Thomas J. Galarneau, Jr.
Department of Atmospheric and Environmental Sciences
University at Albany/SUNY
Albany, NY
22 May 2009, 10:30 AM
National Weather Center, Room 1313
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK
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Tropical Storm (TS) Erin was noteworthy for producing multiple high-impact weather events from Texas to the northern Great Plains, and to the eastern United States during the postlandfall period of 16–22 August 2007. These high-impact weather events include: (1) widespread rainfall > 150 mm over the Texas hill country during 17–18 August, (2) reintensification of Erin to TS strength over Oklahoma on 19 August, (3) a predecessor rain event (PRE) that produced local rainfall totals > 350 mm over the southern Great Lakes region on 19 August, (4) heavy rainfall > 150 mm over Missouri on 19–20 August and Kentucky on 20–21 August, and (5) a severe weather outbreak over North Carolina and Virginia on 21–22 August. The aim of this presentation is to discuss two aspects of Erin’s life cycle. First, the high-impact PRE that occurred over the northern Great Plains and southern Great Lakes region in advance of TS Erin on 19 August will be examined. Second, the physical and dynamical mechanisms that likely contributed to the unprecedented inland reintensification of TS Erin over Oklahoma on 19 August will be assessed. Both parts of this presentation will use available observations and WRF–ARW numerical simulations.
Predecessor rain events are coherent mesoscale regions of heavy rainfall (>100 mm in 24 h) that can occur ~1000 km poleward and eastward of recurving tropical cyclones (TCs). In the case of TS Erin, a southerly stream of deep tropical moisture moved poleward from Erin and intersected a northwest-to-southeast oriented quasi-stationary baroclinic zone beneath the equatorward entrance region of an upper-level jet over the upper Midwest. A slow-moving convective system developed and produced widespread rainfall, with local amounts exceeding 350 mm, that resulted in record flooding in Minnesota and Wisconsin. Observations and numerical simulations indicate that low-level frontogenesis was maximized during the overnight hours of 19 August and provided the forcing for vigorous ascent during the mature stage of the PRE. A finding of interest from a simulation in which the moisture attributable to TS Erin was removed shows a 25% reduction in the total precipitation and a 50% decrease in the maximum precipitation over the PRE region compared to the control simulation. The extent of this decrease in total precipitation in the “dry run” underscores the importance of moisture originating from TS Erin in transforming a heavy rain event into a high-impact, record-breaking rain event.
Also 19 August, the remnant Erin circulation reintensified as it moved northeastward across west-central Oklahoma, producing sustained TS force surface winds and a swath of heavy rainfall (> 200 mm). Observations suggest that reintensification of the remnant Erin disturbance began as a band of deep convection, with embedded intense convective cores, formed along the southern flank of Erin. As these intense convective cores, likely associated with small-scale cyclonic vorticity anomalies, moved westward relative to Erin and axisymmetrized beneath the main mid-level cyclonic circulation, Erin intensified to minimal TS strength and developed characteristics of a warm core tropical disturbance.
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