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Communicating rainfall risk from tropical storms

By Skip Rigney

The federal government is funding research that focuses on improving how the risks from tropical cyclones are communicated to the public. One such project aims at giving people a better perspective on the threat from rainfall from tropical systems.
Hurricane Hanna made landfall in south Texas last weekend with maximum winds of 85 miles per hour making it a Category 1 on the Saffir-Simpson hurricane scale. Despite being in the lowest tier on the scale, Hanna dumped 10-20 inches of rain in many locations in deep south Texas and northeast Mexico. The heavy rain damaged a maternity hospital in Reynosa, Mexico, forcing patient evacuations.
Even though south Mississippi is over 500 miles away from where Hanna made landfall, the broader weather pattern associated with the storm helped produce a rainy period locally that began last Thursday and continued through Tuesday. Locations in the southern part of Pearl River County received four to six inches of rain, while amounts tapered off to around two inches in the northern part of the county over the six day period.
The rain associated both directly and indirectly with Hurricane Hanna is a reminder that, although we usually focus on the winds of tropical cyclones, rainfall can be a major hazard regardless of what the maximum winds are. Storms that move slowly or stall can produce especially heavy rains. The Saffir-Simpson categories are based on maximum wind speed and simply do not capture the risk from rainfall.
With that in mind a team of researchers from the University of Wisconsin-Madison and the NOAA Cooperative Institute for Meteorological Satellite Studies have developed a metric called the Extreme Rainfall Multiplier (ERM). Instead of simply expressing the predicted rain from a tropical system as the number of inches expected, ERM allows forecasters to express the expected rainfall to the heaviest rain events that particular region typically receives in a two-year period. For example, if the ERM is 3, the potential exists for three times more rain than what is normally experienced every couple of years from heavy rain events.
In research published earlier this year in the Bulletin of the American Meteorological Society, Christopher Bosma and his colleagues computed an ERM for all tropical storms and hurricanes affecting the U.S. since 1948. The most extreme was Hurricane Harvey in 2017, whose stalled remnants dumped over 20 inches of rain on the upper Texas coast with some areas receiving 50 inches over six days. Harvey’s ERM was 6.4, indicating that it was over six times more severe than the most extreme rain events normally expected in that part of Texas in a typical two year period.
Hanna’s 15 inches of rain last weekend near Reynosa, Mexico, equates to a 3.0 on the ERM scale, meaning it was three times more than what would normally be that location’s heaviest rain event during a typical two year period.
Even promising research takes time to transition into the operations of the National Weather Service. So, it may be some time before you see the ERM or a similar scale used in conjunction with the categories of the Saffir-Simpson wind scale. In the meantime, it’s a good reminder that wind is not the only threat from tropical systems.