Unexpected High Winds in Northern New Jersey: A Downslope Windstorm in Modest Topography

This study presents the first evidence for the occurrence of a downslope windstorm in New Jersey. During the early morning hours of 4 January 2009, an unanticipated strong wind event was observed. Despite a zone forecast calling for winds less than 4 m s issued 4 h prior to the event, winds up to 23 m s were reported at High Point, New Jersey (elevation 550 m), with gusts to 30 m s in its immediate lee (elevation 311 m). These winds were highly localized; a nearby Automated Surface Observing System (ASOS) station (Sussex, New Jersey, 12 km distant) reported calm winds between 0700 and 1000 UTC, just as the winds were peaking near High Point. High Point is the highest point in New Jersey, and is part of the quasi-two-dimensional Kittatinny Mountain extending from Pennsylvania into New York. This study tests the hypothesis that the topography of High Point, upon interacting with the local atmospheric stability and wind profiles, was sufficient to produce a downslope windstorm, thus causing these unusual winds. The results indicate that the presence of a sharp low-level temperature inversion in combination with a northwesterly low-level jet perpendicular to the ridge provided the key ingredients for the strong winds. Linear theory does not appear to explain the winds. Instead, prior studies incorporating nonlinearity predict a trapped lee wave or possibly a hydraulic jump, and model simulations suggest that High Point was indeed tall enough to generate such a wave along with rotors, although observations were not available to confirm this. Given sufficient model resolution, many aspects of this event were predictable. Similar windstorms have occurred before at High Point, but observations show that this event was the most amplified in recent years.