An Analysis of a High Impact 500-hPa Cutoff Cyclone from the 2008 Warm Season

The forecasting of heavy precipitation and severe weather associated with slow-moving 500-hPa cutoff cyclones is a challenge during the warm season. Operational prediction models often have trouble forecasting the evolution of cutoff cyclones and the associated precipitation distribution and amount. A forecast challenge is to predict the timing of transient short-wave troughs and embedded vorticity maxima, as well as the diurnally varying thermodynamic environment and planetary boundary layer structure. These forecast challenges were on display during an active 2008 warm season over the Northeast that featured numerous cutoff cyclones. This presentation will focus on a case study of the 23–25 July 2008 difficult-to-forecast cutoff cyclone over the Northeast that caused flash flooding and severe weather. Additional forecast challenges were posed by the presence of multiple precipitation modes, including convective lines/bow echoes, heavy-precipitation supercells, and stratiform rain regions, during the three-day period.

The 23–25 July 2008 500-hPa cutoff cyclone developed from a preexisting positively tilted trough over eastern Canada. As this upstream 500-hPa trough advanced southward it elongated and cut off, leading to the formation of multiple cyclonic vorticity maxima. Severe weather tended to cluster in regions of cyclonic vorticity advection near surface boundaries in regions of strong low-level vertical wind shear. Widespread rainfall amounts of 7–9 cm and extensive flooding was associated with this severe weather in a very moist environment (precipitable water anomalies were 2–3 standard deviations above normal). Severe weather on 24 July, including an EF2 tornado, was also associated with low-level positive equivalent potential temperature advection and an 850-hPa low-level jet that was 4–5 standard deviations above normal. The anomalously strong 850-hPa low-level jet and anomalously high precipitable water values were symptomatic of the rapid poleward transport of tropical moisture from the Gulf of Mexico and western North Atlantic and the resulting moisture convergence over the Northeast.