July 7 2009 Night time Supercell

• Convection developed over Central NY and then intensified into two discrete line segments in low CAPE/high shear environment. One of these segments developed a low topped supercell tracking se along the Lower Hudson Valley/NE NJ border into Nassau County. Severe hail and winds were experienced with this storm.

Radar

Reflectivity Storm Relative Velocity

Velocity Tornado Vortex algorithm output

Developing Supercell

Supercell…with nice inflow notch and strong mesocyclone

TVS signature

Instability

Only marginal elevated instability to work with

Marginal to little surface instability

Surface Analysis

Weak surface trough…providing low-level convergence and moisture pooling

Weak surface low providing some low-level convergence and shear

There is not much instability, and only a weak surface low…so what was the forcing for these storms?

100 kt upper level jet streak approaching region!!…with strong upper level divergence

60 kt 0-6 km shear!!

Strong jet dynamics and deep layer shear were able to overcome weak nightime instability to explosively strengthen storms into supercell. Low-level shear and helicity environment was less than ideal for tornadic formation…but the supercell had a subtle surface boundary and surface low to work with…which locally enhanced low level shear and convergence.

Severe Radar Signatures

4 Panel Reflectivity

3 body scatter spike…w/ 75 dbz at 17500 ft...(-12c at 16kft) at 323z – Guaranteed Sign of Severe Hail !!

Nice inflow notch, wer and 3 body scatter spike at 327z

Golf ball size hail reported in Yonkers!

4 Panel SRM

Strong and persistent mesocyclone indicative of strong updrafts…a supercell.

30 kt Vr shear on 0.5deg slice at about 3500 ft at 331z

4-panel Vel

50 kt inbounds on 0.5 deg slice at about 4kft.

Lots of trees down in NE Bergen, S Westchester, and NW Nassau Counties. 100 mph winds estimated in Yonkers. 17000 without power at one point in Westchester.

Derived Radar Products

50-55 Vil. Vil threshold at the time was 40-45.

Echo Tops only 30-35 kft…indicative of low topped convection