During the late afternoon on July 25, 2009, severe weather developed over Western NY between Buffalo and Rochester. The first storm moved across northern Erie County with numerous funnel cloud sitings and hail to 1 inch in diameter. This storm weakened as it moved to the northeast while a second storm formed rapidly to its south, and eventually produced two tornadoes across Western NY. The first occurred at 450 PM EDT, touching down in the town of Darien, just southwest of the village of Corfu in western Genesee County. The tornado, about 100 yards wide, moved over Corfu around 5PM and then lifted 5 minutes later as the parent storm continued off to the northeast, leaving a 4.2 mile long path of destruction over a 15 minute period. Numerous buildings were damaged and trees uprooted. A barn and greenhouse were destroyed. Fortunately, there were no injuries. The storm was rated an EF1, with peak winds estimated to be near 100 mph.
The storms developed in a well anticipated unstable airmass. A day prior to the event, showers and thunderstorms with gusty winds were in the forecast for Saturday, with the probability of measurable rain increasing from 50% to 90% during the late afternoon and evening hours. The Storm Prediction Center (SPC) placed western NY in a slight risk area for Saturday afternoon and evening. Initially, the concern was for an approaching cold front moving into this unstable airmass. However, as the day unfolded, it became apparent that the cold front would still be well upstream.
These thunderstorms weakened into a thin line while continuing eastward across IN overnight and into OH by Saturday morning. Notable on radar was a bookend vortex on the northern extent of this line.
This vortex, commonly called a Mesoscale Convective Vortex (MCV) moved across Lake Erie during the day on Saturday with only showers. The vortex, by definition, a spinning airmass, had a significant amount of rotational shear. By Saturday afternoon explosive thunderstorm development occurred over eastern Lake Erie. Convection quickly evolved into supercells, or rotating thunderstorms commonly associated with severe weather, including tornadoes.
At 332 PM, the National Weather Service in Buffalo, NY (NWS BUF) issued the first severe thunderstorm warning of the day for a supercell about 6 miles west of Sturgeon Point. This storm had a classic supercell structure, with a kidney bean shaped reflectivity and an appendage stretching off to the south of the storm. Storm relative motion showed significant rotation as well.
This storm quickly intensified, and by 446 PM, NWS BUF updated the severe thunderstorm warning to include the possibility of tornadoes. At 453 PM, the storm had a well defined hook and rotation with a tornado now on the ground and moving across Corfu. Estimated wind speeds about 700 ft above the ground were estimated to be near 91 kts, or 105 mph. Another tornado warning was issued for portions of Genesee, Orleans, and Monroe counties at 503 PM.
As reports started to come in with damage near Corfu, NWS BUF issued a third tornado warning for northwestern Monroe County at 546 PM. The storm which had produced the tornado over Corfu still had a funnel cloud and had moved over the Thruway. At this time, the storm did not have strong rotation, but it did have a notable hook echo. This is the storm that produced a brief tornado in Hilton.
The NWS BUF staff was extremely busy over a four hour period beginning around 330 PM. Three tornado warnings were issued along with five severe thunderstorm warnings, and a 16 severe weather statements. (This does not include a number of other warnings and statements that were issued after sunset). Of course, the forecast office was also issuing regularly scheduled forecast products, numerous storm reports, and nowcasts as well throughout the event.
The NWS conducts a thorough damage survey of significant severe storms. For tornadoes, the NWS looks at damage and assign 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 storm surveys of the Corfu and Hilton tornadoes.