1A.8 The “Perfect Storms” of 1991: An example of linking high-impact weather and climate on intraseasonal time scale

Tuesday, 26 June 2007: 9:45 AM
Summit A (The Yarrow Resort Hotel and Conference Center)
Jason M. Cordeira, Univ. of Albany/SUNY, Albany, NY; and L. F. Bosart

The high-impact weather events in the US that occurred in late October and early November of 1991 known as the “Perfect Storms (PSs)” transpired during a highly amplified large-scale flow regime transition. The Pacific North American index exemplified the transition from +1.9 to –2.8 and –2.8 to +1.9 standard deviations over 25 days. Flow remained amplified during the transition and assisted in the development of two meteorologically significant and “PSs”. The purpose of this presentation is to document the life cycles of these twin high-impact weather events via a multi-scale diagnostic analysis and examine the linkages between the storms and the intraseasonal variability of the observed large-scale flow regimes.

The first of the PSs (PS1) began with an extratropical, baroclinic cyclone east of Nova Scotia on 28 October embedded in a downstream trough of a highly amplified mid-level trough-ridge pattern over the US. PS1's deep circulation quickly became the foremost feature in the northwest Atlantic as it partially absorbed Hurricane Grace to its south. PS1 continued to strengthen to 972 hPa as it propagated westward towards the US, with resulting widespread wind and wave damage along the coast, and then subsequently weakened as it performed a counter-clockwise loop in the far western Atlantic beneath an amplifying ridge. The warm waters of the Gulf Stream then excited convection within the center of PS1, leading to the development of the Unnamed Hurricane on 2 November.

The second of the PSs (PS2) began as a cyclonic upper-level potential vorticity anomaly rotating through the trough in the western US. A surface cyclone formed ahead of this trough in the extreme western Gulf of Mexico. PS2 subsequently strengthened and propagated northward as the triggering mid-level trough acquired a negative tilt on 1 November. A strong (1051 hPa) surface anticyclone propagating southward from Canada provided anomalously cold air (~ -3 std. dev.) across the Upper Midwest, contributing to the enhanced low-level baroclinicity on which PS2 fed. PS2 gave Minnesota its earliest and heaviest snowfall totals, setting the state record of 93.7 cm in Duluth, MN.

Results suggest ridge eruptions in the NE Pacific associated with regime transitions highly influence downstream anticyclone and cyclone development and propagation. In addition, amplified flow supported large-scale meridional transports of potential vorticity that aided in key mid-latitude interactions conducive for cyclogenesis. Accordingly, the presented research will focus on (1) the development and life cycles of PS1 and PS2, (2) linkages between the two Perfect Storms and the large-scale flow regimes, and (3) impacts of the observed intraseasonal high-impact weather events on seasonal climate anomalies.

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