Tornadic convection in the New York City Metropolitan Region: The 8 August 2007 event and a composite analysis

There has been little research on how severe convection evolves in an urban-coastal environment, such as New York City. Nowcasting convection in this area can be difficult, since storms evolve rapidly as they encounter the marine boundary layer, urban heating, and land-sea breezes. Warm season convection often weakens approaching the coast as a result of the cooler sea surface temperatures, but occasionally severe convection will persist across the coast. For example, early in the morning on 8 August 2007, a mesoscale convective system (MCS) produced two (EF1 and EF2) tornados in the New York City area. This was the first documented tornado in Brooklyn in over 100 years. Both tornados occurred well before the thermodynamic peak of the day (1000-1100 UTC), and caused tens of millions of dollars in damage and the associated severe flooding that disrupted all modes of transportation throughout the city.

This talk will highlight the synoptic and mesoscale evolution of the August 2007 tornado event that impacted NYC using the data-rich observing network around the New York City area. Two Terminal Doppler Weather Radars (TDWR's) at JFK and EWR airports provided superior observational capability as compared to surrounding WSR-88D radars, given their proximity and 1-minute low-level scan strategies. Real-time mesonet surface observations, and ACARS soundings also provided additional spatial and temporal detail of the low-level winds and instability. This event developed as a mid-level (700 mb) trough approached the East Coast. There was marginal instability upstream over Pennsylvania (PA), but the convection intensified during the night as a nose of moderate instability (CAPE ~1500 J/kg) advected northward along the coast. A surface trough (mesolow) developed with the convection over eastern PA and New Jersey during the night, which increased the shear in the lowest 1 km to ~40 kts near NYC and the surface winds backed more to southeasterly. The TDWR radar data illustrate the rapid evolution of the tornadic mesocyclones near the coast. The mesocyclones developed along a wind shift boundary associated with the surface trough and a weak baroclinic zone. A composite of 15 tornado events over NYC-Long Island during the past 20 years will illustrate that the synoptic evolution during the August 2007 event is similar to many other tornado events in this area.