89th American Meteorological Society Annual Meeting

Tuesday, 13 January 2009: 2:00 PM
Evolution of severe convection in the New York City Metropolitan Region
Room 124A (Phoenix Convention Center)
Brian A. Colle, SUNY, Stony Brook, NY ; and K. Lombardo and J. Murray
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 first highlight the spatial distribution and evolution of deep convection over the New York Metropolitan Region during the warm season (April through September). The frequency of convection at each grid point over the Northeast U.S. was obtained by summing every 15-min the composite reflectivity values that are at least 45 dbZ. There is a sharp gradient in convective frequency immediately west of the coast (around New York City) as a result of the cooler marine boundary layer. As the warm season progresses, the convective activity shifts more towards the coast, which is consistent with the warming sea surface temperatures.

Using the data-rich observing network around the New York City area, the synoptic and mesoscale evolution of the August 2007 tornado event that impacted NYC will be highlighted. Two Terminal Doppler Weather Radars (TDWR's) at JFK and EWR airports provided superior observational capability as compared to the 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 with an approaching mid-level (700 mb) trough. A surface trough (mesolow) de. veloped with the convection over eas

tern 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 to south-southeasterly. The TDWR radar data illustrate the rapid evolution of the tornadic mesocyclones near the coast.

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