SPOT
CHASE: July 19, 2007
On
July 19, 2007, clusters of heavy thunderstorms formed in advance of a
cold frontal boundary trailing southwesterly from a surface low in southern
New York state. Moving ENE, they reached eastern Pennsylvania by about
2130Z.
21Z
Surface analysis (HPC)
The
SPC
Severe Thunderstorm Event archive is a good source of data for this
event. The 1630Z Day 1 Convective
Outlook placed NJ in a SLGT risk area, with a 2% tornado probability.
2141Z
(~ 1741 EDT) The northernmost cell in the Nexrad image at right has developed
a mesocyclone, which is not very pronounced in the 0.48
radial velocity scan. The beam altitude in the vicinity of the cell
is about 900m. In the 1.45
degree scan, the velocity signature is clear.
The
1.45 degree reflectivity scan is unremarkable, and the entire cluster
appears quite disorganized. At the surface, the only hint of rotation
are low-contrast striations on the leading edge of the cloud base. (Move
the mouse over the reflectivity scan to switch to a velocity scan overlay.)
Based on the location of the mesocyclone (more or less on the front flank of the cell), this cell has an HP-like character.
Based on the location of the mesocyclone (more or less on the front flank of the cell), this cell has an HP-like character.
2152Z
(~1752 EDT)
At right: Nikon Coolpix 950 photograph looking NW from a stop in Lawrenceville, NJ. A wall cloud with pronounced rotation is visible. To the right of the photo (not visible), is the front flank downdraft. Heavy rain had fallen a mile or so north of the camera location around 2140Z.
At right: Nikon Coolpix 950 photograph looking NW from a stop in Lawrenceville, NJ. A wall cloud with pronounced rotation is visible. To the right of the photo (not visible), is the front flank downdraft. Heavy rain had fallen a mile or so north of the camera location around 2140Z.
2152Z
(~1752 EDT)
At an elevation angle of 1.45 degrees, the weak mesocyclone is visible in the radial velocity scan, but the reflectivity scan (also a mouse-over image at right) is unremarkable.
At an elevation angle of 1.45 degrees, the weak mesocyclone is visible in the radial velocity scan, but the reflectivity scan (also a mouse-over image at right) is unremarkable.
2154Z
(~1754 EDT)
At right: Nikon Coolpix 950 photograph looking NW. The wall cloud is lifting and becoming less cylindrical, although the rotation is still visble.
At right: Nikon Coolpix 950 photograph looking NW. The wall cloud is lifting and becoming less cylindrical, although the rotation is still visble.
2158Z
(~1758 EDT)
By this time, the mesocyclone is dissipating and in the next scan (2204Z, not shown) will be barely discernible. The road and terrain conditions would not favor chasing this cell further even if it remained intact
By this time, the mesocyclone is dissipating and in the next scan (2204Z, not shown) will be barely discernible. The road and terrain conditions would not favor chasing this cell further even if it remained intact
