The interactions of slow dynamical processes with synoptic time scales produced excessive rainfall along the USA East Coast during 24-29 June 2006. An empirical Global Synoptic Dynamic Model of subseasonal variability (GSDM; Weickmann and Berry, 2006) provides a framework to interpret the sequence of events that led to the rainfall. The GSDM considers multiple time scales that have both stochastic and quasi-oscillatory behavior.

A super-position of four time scales contributed to the circulation anomalies including: 1) persistent tropical convection across the western Pacific Ocean due to warm SSTs from global warming and the ENSO cycle, 2) a ~50-day mountain-frictional torque index cycle that has ~ a 6-day decay time scale and large amplitude around the latitude bands of major north-south mountain ranges with east-west slopes (e.g., 10-50 N in the Northern Hemisphere), 3) 20-30 day tropical convective variations that induce jet stream fluctuations over the mid-latitude west and central Pacific and 4) baroclinic life cycles and mobile wave packets. The MJO signal was weak.

The circulation response to (1) was for twin subtropical anticyclones centered ~150E, with their amplitudes being modulated as they interacted with extratropical baroclinic wave packets. These anticyclones supported a western Pacific wave train favoring ridges across western North America with downstream troughs across the central and eastern USA during June. The trades increased across the subtropical North Atlantic Ocean basin by the middle of the month with anomalies ~ 5 m/s, as part of a strong positive global frictional torque due to (2). An intense tropical convective flare-up occurred across the central equatorial Indian Ocean by around 16 June linked to (3). The divergent outflow from this event interacted with a passing baroclinic wave packet, (4), moving through Asia on about 18 June, allowing rapid downstream amplification of already existing circulation anomalies at an energy propagation speed of ~36 m/s. By 24 June retrogression and amplification of the wave train occurred across North America, supporting an anomalous trough across the central USA with a large anticyclone just east of New England (a summertime version of Stage 2 of the GSDM). The 250-mb anticyclone, with daily mean vector wind anomalies in excess of 50 m/s at times, dynamically induced the moisture laden enhanced trades to spread northward across the eastern USA. With a stationary north-south front in place and the central states trough jet streak dynamics passing through, several days of tremendous rainfall occurred. Week 1-2 predictability of this event will be addressed in the context of the GSDM and available ensemble output.