2007-07-06 Outflow Boundary Thunderstorm in NYC



  Here's what happened on Friday afternoon/evening. The 2007-07-06 21Z HPC surface chart shows the trough overhead and the approaching cold front. The 2007-07-06 critical fields from the SPC told us the air was quite unstable directly over us and southern Connecticut. But since the axis of the 500mb trough had passed just east of us, and the highest 500mb vorticity and 300mb divergence had moved over west and south CT, it appeared the chances for a lot of lift over us were decreasing by 21Z. The NWS had only a slight chance for scattered severe thunderstorms for the evening, and we thought the chances too small. True to the data we were looking at, a large area of severe thunderstorms developed and continued over southern CT, NYC had nothing, and the few storms to our northwest were dying out.

  But just as synoptic scale lift from above can fire up thunderstorms in unstable air, we can lift it from the bottom in the mesoscale to get it rising, and that's what happened in this case. Although we saw the southwestward-moving outflow boundary blow out from the CT thunderstorm cluster, I remember thinking that you don't see them moving in that direction that often around here, and that it would either ignite thunderstorms to our east or die out before it got here.


Figure 1 (left). Our notorious outflow boundary on 2007-07-06 at 22Z. Courtesy RAP/UCAR.


But, as we know now, it held together and moved a lot further west than I thought it would. It just kept on going, and as it undercut the unstable air over NYC and Long Island, it managed to lift the air and ignite a whole line of thunderstorms from Brooklyn out into the Atlantic. I would call it a little "mesoscale back door front."

Here's the 2007-07-06 radars showing the outflow boundary and the subsequent ignition of a new line of thunderstorms from Brooklyn out into the ocean. By the way, just after I got home from Manhattan, we had, here in central west Brooklyn, 1 minute of thunder and raindrops the size of quarters (had to be melting hail), but that was it; we were on the northwest edge of the strong storm that blossomed over central Brooklyn and moved southeastward. But in south central Brooklyn they had torrential rain and penny sized hail. 

  So in the other 4 boroughs they'd say our forecast was perfect, but in Brooklyn they said "throw the bums out".