The Brooklyn, NY Tornado of August 8th, 2007
by Philip Lutzak – August 2007
From the evidence previously discussed, the merging of these two systems resulted in a backward propagating SCS, with its maximum convergence zone and thus heaviest precipitation at the rear of the system. This type of slow-moving MCS often produces flash flooding, as indeed occurred here - rainfall was 3.47 inches at JFK, most of which (3.18 inches) fell in the 2 hours from 6-8AM (JFK metars). In addition, the back (northwestern) edge of the system eventually intersected an area of strong vertical wind shear and significantly high SRH, causing supercell thunderstorms to develop, one of which produced the EF2 tornado in New York City.
In Figures 1 and 2 below are the reflectivity and velocity radar images from the Mount Holly, NJ radar, 76 miles southwest of New York City, as the tornado moved through Staten Island and Brooklyn. These images have been enlarged for close-ups of the New York City area. Understanding that green colors represent winds moving towards the radar (northeast) and reds represent winds moving away from the radar (southwest), Figures 1b and 2b clearly show the signature of a rotation of the mesocyclone within the supercell thunderstorm. In the same images we can also see the eastward motion of the mesocyclone as it moved over extreme northeast Staten Island, across New York Harbor, and into western Brooklyn. A tornado warning was issued at 6:28AM, just as the tornado was moving across Staten Island as an EF1. After touching down on Staten Island, the tornado lifted up briefly over the waters of New York Harbor, but touched down again as an EF2 in Brooklyn two minutes later at 6:30AM.
It should be noted here that the supercell that produced the tornado was an HP supercell, or High Precipitation supercell. From the 2007-08-08 1030 radar, it appears that there may have been as many as three HP supercells in close proximity. Note that they are not discrete supercells, but rather embedded within the surrounding convection. Note also the "kidney bean" shape, a common configuration of HP supercells on radar reflectivity images. These are particularly dangerous when accompanied by a tornado, since the heavy precipitation usually obscures the tornado from view. Eyewitness accounts from Brooklyn, NY support this observation.
Summary The difficulty in forecasting this situation was trying to ascertain the resulting conditions when a fast-moving DCS, associated with derechos and downbursts, was intercepting a slower-moving SCS associated with heavy, possibly flooding rains. From the evidence presented here, the final, merged MCS was an SCS, or Severe Convective System, characterized by slower movement, abundant mid level moisture, and convection concentrated at the rear of the system. It is likely that the faster moving cold pool and gust front of the DCS enhanced convection along the gust front of the SCS as it intercepted it.
As the already energized, highly convective back edge of this system moved over New York City shortly after 5AM, a wind shift from southerly to strong west-southwesterly due to a developing warm front in the low to mid levels may have enhanced the convection along the convergence zone. At this time, as the flooding rains began, an area of vertical shear of 35-40 knots intersected the northwestern side of the SCS, causing supercell thunderstorms to develop within the already heavy convection. As was previously shown, the SRH values were high enough to produce tornadogenesis in one of the supercells moving over New York City. Also, because this was an SCS type of MCS, the system moved more slowly, causing higher rainfall amounts than otherwise might have occurred. What is interesting about this scenario is that while most MCSs that produce flash flooding events occur in low-shear environments, in this case it appears that New York City was in the unfortunate and somewhat rare location during the summer season where an MCS with flooding rains in a generally low-shear environment became impinged upon by enough vertical shear on its back edge to cause a tornado along with the flash flooding.
As a final note, I would like to add a quote/comment from Steve Corfidi of SPC: "Concurrent flash flood and tornado events are most common during the cool season when the mean flow may be strong but largely unidirectional. In these situations deep shear may be limited, but low level shear may be heightened near storm outflow boundaries. Throw in a nearly saturated environment (weak cold pools), and you have the recipe for isolated tornadoes in addition to back-building and/or training storms with flash flooding." So it should be emphasized that this was an event more commonly expected in the cool season, although in this particular situation the impetus for the shear in a nearly saturated environment with strong unidirectional mean flow came from a developing warm front rather than from storm outflow boundaries.
Unusual aspects of this event:
1. The merging of two MCSs of different types (DCS and SCS).
2. Flash Flooding with a strong tornado due to the juxtaposition of low and high shear environments.
3. Frontogenesis (in this case warm frontogenesis) within the area of an MCS.
4. An event during August more commonly associated with the cool season.
For improvements in future forecasting techniques, it seems that a case study of situations where one MCS has overtaken another could be quite fruitful if it could help to determine rules on the outcomes when different MCS types combine.
Author's Note: Many thanks to Stephen Corfidi of SPC for the use of his analysis on MCS types and especially for his review of this work. Also many thanks to Professor Lee Grenci of Penn State for his keen insights into flash flooding and low-shear environments. Finally, thanks to Lee Grenci and Mark Thornton for providing critical radar images to me. I couldn't have done the correct analysis without them.
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