BACKGROUND
Early on Thanksgiving Day 2004, the air mass over the NE was relatively moist, with surface dew points in the 50F-60F range ahead of a cold front trailing south from a surface low in NW PA. In advance of the cold front a low-topped squall line formed. Winds ahead of the front were southerly and brisk, and vertical wind profiles indicated strong unidirectional shear. The SPC 1200Z Day 1 Convective Outlook called for a Slight risk of severe thunderstorm formation in the area (link.) Since severe thunderstorms did form in the New England states this day, the SPC archive does have an organized data set for this date (link).
The squall line passed through NJ around 1200Z, but a few hours earlier an isolated cell formed in the warm sector and moved NE along the New Jersey coast. This long-lived cell showed no evidence of rotation while making it's tansit of New Jersey, so although this cell was an example of organized convection, it was probably not a supercell.
1231Z 0.44˚ reflectivity scan from KDIX. The cell looks somehat incoherent but hangs together until well after it disappears from the field as it proceeds NE. We elected not to chase this cell which looked like it would clear the coastline too far for us to intercept it.
GOES-EAST image over NE US (courtesy of the College of DuPage) at 1331Z. The organized convective cell in southern NJ is not as well-defined as the squall line to the west, which was described by SPC forecasters as "low-topped."
as the squall line which was descr
AniS animation of thumbnail reflectivity images for the 0.5˚ elevation from KDIX, from 1230Z on 11/25 to 1430Z on 5/16/2006. The squall line enters the field during this period as the cell moves offshore (see static image to left.)
1100 EST (1500Z)
GOES-EAST image over NE US (courtesy of the College of DuPage). The low-topped squall line appears as a bright band in this image. A satellite imagery loop is linked here.
At this
point, Spencer and I gathered our cameras and prepared to head
out and observe the squall line passage. Lightning was being
reported in association with this line further north, but near
central NJ we expected to see none of that.
AniS animation of thumbnail reflectivity images for the 0.44˚ elevation from KDIX. The squall line becomes disorganized as it proceeds downslope and crosses the Delaware River into NJ.
STOP 1 ~1120 EST (~1520 Z )
We arrive at West Manor Way about the time the squall line is passing overhead. The site does not live up out expectations, as some new construction has been taking place and visibility is obstructed. (Scratch one viewing site.)
Above: Looking north; we can see the shelf cloud associated with the leading edge of the line.
Above: Looking north; a few minutes later we can see into the "vault" behind the squall line.
Above: Looking south; the line curves off into the distance: it is narrow, and moving fast.
Above: 1545Z GOES-E satellite image showing the well-defined line just east of Mercer County, NJ. Below: 1555Z KDIX 0.5˚ data (reflectivity).
KDIX 0.44˚ Reflectivity animation showing the approach and passage of the low-topped squall line. Click here for an animation of images obtained from the CoD web site.
STOP 2 1155 EST (1555 Z )
The squall line having passed us while stopped on West Manor Way, we pack up and head to another stop we had scouted which gave a good view NE-SE. The line is moving fast, so by the time we travel the 2-3 miles east to Stop 2, we are looking at the cathedral-like ceiling of the underside of the squalls.
Above: Looking E at the ceiling behind the sharp leading edge of the squall line. BKN-SCT CU in the warm air ahead of the line is still visible in the distance. Below: A wider view of the receding line. The the cloud base of cumulus congestus clouds in the leading edge was lower than the ceiling behind it, giving a vaulted impression.
Below: The fish-eye lens captures (to a degree) the three-dimensional view of the passage. From the northern to the southern horizon, the leading edge of the line was visible as it receded eastwards. At the same time, the clouds were breaking up just west of the zenith.
This site offers an unparalleled view of the eastern horizon, at least for the Robbinsville area. In the fish-eye view, due east is approximately aligned with "4:30 on a clock."
Below: Zooming in on a portion of the leading edge of the squall, in an attempt to capture the step-like transition in the cloud base, reminiscent of the "whale's mouth" formations sometimes described during the passage of shelf-clouds.
The ceiling soon breaks up, as seen below.
Below: The sun makes a dramatic entrance. Behind us (westward) a faint rainbow hints that rain may yet be in the forecast.
Shockwave Flash animation of visible satellite images (acquired from CoD website upon return from field)
