OVERVIEW

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.

Figure 1: Path of tornado moving across southeast Allegany County and southwest Steuben County	Disclaimer	Figure 2: Path of tornado starting near Paynesville, continuing through the town of Willing and Independence, just south of the village of Whitesville, and into the town of West Union.	Disclaimer

METEOROLOGICAL BACKGROUND

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.

Figure 3: Observations of sea level pressure and fronts for the Eastern Great Lakes Region at 21Z (5PM EDT)

Figure 4: Day 1 Outlook from SPC showing a slight risk for severe thunderstorms over Western NY

As the afternoon and evening progressed, a few severe thunderstorm warnings were issued. By 8PM, a new Buffalo sounding was sent up, showing abundant shear, as seen in the profile, which showed a clockwise turning of the wind direction with height (Figure 5). The sounding was quite moist and unstable as well. Instability and wind shear are particularly important for the development of tornadoes, while moisture of course is necessary for any thunderstorm development. Some of SPC's algorithms supported supercell development.

Figure 5: 00Z (8PM EDT) KBUF Sounding (at the Buffalo International Airport)

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.

Figure 6: Storm Relative Motion (SRM) from the KBUF WSR-88D at 0233Z (1033PM EDT) showing a strong velocity couplet in the extreme northeast corner of McKean County, PA.

Figure 7: Storm Relative Motion (SRM) from the KCCX WSR-88D at 0233Z (1033PM EDT) showing a strong velocity couplet in the extreme northeast corner of McKean County, PA.

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).

Figure 8: Reflectivity from the 0.5 degree slice of the KBUF WSR-88D at 0233Z (1033PM EDT) showing hook in the extreme northeast corner of McKean County, PA.

Figure 9: A bounded weak echo region (BWER) seen from the KBUF WSR-88D at 0233Z( 1033 PM EDT). The upper left image shows the 1.3 degree slice of the reflectivity, with 50+ dBZ echoes surrounding an area of 40-50 dBZ echoes. A similar "donut" can be seen in the upper right image, which is set at a constant altitude of 15,750 ft. The lower left image shows a cross section of the BWER with the cross section taken from southwest to northeast as indicated by the dashed arrow in the upper right hand and lower left hand image. The lower right hand image shows both a cross section and a "fly over" view of the reflectivity.

Figure 10: Echo tops from the storm at 0242Z (1042 PM EDT), or the height of the 18 dBZ echo, showing a broad area reaching 50,000 ft

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.

DAMAGE SURVEY

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).

Figure 11: NWS storm survey - Whitesville - Small Rd. Barn and 2 smaller farm buildings destroyed.	Figure 12: NWS storm survey - Whitesville - Small Rd. Closer view of barn. The roof was partially intact. The trailer next to the barn was moved off its blocks.
Figure 13: NWS storm survey - Whitesville - Small Rd. Another view of the barn. The old jeep, although sustaining damage, was inside the barn and did not move. Several other pieces of equipment, including ski mobiles were heavily damaged, as well as numerous pieces of farm equipment, including a tractor.	Figure 14: NWS storm survey - Whitesville - Saunders Rd. Pieces of the barn were thrown 1000s of feet from the barn in both directions.
Figure 15: NWS storm survey - Whitesville - Saunders Rd. More debris from the barn	Figure 16: NWS storm survey - Whitesville - Small Rd. An example of circular areas where grass was flattened.
Figure 17: NWS storm survey - Whitesville - Saunders Rd. This small tree was twisted but still intact.	Figure 18: NWS storm survey - Whitesville - Saunders Rd. Many nearby trees were uprooted and or topped in a variety of directions. The fallen trees shown here were generally facing the same direction, however others were uprooted and in different directions.