EXTREME RAINSTORMS IN ADVANCE OF TROPICAL CYCLONES

Extreme rainstorms that occur well in advance of tropical cyclones (TCs) are known as predecessor rain events (PREs). The societal impact of PREs is that they can result in devastating inland flooding well before the arrival of the main rainshield associated with the TC. A well-known example of a PRE (before the term "PRE" was coined) occurred ahead of TC Agnes in June 1972 when more than 300 mm of rain fell in the Chemung River Valley of southwestern New York and resulted in destructive flooding. The deployment of the operational NWS Doppler radar network in the early 1990s along with the many recent active TC seasons has made it possible to conduct systematic studies of PREs. Between 1995 and 2008, 28 PREs were identified over the central and eastern United States, based on a rainfall rate of at least 100 mm in 24 h. PREs were most common in August and September during this period and -30% of all western North Atlantic TCs were associated with at least one PRE. PREs developed -1000 km poleward of the TC, occurred 24-36 h before the passage of the TC at the latitude of the PRE, and were clustered preferentially to the left of the track of the TC.

PREs develop in response to a deep tropical moisture plume that emanates from the TC and is carried well poleward. PREs occur where this moisture plume encounters a surface frontal zone and is forced to ascend, resulting in copious and prolonged heavy rain. Based on a PRE-relative composite analysis, we found that PREs typically form: 1) east of a 700-hPa trough; 2) along the western flank of a 925 -hPa thermal ridge axis; 3) near the intersection of a low-level jet with a surface frontal zone; and 4) beneath the equatorward entrance region of a 200-hPa jet. Case studies of PREs associated with TCs Rita (2005), Erin (2007), and Ike (2008) confirm the dynamical importance of jet-driven tropospheric-deep vertical circulations to PRE formation and evolution. Composite analyses of the 28 PREs identified between 1995-2008 along with the results of the case studies of these three TCs have revealed that PREs mature and intensify where a strong low-level jet intersects a surface frontal zone and air parcels in the associated tropical moisture plume ascend in conjunction with warm-air advection beneath the equatorward entrance region of an upper-level jet. PREs also are distinguished from more "ordinary" heavy rainstorms by the presence of a moisturerich tropical air mass that can sustain extraordinary rainfalls, as evidenced by the more than 500-mm storm-total rainfall that occurred in Houston, Minnesota, during the TC Erin PRE on 18-19 August 2007.

Ongoing research on PREs is focused on additional case studies to document the spectrum of PREs that can occur ahead of TCs impacting the central and eastern United States. We are also in the process of examining whether PREs can be a factor in producing heavy rains in the southwestern United States ahead of landfalling eastern Pacific TCs. Additionally, a study of possible southwestern U.S. PREs and PREs that occur in the vicinity of the Appalachians over the eastern United States affords an opportunity to determine how PREs respond to, and are influenced by, the terrain-forced ascent of warm, moist tropical air. - LANCE F. BOSART (THE UNIVERSITY AT ALBANY), T. J. GALARNEAU, JR., J. M. CORDEIRA, AND B. J. MOORE. "Extreme Rainstorms in Advance of Tropical Cyclones," presented at the 22nd Conference on Climate Variability and Change, 17-21 January 2010, Atlanta, Georgia.

[Sidebar]

Meteorological features associated with a PRE ahead of a TC: I) downstream 200-hPa jet ("J"; gray shading); 2) upstream 700-hPa trough axis (dashed black line) and height contours (black); 3) low-level jet (large arrow); 4) tropical moisture plume (light blue shading representing precipitable water values > 50 mm) emanating from the TC (red conventional TC symbol); 5) surface frontal boundary; 6) low-level wind pattern near the frontal boundary and PRE in the warm (red-arrowed lines) and cool (blue-arrowed lines) air; and 7) surface cyclone and anticyclone locations given by the red "L" and blue "H" symbols, respectively. (BOSART)

[Author Affiliation]

LANCE F. BOSART (THE UNIVERSITY AT ALBANY), T. J. GALARNEAU, JR., J. M. CORDEIRA, AND B. J. MOORE. "Extreme Rainstorms in Advance of Tropical Cyclones," presented at the 22nd Conference on Climate Variability and Change, 17-21 January 2010, Atlanta, Georgia.

EXTREME RAINSTORMS IN ADVANCE OF TROPICAL CYCLONES

Extreme rainstorms that occur well in advance of tropical cyclones (TCs) are known as predecessor rain events (PREs). The societal impact of PREs is that they can result in devastating inland flooding well before the arrival of the main rainshield associated with the TC. A well-known example of a PRE (before the term "PRE" was coined) occurred ahead of TC Agnes in June 1972 when more than 300 mm of rain fell in the Chemung River Valley of southwestern New York and resulted in destructive flooding. The deployment of the operational NWS Doppler radar network in the early 1990s along with the many recent active TC seasons has made it possible to conduct systematic studies of PREs. Between …