"New Jersey Flood: Terrain-Locked Convection"
12 August 2000

On 12 August 2000, flooding rains hit the state of New Jersey. The rains were associated with regenerative warm-topped convection, and in many areas, including the hardest hit Sparta, roads were closed, bridges washed out, mudslides occurred and motorists were stranded. Terrain-locked convection over Sparta resulted in over 9 inches of rain in 8 hours, with media reports of as much as 14 inches. Rainfall rates were as high as 4 inches per hour.

The rains were associated with a trough over the Northeast U.S., digging south into the mid-Atlantic. A vorticity max on the back side of the trough moved slowly down and strengthened, while the trough cut off around 0800Z. The vorticity max strengthened additionally around 1700Z, as it rounded the base of the trough, while the cutoff low sat in central Pennsylvania. The position of these features put northeast Maryland, southeast Pennsylvania and southern New Jersey in the area under the best positive vorticity advection. As the cutoff low moved south, the support aloft moved along with it.

At the surface, an area of low pressure sat well off the Delmarva peninsula throughout the period. There was not much change in pressure, though the gradient over the affected area strengthened slightly through the day on the 12th. The center of low pressure shifted south slightly, and sat off the Virginia coast by 0000Z 13 August. Surface convergence at 1300Z can be noted in central New Jersey, with northeast winds into the eastern part of the state, and northerly winds into the west. This boundary shifted slightly west later in the day. The surface dewpoints were not extremely moist, mainly in the mid-60's throughout the region. Also, the precipitable water amounts were around 1.25 inches, not excessive for the mid-Atlantic coast. There were no excessive moisture parameters to be found in relation to these high rain totals. A weak theta-e ridge did exist over New Jersey and Eastern New York on the 12th, though.

The radars show how the rains evolved. The terrain-locked area of precipitation began around 1130Z in northern New Jersey. The radar began to fill in from northern New Jersey southwest to the PA/NJ border and from there, southeast to the coast around 1530Z. The precipitation became heavier around 1700Z, about the time when the area was beneath the best support aloft. The heavy precipitation remained mainly stationary in the north part of the state, while the precipitation shifted east in the southern half of the state.

The flooding rains caused damages estimated in the hundreds of millions across the state, about $166 million in Sussex County alone.

Case Study 34