OVERVIEW
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.
| This rotating storm then produced a second tornado at 555 PM in the town of Hilton in western Monroe County. The damage with this tornado was less extensive, with primarily tree damage and broken outdoor furniture, although one home did sustain minor structural damage. The tornado was on the ground for about 10 minutes, had a 1.5 mile long path, 30 yard width, and an EF0 rating with peak winds estimated at 75 mph. |  |
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Path of two tornadoes across Genesee and Monroe Counties |
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Path of first tornado moving through village of Corfu in Genesee County |
Path of second tornado moving through Hilton in Monroe County |
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METEOROLOGICAL BACKGROUND
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.
 Day 2 Outlook from SPC showing a slight risk for severe thunderstorms over Western NY |
 24 hr forecast of sea level pressure and fronts for the Eastern Great Lakes Region |
| Complicating matters were the remnants of severe weather from a Mesoscale Convective System (MCS) that moved from southwest MN into eastern IA and southeastward across central IL during the preceding afternoon and evening (July 24, 2009). This severe weather produced baseball sized hail and two tornadoes across southwest WI and northwest IL. Often, a severe weather outbreak in this portion of the Midwest is a precursor to severe weather over some part of the Northeast US. |  Storm reports for Friday July 24, 2009,
showing a swath of damage from an MCS moving from MN southeast through IL |
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.
 Mesoscale Convective Vortex moving across Lake Erie |
 hourly imagery of regional radars and lightning showing weak bookend vortex moving along Lake Erie |
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.
 Reflectivity showing kindey bean shaped reflectivity off Sturgeon Pt. |  Significant Rotation with storm off Sturgeon Pt. |
At 411 PM, the storm showed increasingly strong rotation with the appendage becoming more hook-like, a typical indicator of a supercell about to drop a tornado, and a tornado warning was issued for Erie County. Unfortunately, the core of the storm passed directly over the radar, known among forecasters as “the cone of silence”, as the radar is not designed to tilt vertically. Thus it was difficult to continue to see the storm’s rotation move just to the south of the forecast office in Cheektowaga. After the storm moved northeast of the forecast office, there was little rotation left, and at 429 PM, the tornado warning was canceled. Although no tornado occured in Erie County, there were several reports of funnel clouds, a well as 1" diameter hail at the NWS BUF office. This was clearly a dangerous storm. Meanwhile, a new storm was just beginning to form to the south of the first storm, just southwest of Alden, with a new severe thunderstorm warning issued for an area from Clarence and Alden northeast across Corfu, Batavia, and north to Point Breeze.
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 hook over Erie County |
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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.
 reflectivity showing hook near Corfu |
 Estimated Actual Velocity of 91 kts or 105 mph near Corfu |
In order for tornadoes to form, storms needs to have a number of weather factors come together. There needs to be ample moisture and instability. Something needs to trigger the storms into forming. Finally, you need wind shear, which comes in two forms: directional and speed. The atmospheric environment that spawned tornadoes in Western NY had all of these factors. Forecasters look at atmospheric profiles showing wind shear, instability, and moisture over various points over the ground. The profile over Corfu at about 5PM showed significant instability, and impressive wind shear. In particular, it was notable that the low level wind just off the ground was from the southeast at about 5 to 15 mph while winds near 10000 feet (roughly 3km) were much stronger...near 40 mph from the southwest. |
 Local Analysis and Prediction System Sounding (LAPS) over Corfu at 5PM showing both strong directional and speed shear |
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.
hook over Northwestern Monroe County
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.
 severe thunderstorm warnings and tornado warnings issued by NWS Buffalo between 330 PM and 740 PM on July 25, 2009 |
DAMAGE SURVEY
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.
| Select NWS storm survey images of damage resulting from the Corfu tornado | |
 NWS storm survey - Corfu - Alleghany Rd, Corfu, west side |
 NWS storm survey - Corfu - Petals & Plants, East Main St, Corfu, south side |
 NWS storm survey - Corfu - Alleghany Rd, Corfu, east side |
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 NWS storm survey - Corfu - East Main Street, Corfu, looking east |
Select NWS storm survey images of damage resulting from the Hilton tornado |
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 NWS storm survey - Hilton |
 NWS storm survey - Hilton - some roof damage in background. |
 NWS storm survey - Hilton |
 NWS storm survey - Hilton |
