Around 1000 PM EDT on Sunday June 27, 2010, several small storms within a disorganized storm cluster began showing signs of rotation in Warren County, PA. One storm in particular became a supercell and eventually dropped a tornado over the far southeastern corner of Allegany County, NY and continued into Steuben County. The tornado, moving east northeast at about 30 mph, began producing damage at 1105 PM and left 7 mile long path of destruction over a 15 minute period (Figure 1 and 2). A large barn and three small buildings were destroyed, and numerous trees were uprooted, topped off, and twisted in multiple directions. One car was damaged by a falling tree, and one power pole was knocked over. Several area residents were without power due to fallen wires. Fortunately, there were no injuries. The storm was rated an EF1 by a NWS storm survey, with peak winds estimated to be between 100 and 110 mph. The maximum path width was nearly 250 yards.
The afternoon surface pattern featured a double cold front upstream of NY with the initial front over eastern OH and stretching southwest through central KY and TN, while the main cold front was hung up across southern Ontario stretching westward into southern MI and northern IL (Figure 3). Dewpoints were high, with several locations in the area topping 70F. The Storm Prediction Center (SPC) had placed the area east of the primary cold front in an area for a slight risk for severe weather earlier in the day (Figure 4), coordinated with the area forecast offices around 4 PM and together with the forecast offices issued a Severe Thunderstorm Watch.
By 1030 PM, one storm in particular showed strong rotation aloft on both the Buffalo NY and State College PA WSR-88D radars (Figures 6 and 7). While the radar cannot directly detect a tornado, the presence of strong rotation is one of many radar signatures used in predicting the possibility of a current or future tornado.
Another radar signature, commonly known as a hook echo, also implies rotation and a strong updraft. This was seen for a few scans (Figure 8). A bounded weak echo region, or BWER, nearly coincident with the strongest rotation suggested a strong updraft (Figure 9). The radar also indicated a storm top nearing 50,000 ft (Figure 10).
With coordination between the two NWS offices, two tornado warnings were issued – one along and south of the PA border and another one along and north of the NY border. The storm continued to show signs of rotation as it moved across the NY-PA state line into south central Allegany County at about 1045 PM, then showed signs of weaker rotation as it moved across the extreme southeast portion of the county at 1105 PM and into Steuben County by 1120 PM. In a classic supercell, the rotation will lower with time. In this case, the strongest rotation was probably found below the radar's lowest elevation at this location. At this distance from the radar, the radar could not see below 5000 ft above the ground.
The NWS conducts a thorough damage survey of significant severe storms. For tornadoes, the NWS looks at damage and assigns a rating as defined by the new Enhanced Fujita Scale, which looks at a number of damage indicators (ex homes, trees, mobile homes, schools etc), and then looks at the degree of damage for each case, and then assigns a lower and upper bound wind speed. Once the survey is completed, an EF rating is assigned to the tornado.
Here are some of the images taken by NWS personnel during the storm surveys of the Whitesville tornado Figures 11-18).