Summary

The severe thunderstorms of June 2, 1998 occurred in a very unstable synoptic scale environment across Western New York and Southern Ontario. Some of the most important ingredients that produced the storms included:

1. Favorable location at the left entrance region of the upper level jet.
2. Diffluent flow at 500 mb.
3. Strong mid level forcing as indicated by the 700mb omega field.
4. Strong low level forcing and moisture availability as indicated by the 850mb
wind max and theta-e ridge.
5. Strong convergence along surface trof..

In addition, the veering wind profile favored supercell development. Helicity values were augmented by the development of a right moving storm and the backing of the low level wind field along the lake breeze front.

Finally, the lake breeze boundaries that set up during the afternoon hours as a precursor to the severe weather may have had an additional role in enhancing the potential for tornadic storms. Pearce et al. (1998), Maddox et al. (1980) and Doswell et al. (1980) all describe how thermal boundaries can concentrate moisture convergence and horizontal vorticity as a source for thunderstorm updrafts. Updrafts that interact with a thermal boundary (such as a lake breeze boundary in our case) can tilt baroclinically generated horizontal vorticity into vertical vorticity, causing storms to rotate.

Pearce, Matt and G. Forbes, 1998: "Mesoscale boundaries, intersections, and storm-relative environmental helicity as factors in weak supercell and tornado formation over Pennsylvania. Preprints, 16th Conf. on Weather Analysis and Forecasting. Amer. Meteor. Soc., 109-111, Phoenix, AZ.

Doswell, C. A. III, A.R. Moller and R.W. Pryzbylinski, 1990: A unified set of conceptual models for variations on the supercell theme. Preprints, 16th Conf. Severe Local Storms, Kananaskis park, Alberta, Canada, Amer. Meteo. Soc., 40-45.

Maddox, R. A., L.R. Hoxit and C. F. Chappell, 1980: A study of tornadic thunderstorm interactions with thermal boundaries. Mon. Wea. Rev., 108, 322-326.