Estimating boundary-relative flow in forecasting convection

Recent evidence has suggested that low convective cell bearing flow orthogonal to the orientation of a surface boundary (low U_b ) is a favorable condition to initiate and maintain deep convection under relatively benign large scale forcing. Recent techniques have been developed to help forecasters estimate U_b upon knowing a boundary motion and mean flow vectors. However, the time needed for calculating U_b has made its application to short-term convective forecasting problematic. In this study, a technique has been generalized to a web-based tool to help expedite the process of estimating U_b for boundaries moving in any direction and speed.

This paper uses a method a forecaster can use to estimate U_b for any boundary and access to the boundary-relative flow web applet. This method can be used as a consideration in assessing the potential of a boundary to initiate and maintain convection under relatively weak large scale forcing. In addition, this technique for estimating U_b may help to anticipate how a boundary influences the mode of organized convection in stronger large scale forcing. Additional applications of this technique can be used to estimate the orthogonal component of shear to a boundary. Several examples show the application of this technique in both weak and strong synoptic forcing with the help of the Advanced Interactive Weather Processing System (AWIPS). The boundary-relative flow applet can be found at http://www.osf.noaa.gov/otb/working/boundary.