A global climatology of 500 hPa cutoff cyclones has been constructed objectively from gridded, twice-daily (0000 and 1200 UTC) 2.5 degree NCEP/NCAR reanalyses for the period 1953-1999. Maps of cutoff cyclone frequency and cutoff cyclone 12 h periods were produced for the Northern and Southern Hemispheres and then for the northeastern US. The maps were compared with previous climatologies (e.g., Bell and Bosart 1989) for representativeness and calibration purposes. Additionally, maps of accumulated total 24 h precipitation (1200-1200 UTC) were also generated from the NCEP Unified Precipitation Dataset (UPD). The UPD dataset is available on a 0.25 by 0.25 degree resolution grid over the conterminous US for the period 1948-1998. The UPD dataset is used to facilitate the comparison of the precipitation distribution in association with 500 hPa cutoff cyclones as these cyclones interact with the complex terrain in the northeastern US.

A total of 897,472 cutoff 500 hPa cyclones were identified globally in our 47-year dataset. Cutoff cyclone frequency is maximized in winter (December-February) across the northwestern Pacific Ocean, with the highest frequencies over the Sea of Okhotsk. This favored area extends eastward across the North Pacific and the Aleutian Islands. Cutoff cyclone frequency is also maximized over the southwestern US, in the vicinity of Hudson Bay, over the Canadian Maritimes, near the southern tip of Greenland, and in a band stretching from northern Africa eastward to the Turkish Plateau.

In late autumn (November), 500 hPa cutoff cyclones are most frequent over the US in a southwest-to-northeast band from Kansas to the Great Lakes and northern New England, with distinct maxima farther north over Hudson Bay and eastern Labrador. In midwinter (January), cutoff cyclones are somewhat less common over the US equatorward of 40°N but increase in frequency over Canada where the Hudson Bay and Labrador maxima shift equatorward. By early spring (March), a "cutoff freeway," hinted at in late autumn, is quite evident from the Rockies and central Plains eastward across the Great Lakes to Atlantic Canada. This cutoff cyclone distribution is consistent with the well known active spring storm track found between 35-40°N over the US east of the Rockies.

Illustrative examples of cutoff cyclone behavior over the northeastern US and their associated precipitation distributions as derived from the UPD dataset will be shown at the conference.