Figure 5. Rainfall totals associated with Tropical Storm Irene. Courtesy National Weather Service, Burlington, VT.
National Weather Service in Gray, Maine, showed that in excess of five inches of rain fell within three hours (Fig. 4) from a slow- moving thunderstorm. Comparing this rainfall rate to known climatological infor- mation showed this as a 200-year rainfall event, or as being quite rare. This infor- mation helped the defense reach a settle- ment.
In addition to the rainfall amount esti- mates, surface observations were used to corroborate the onset and timing of the heaviest rainfall. A weather station operat- ed by the State of New Hampshire along I-93 in Ashland, only a few miles from the incident, was able to provide this informa- tion every five minutes, greatly aiding the analysis.
As in the Indiana State Fair stage col- lapse, National Weather Service weather radar data was invaluable in determining the timing and impact of adverse weather conditions. This data is likely not used to its full potential as a diagnostic forensic tool in reconstructing weather, as non-meteorolo- gists may not be aware of its capabilities or existence.
20
Tropical Storm Irene Historic Rainfall Estimates
Tropical Storm Irene was Vermont’s the
largest natural disaster since the Great Flood of 1927, producing hundreds of mil- lions in property damages. Irene dumped three to seven inches of rain (Fig. 5), pro- ducing catastrophic flooding in many areas, especially in small, steeply sloped drainag- es in the southern two-thirds of the state. While meteorologists were certain a lot of rain was coming, they were less certain about the river and stream responses from this rainfall input. A variety of factors con- trol river/stream runoff characteristics, in- cluding the drainage size, slope, soil and development characteristics, and vegeta- tion. It turns out that the most rapid run- off response occurs in small, steeply sloped drainages with wet precursor soils and with vegetation in a dormant state. Nearly all of these conditions were met with Irene; the rainfall simply had nowhere to go but down river and stream channels. The short time- frame (approximately twelve hours) over which the heaviest rainfall occurred exacer- bated the flooding; if seven inches of rain
THE VERMONT BAR JOURNAL • WINTER 2012
had occurred in thirty-six hours, damages would have been notably less. From a meteorologist and hydrologic engineering perspective, it is useful to ex- amine the historical significance of the ob- served rainfall. This analysis can be used to better predict and prepare for future events. Comparing observed Irene rainfall to climatological rainfall extremes reveals what regions statewide featured the most extreme precipitation relative to expected climatology extremes. You have probably heard terms like the 100-year or 500-year flood, but what do they mean and how are they derived? In or- der to determine the expected likelihood of precipitation extremes, a variety of sta- tistical analysis methods are used to inter- polate from historical observations. For ex- ample, if at Montpelier there are fifty years of record and, on average, two inches of precipitation in twenty-four hours occurs once a year, then we would call this thresh- old as a “1-year event.” There would be a 100% chance of seeing two inches of pre- cipitation within twenty-four hours within any given year. The corresponding 25-year rainfall
www.vtbar.org
The Forensic Meteorologist—“Reconstructing Weather”
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