A circular image hovered ominously over a map of the Northeast as National Weather Service Hydrologist Stephen DiRienzo described the challenges forecasters faced as Tropical Storm Irene bore down on the region in August.
The image depicted 24 hours of constant rainfall that fell in amounts beyond the current forecast system’s prediction capabilities.
In fact, the rainfall totals show Irene dropped at least four times more rain than the computer models can even show.
DiRienzo was one of several speakers at the Fourth Annual Mohawk Watershed Symposium held Friday at Union College, an event dominated by the devastating storms Irene and Lee, which battered the region’s infrastructure and numerous communities.
DiRienzo described how one computerized model used to help forecast the amount of rain can go no higher than 4 inches. Above that it simply displays a 4 with a plus sign after it.
Experience led meteorologists in Albany to double that number and call for 8 inches or more of rain instead of listening to the computer.
In the end, the rainfall totals in some places doubled those guesstimates, with some spots receiving between 16 and 18 inches of rain during the 24-hour period on Aug. 28.
“Unfortunately we only forecast about half the amount of rain,” DiRienzo said.
Tropical moisture wasn’t the only factor that led to record rainfall in the region — about half of the rain that fell did so because of a driving wind that was slamming into the mountainous, 3,000-foot-high escarpment east of the Schoharie Valley, DiRienzo said.
This actually created a wall of wind that pushed rain back into denser concentrations. Areas directly downhill from that escarpment received the greatest amount of rain due to the phenomenon, which is called “upslope enhancement.”
The forecast challenge became even more difficult as more-tangible signs, such as the precise amount of rainfall that landed at any particular place, didn’t make it to the forecast center or turned out to be inaccurate.
Rain gauges don’t work as well under tropical storm-force winds, DiRienzo said.
Power outages left individuals who were measuring rainfall without the ability to send in their readings, and a technician at the National Weather Service couldn’t reach people with data using a ham radio.
Despite technological difficulties, DiRienzo said a review of Hurricane Center data tells him hurricane-related storms are hitting the region roughly once a decade.
Britt Westergard, another National Weather Service hydrologist working during that hectic weekend, said just believing what she saw “was actually happening” was one of the most difficult parts of the job.
With modern history in mind, forecasters were “going out on a limb” to even suggest 10 inches of rain might fall in a particular region, she said.
Geologist Jaclyn Cockburn of the University of Guelph, Ontario, pointed out the need for researchers and government to refine their data collection or planning in order to minimize damage from severe flooding events.
In a paper submitted for the symposium, Cockburn and Union College Geology Professor John Garver propose that instead of checking gradual weather changes, government and developers should consider extreme weather events a priority.
Policy and planning decisions made today are based in part on global climate models that use greenhouse gas emissions to gauge climate changes — and these models don’t recognize extreme weather events like tropical storms Irene and Lee.
But it’s events like these storms that have the most impact on the region’s infrastructure — roads and bridges and canals.
“We need to be ready for those, they’re going to happen,” Cockburn said.
The daylong conference included numerous discussions focusing on the science and history of flooding in the Mohawk River Basin; a description of the Schoharie River Center’s post-storm research that helps bring real-life science to local youths; and calls by Dam Concerned Citizens to get more stream monitoring gauges in the Schoharie Creek and its tributaries.
Canal Corp. Director Brian Stratton gave an overview of the massive damage sustained by the canal system and the work crews did to get stranded boats out.
Roughly 160 people attended Friday’s event, the fourth in a series first organized following the 2006 flooding that caused severe damage along Mohawk River communities.
Agencies and groups attending included the state Department of Environmental Conservation, the U.S. Geological Survey, the Canal Corp., Dam Concerned Citizens, and the state Department of State.
State Assemblyman Peter Lopez, R-Schoharie, and U.S. Rep. Paul Tonko, D-Amsterdam, were among guest speakers.
Research papers detailed at the event can be found on the Internet at minerva.union.edu.