Lowering values of U b possibly increased the residence of the cumulus line over the ascending air of the retreating east-west outflow boundaries to help increase the chances of new convection. The inputs into the tool are simply boundary speed and some mean steering-layer flow or an observed cell motion. The applet then shows U b for all boundary orientations. The forecaster need only pick the U b for the boundary orientation closest to that observed. The case presented here showed supporting evidence that U b had some influence in the initiation of outflow initiated convection for a case with near zero shear and a relatively uniform 0-6 km wind field.
Convective initiation remains one of the most difficult of forecasting problems.
Of the many considerations that explain a given propensity for a quasi-two dimensional boundary to initiate convection, the U b proposed by WM97 offers some observational support both in their study and in this one case. None of these considerations can be completely accepted or rejected based on the evidence that exists to date. For weakly forced convection, U b may apply best. However when shear increases, the other considerations may apply over that of U b . Furthermore, there is little understanding on the fundamental mechanisms of convective initiation.