The 1 July 2017 Maine Tornadoes: A Brief Mesoscale Analysis and Dual Polarization Perspective

The afternoon and evening hours of 1 July 2017 presented many challenges for the operational staff at the National Weather Service Forecast Office in Gray, Maine. Five tornadoes occurred in southern Maine which is a record for a single day in the state. In addition, numerous damaging straight line wind events occurred as well as significant flash flooding that resulted in a disaster declaration.

A warm frontal boundary draped in a west-east fashion across the forecast area provided a focus for thunderstorm development in the presence of moderate to strong deep and low level bulk shear, moderate surface-based CAPE, and low LCL heights. The juxtaposition of these indices aided in a favorable environment for supercells and tornadoes by early afternoon in the vicinity of the warm front. A brief mesoanalysis will be presented to highlight some of the important ingredients.

Supercell thunderstorms did indeed develop near the warm front across southern Maine and central New Hampshire during the early afternoon hours as a short wave trough approached from the west. Several supercell structures were noted on radar imagery for several hours. In addition, two QLCS tornadoes occurred after the more discrete supercells either weakened or merged with the approaching QLCS. This presentation will focus mainly on dual polarization (dual-pol) and other base data signatures that were witnessed on Gray, ME WSR-88D imagery. These include potential tornado precursors such as rapidly developing front flank hydrometeor size sorting in differential reflectivity (Z_DR) data, Z_DR arcs, and specific differential phase (K_DP) - Z_DR separation.

Events like 1 July 2017 are rare in northern New England. The dual-pol signatures offered Gray, ME forecasters a real-time look at and subsequent training opportunity on potential tornado precursors in the relatively young dual-pol era.