Cold Season 500 hPa Cutoff Cyclone Precipitation Distribution and Case Studies

Anthony Fracasso, Lance F. Bosart, Daniel Keyser
University at Albany/SUNY
Dept. of Earth and Atmospheric Sciences
1400 Washington Avenue
Albany, NY 12222

Michael Evans
National Weather Service
Binghamton Weather Forecast Office
32 Dawes Drive
Johnson City, New York 13790

Cutoff cyclones pose a challenge to forecasters, especially in the northeastern United States.? The purpose of this research is to diagnose and understand the distribution of precipitation associated with the passage of cutoff cyclones in the Northeast by means of composite and case studies, and to identify whether there are characteristic precipitation signals associated with particular cutoff cyclone tracks.

Composites were constructed from the gridded reanalysis datasets available from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) along with the NCEP Unified Precipitation Dataset (UPD), a once-daily (1200 ? 1200 UTC) gridded precipitation dataset available on a 0.25 deg grid.

A climatology of cold season (October ? May) 500 hPa cutoff cyclones has been produced for the period 1948 ? 1998.? Monthly maps showing average daily precipitation for all cases where a cutoff cyclone was present in the northeast US and vicinity were created.? In addition, these average daily precipitation amounts were then compared to climatology, showing that a variation from about 60% to 30% to over 60% from October to January to April, respectively, of the climatological precipitation throughout the Northeast is associated with cutoff cyclones.?

A 19-year (1980 ? 1998) subset was used to subjectively choose cutoffs that followed one of four favored tracks near the Northeast (Mid-Atlantic, Southwest, Clipper, and Hudson Bay track).? Average daily precipitation maps for all cutoffs within a track for the entire cool season (except May) were then produced.? Also, storm-relative maps were constructed for each track using storms that fell within or close to the U.S. border, showing the previous 24 h average cumulative precipitation.?

One case study, which occurred in late May 2003, representative of two cutoff cyclone tracks (Southwest to Mid-Atlantic track) is examined.? The cutoff tracked across the Great Lakes and then looped around and passed through New York State and Northern New England.? Although heavy rains were forecast for most of the area, some locations received well under 25 mm, while other areas received closer to 100 mm over a period of about one week.? The structure and evolution of this cutoff cyclone and its associated precipitation shield are examined.? Vorticity maxima rotating around the cutoff seemed to be the best indicator of where precipitation would fall, especially when the cutoff is nearly stationary and barotropic.