The Cape Cod Tornadoes of 23 July 2019: Integrating Research on Northeast Tornado Environments and Dual-Polarization Radar to Provide Increased Warning Lead Time

On 23 July 2019 two EF-1 tornadoes struck Cape Cod, Massachusetts. The first tornado touched down near Kalmus Beach in Barnstable at 15:57 UTC (11:57 AM EDT) and the second tornado struck Harwich around 16:10 UTC (12:10 PM EDT). The tornadoes uprooted or snapped nearly two hundred trees and caused extensive damage to homes and a motel. These tornadoes were produced by a supercell thunderstorm that developed south of Long Island several hours prior and likely produced waterspouts as it tracked over the coastal waters into Vineyard and Nantucket Sounds. Aside from a weak EF-0 tornado in Woods Hole in October 2018 these were the first tornadoes to significantly impact Cape Cod since August 1977 when a F-1 tornado struck Barnstable County Airport in Hyannis.

A recent NOAA Hollings Research Project at the National Weather Service (NWS) Weather Forecast Office (WFO) in Boston/Norton found that favorable environments for tornadoes in the Northeast include relatively low instability, high shear, a tropical air mass with precipitable water values in excess of 50 mm, and the presence of a nearby low-level boundary. The project also studied radar signatures and a Warning Guide was developed to assist WFO Boston/Norton forecasters in the Warning Decision-Making process. Staff training was reinforced through group workshops and self-paced exercises.

Forecasters at WFO Boston/Norton, Massachusetts integrated this training into operations on 23 July 2019. Tornado Warnings were issued for Cape Cod with lead times of 16 minutes and 31 minutes, respectively.

This presentation will review of the synoptic and mesoscale environments prior to tornadogenesis and how they are typical of Northeast tornado cases. An examination of radar images, including dual-polarization products, will be used to show the evolution of this event and how the Warning Guide was used operationally to provide increased lead time for the Tornado Warnings.