 miles, respectively. The WSR-88D may help pinpoint the frontal location. Unfortunately, the base velocity gave only a hint of the surface trough location (Figure 1), a small area of convergence near Davenport (DAV). Surface east wind were present at the RDA site, but the 0.5ø angle radar beam quickly rose above the inversion about 15 miles away. Hence, the velocity data showed little east wind ahead of the front near Davenport. The PUP operator may not conclude that the trough was at Davenport by examining the base velocity data.

When the default storm speed and direction is subtracted, the location of the surface trough on the SRM chart (Figure 2) becomes more evident. The zero isodop cuts across eastern Washington on a N-S line from Colville (CQV) to northeast of Dayton (DAY). The storm track algorithm calculated an average storm motion of 214ø at 21kts. Since wind speed and direction were different ahead of the front, the SRM chart clearly showed the surface trough. The Composite Reflectivity (Figure 3) showed an area of moderate rain behind the surface trough. A storm center was identified west of Davenport with a forecast motion of 245ø at 32kts, which differed from the average storm motion used in the SRM. (Recall that the average storm motion is determined by all storms detected from the previous volume scan).

The VAD wind profile data from 0814Z to 0913Z (Figure 4) showed east wind near the surface at 0814Z but veered to south at 5000 feet. The surface trough passed the RDA site around 0845Z, and the wind above ground level shifted to west at 20kts. For this particular event, the SRM chart was occasionally used to track the trough into the Idaho panhandle when velocity data failed to show any boundaries.

Before using the SRM chart to locate boundaries, remember the limitations. Recall that the SRM chart is derived by vector substraction of the estimated storm speed and direction. The storm track algorithm provides the default value for average storm speed and direction; however the PUP operator may enter a storm speed and direction on an one-time request only (see JS2-3-25). The purpose of the SRM chart is to detect shear regions, but the PUP operator may use it to find boundaries if other techniques give inconclusive information. In this case, the SRM graphic was used to find a boundary, and not to determine the shear for a particular convective storm.