Poster Session P9.7 The 2004 April Fool's New England Flooding Event: Analysis of Three Heavy Precipitation Episodes Associated with a Slow Moving Cutoff Cyclone

Wednesday, 6 October 2004
David R. Novak, NOAA/NWS/HPC, Camp Springs, MD; and A. Ayyier

Handout (489.7 kB)

During the three day period from 31 March – 2 April 2004 as much as 19.7 cm of rainfall was recorded in New England, causing widespread urban and small stream flooding. The heavy rain occurred in three precipitation episodes associated with the passage of a slow moving cutoff cyclone. This presentation will explore the relative roles synoptic and mesoscale forcing, stability, moisture availability, and orography played in contributing to the heavy rains of each precipitation episode, and will document the accuracy of quantitative precipitation forecasts (QPF) from numerical model guidance.

Initial results show that the precipitation episodes occurred in the northeast quadrant of the cutoff cyclone in an environment of weak conditional stability and anomalously large precipitable water values. Forcing for the first precipitation episode was dominated by lower-tropospheric warm air advection, while the second and third episodes exhibited a combination of warm air and vorticity advection associated with transient disturbances rotating around the cutoff cyclone center. The heaviest rain fell during the third precipitation episode when synoptic and mesoscale forcing remained focused over southern New England for several hours, as the cutoff cyclone stalled over Virginia. Deep easterly flow during this period contributed to orographic precipitation enhancement (suppression) on the east (west) slopes of north-south oriented terrain features in New England.

Operational model guidance during the event alerted forecasters to the potential for heavy rainfall over New England, however, precipitation forecasts exhibited notable timing, placement, and intensity errors, even at forecast projections less than a day. In particular, the Eta and GFS model forecasts did not adequately resolve the transient disturbance responsible for the second precipitation episode, and suffered from significant phase errors in forecasts of the third precipitation episode. However, high resolution models were able to predict the orographic enhancement of precipitation during the third precipitation episode. These results highlight the multi-scale nature of cutoff cyclones, and the challenge these cyclones present to operational quantitative precipitation forecasts.

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