An application of a cutoff low forecaster pattern recognition model to the 30 June–2 July 2009 significant event for the Northeast

Cutoff lows remain a challenge to operational forecasters. Predicting the potential for severe weather, flooding, heavy precipitation, or non-impact sensible weather relies heavily on the track of a cutoff low, shear and instability profiles down stream, and various synoptic and mesoscale meteorological parameters. The Collaborative Science Technology and Applied Research (CSTAR) program has studied warm season cutoff lows impacting the Northeast for nearly a decade. Most recently, results have yielded an expanded precipitation climatology with cutoffs in the months of May to September across the Northeast, as well as five key patterns of cutoffs based on the tilt of the longwave 500 hPa trough. These five distinct conceptual or pattern recognition models examine lower-, middle-, and upper-level synoptic and mesoscale features such as temperature and moisture profiles, low-level jets and mid- and upper-jet streaks associated with the cutoff and the sensible or extreme weather it produces.

A Great Lakes cutoff low impacted the Northeast from 30 June to 2 July 2009. On 30 June the cutoff resembled a neutral tilt “Type A” pattern developed in previous CSTAR work. Over 40 severe weather reports of damaging winds in excess of 50 knots (58 mph), and large hail (greater than 1.9 cm) occurred from Pennsylvania and New Jersey northeast into New York and New England. The majority of the severe reports were large hail. The severe convection was focused ahead of a surface trough and a potent short wave trough rotating around a strong 500 hPa cutoff low, meandering eastward across Michigan. Strong differential cyclonic vorticity advection along with a potent upper level jet streak helped initiate the severe convection with steepening mid-level lapse rates, lowering wet bulb zero heights, appreciable instability (surface based convective available energy of 1000-2000 J kg-1), and low level moisture (precipitable water values of 2.5-4.0 cm) in place.

A multi-scale analysis approach will be utilized by applying the cutoff low conceptual model for the event. This application will be done in order to understand the convective environment that produced the severe weather and isolated flash flooding on the first day. There will be a heavy emphasis on the utilization of observational data to find clues that led to the active weather with the Great Lakes warm season cutoff low.