Forecasting Tornado Outbreaks vs. Derecho in the Northeast

Using Storm-relative Flow 

Forecasting Tornado Outbreaks vs. Derechos in the Northeast 

Introduction

Data and Methodology

Patterns

Storm Relative Wind Flow

References

According to Brooks et al. (1994) intensifying mid-level storm-relative winds increase the amount of rain blown away from a thunderstorm updraft, thereby lessening low-level baroclinic generation and the development of strong outflow. For weak storm-relative winds, outflow will be dominant and there will be less chance for low-level tornadic meso-cyclones to develop.  Atmospheric profiles (soundings) were created, estimating general synoptic scale conditions for the 11 tornado outbreak cases and the 10 derecho cases in this study.   Storm-relative winds at 6500 m MSL (mid-level) were calculated  and plotted vs. Bulk Richardson Number (BRN) for each case.  The storm-relative winds were an excellent discriminator between the tornado and derecho cases.   In this study, tornado outbreaks occurred with stronger storm-relative mid-level winds and lower BRNs.  Derechos occurred with lighter storm-relative mid-level winds and a wide range of BRNs.   It should be noted that the cases in the study were major tornado outbreaks and derechos.   Also,  there was some overlap in the two types of events. Some derechos produced weak tornadoes. Bow echoes and squalls lines were also observed during some tornado events.    

From Weisman and Klemp (1986):                                     

        BRN  =    B   
                1/2 U2  

where B is the buoyant energy of the storm environment and U is the density-weighted difference between the mean wind over the lowest 6 km and a representative surface layer (0-500 m) wind.  Supercells were favored with BRNs less than 50 while multicellular storms were favored with BRNs greater than 35.