Five Linked September Northern Hemisphere Tropical Cyclones during September 2011: Noru, Talas, Lee, Nate, and Katia

Department of Atmospheric and Environmental Sciences The University at Albany 1400 Washington Avenue Albany, NY 12222

The purpose of this presentation is to use potential vorticity (PV) thinking to document the structure and evolution of the large-scale flow from eastern Asia to the western Atlantic from 1-10 September 2011. This period featured five tropical cyclones (TCs), two over the western Pacific (Noru and Talas) and three over the Gulf of Mexico and western Atlantic (Lee, Nate, and Katia), that could be linked to one another through mutual interactions and downstream development.

Slow-moving, large TC Talas produced widespread heavy rainfall over western and central Japan in early September 2011. An apparent predecessor rain event that occurred northeast of TC Talas culminated in the formation of a diabatic Rossby vortex (DRV) that raced northeastward toward the Aleutians and transitioned to a prominent extratropical cyclone (EC) over the North Pacific. Anticyclonic wave breaking (AWB) northeast of TC Talas enabled a potential vorticity (PV) streamer to form to the south over the subtropical western Pacific. TC Noru formed southeast of TC Talas by the tropical transition process along this PV streamer. A second AWB event occurred over the northwestern Pacific as TC Noru engaged in a binary interaction with TC Talas and both storms subsequently underwent extratropical transition along the northeastern coast of Asia.

The second AWB culminated in the formation of a deep downstream trough over the Gulf of Alaska. This deep trough was associated with an intense EC that originated from the aforementioned DRV. Farther downstream, TC Lee formed over the southeastern Gulf of Mexico on 1 Sep, drifted northeastward, and made landfall in eastern Louisiana on 5 Sep at which time it was experiencing ET ahead of a meridionally elongated trough over the central U.S. This trough became meridionally elongated in response to the aforementioned downstream trough development over the Gulf of Alaska and subsequent ridge amplification over western North America. TC Nate formed over the Bay of Campeche at the southern end of a wind shift line that marked a remnant cold front that stretched southward from TC Lee and was associated with the meridionally elongated trough. TC Nate produced very heavy rains over eastern and southeastern Mexico and subsequently made landfall over eastern Mexico.

The meridionally elongated trough west of TC Lee collapsed to a PV tail as the southern part fractured from the main PV reservoir while the northern part of the PV tail continued eastward toward New England. The remnants of LC Lee moved northeastward along a low-level frontal boundary ahead of the fractured trough and produced very heavy rainfall. Diabatic outflow-induced upper-level ridging over the western Atlantic ahead of Lee helped steer TC Katia, an Atlantic storm that was active at the time that TC Lee made landfall in Louisiana, toward extreme southeastern New England. Deep tropical moisture ahead of TC Katia flowed northward and north-northwestward around the western side of the western Atlantic ridge. This moisture, in conjunction with moisture flowing north-northeastward from the remnants of TC Lee, contributed to a brief PRE over parts of New England and Atlantic Canada.