SPOT CHASE: August 17, 2007 Several small supercells passed through central NJ, moving W to E. They formed in advance of a surface low in E Pennsylvania, in an area of strong wind shear. Right: 1630Z Day 1 Convective Outlook placing NJ in a SLGT risk area, with a 2% tornado probability. See the SPC  Severe Thunderstorm Event archive for data, image loops, etc. documenting severe weather events for this date. See this page with supporting data here.
	SYNOPSIS: A pronounced flanking line associated with a small supercell passes over Robbinsville at about 2315Z. At least two small wall clouds develop in this line. The parent cell remains N, moving into an area of unfavorable viewing, but develops a mesocyclone with 2-3km vertical extent. A trailing cell appears promising, and this too develops a mesocyclone just about sunset. A few pictures are managed before it is too dark. Right: HPC Surface Analysis of conditions at 21Z on August 17, 2007. A surface low in western PA with trailing cold front and warm/stationary front through NJ dominate the synoptic situation.
	Preliminary 2250Z (1850 EDT) lowest elevation NEXRAD scan from Mt. Holly, NJ (KDIX). An outflow boundary trails the supercell now exiting the frame to the east. Although not visible in the 0.5 degree scans, a mesocyclone persists for 2-3 km. Camera location is approximately at the magenta marker. A mouseover displays the radial velocity scan and the convergence line associated with the long outflow boundary. Trailing cells move into a modified air mass approximately 5-10F cooler than the air mass in southern NJ.
	Flanking Line Passage Having missed the classic supercell which passed north of Mercer County around 2230Z while at work, kept an eye on the sky. The cells moving in appeared disorganized and multicellular in nature, but short-lived supercells have developed under worse conditions in central NJ. In the radar scan at right, a tail-end supercell with a trailing outflow boundary is followed by a train of small cells. The leading cell will develop a classic supercellular structure with a mesocyclone and RFD/Flanking Line. Roughly 50 minutes later, the flanking line will pass overhead. Due to the proximity of the cell to KDIX, the mesocyclone is not visible in the lowest scan. This link opens in a new window.The existence of a a mesocyclone, RFD, and flanking line identify this as a classic supercell, although a small one.	2337Z. A small flanking line cell labeled "A" is roughly in the position of an updraft base in the photographs below. The curved "flanking line" coincides with the convergence line in the radial velocity scans.
	2333Z (1933 EDT) Looking NNE. Initially this appeared to be a shelf cloud, but it quickly became evident it was not leading a cell. Later it is clear this is a flanking line forming as the RFD from a cell to the N/NE interacts with southerly flow. The smooth leading edge suggests lifting of the warm moist airmass but what appears to be a wall-cloud-like feature is developing under the cloud base.
	~2333Z (~1933 EDT). Moving along the cloud band, looking approximately W. The band curves from NNE to SW. The one section missing from the sequence is "right overhead", but there is really no information in that view.
	2336Z (1936 EDT) Looking W. The flanking line (if that's what we have here) is passing overhead. A turbulent cloud base lowering is virtually overhead, contrasting with the smooth surface of the flanking line, which exhibits a "whale's mouth" profile.
	2339Z (1939 EDT) (Looking E) The discrete updraft base is "clearly" visible now that the line has cleared the trees. Due to the approach of a new cell that appears visually to offer promise (see below), the cameras are packed up and moved to one of several nearby sites which offer a good view of the W/SW.
	2333Z (~1933 EDT) Between the trees and the cloud band overhead, a vertical CB tower suggests the approaching cell esxhibits a strong updraft. This shot was taken as part of the sequence documenting the flanking line, but this cell is the target cell. With only about 20 minutes until sunset, only a few more pictures would be taken before hitting the road.
	Back to Chasing	Go to Stop 1