OVERVIEW

Around 530 PM EDT on Sunday July 26, 2009, a weak storm developed over Warren County, PA and moved northeast over southern Chautauqua County.  By 630 PM, the storm moved into southern Cattaraugus County, across the Onoville Marina and into Allegany State Park, producing a tornado. The storm then continued through the park to Limestone with more reports of tornadic damage.  A NWS Buffalo (NWS BUF) storm survey estimated that the storm produced EF-1 damage, a broken track about 20 miles long, and peak winds near 100 mph.   The storm moved generally to the east between 20-25 mph over the course of about 50 minutes.  Due to the heavily forested terrain, it is difficult to determine if the tornadic storm track was continuous or if there were several tornado touchdowns. However, the storm survey did note that the width of the tornado was about 100 yards wide at one point due to extensive tree damage.

Most of the property damage was within the Onoville Marina where several boats were overturned.  There was one mobile trailer that was similarly overturned.  Several other trailers were pushed off of their supports, and numerous awnings were destroyed.  In Allegany State Park and along Guernsey Hollow road, hundreds of trees were knocked over, although most of the damage occurred in heavily forested areas.  There was notable tree damage near the Cain Hollow Campground and within the Rice Brook area of the park.

Path of tornado moving across southern Cattaraugus County

 

Path of tornado moving across southern Cattaraugus County - detailed and zoomed in view
Path of tornado moving across southern Cattaraugus County Path of tornado moving across southern Cattaraugus County - detailed and zoomed in view

METEOROLOGICAL BACKGROUND

The afternoon surface pattern featured an approaching cold front. Interestingly, this cold front had moved eastward only about 125 miles over a 24 hour period, or roughly 5 mph. In fact, it was the same cold front that was upstream during the preceding tornadic day. An analysis by the Hydrological Prediction Center placed the cold front from Toronto southeast through Cleveland, OH at 400 PM.

 

HPC analysis of sea level pressure and fronts for the Eastern Great Lakes region at 400 PM Sunday July 26, 2009
HPC analysis of sea level pressure and fronts for the Eastern Great Lakes region at 400 PM Sunday July 26, 2009
 
The storm that produced the relatively long track was unusual because the storm environment, although unstable, had little apparent directional shear aloft.  The storm was also rather weak at the time of tornadogenesis. In fact, the storm itself was not even producing lightning at the time of the tornado touchdown.  Radar and satellite data estimated that the storm top was near 20000 feet, with an air temperature at that altitude near -10C or 14F.  At this temperature, it is barely cold enough to produce hail.  In comparison, the tornadic storms on the preceding day (Corfu and Hilton, NY) had storm tops near 25000 feet, corresponding to temperatures near -22C or -8F.  Hence the prior day’s storms were in a much more unstable environment as it was much cooler aloft. Another difference between the two days was a drier low level airmass. The Local Analysis and Prediction System Sounding (LAPS) showed a subtle inverted-v signature and higher Lifting Condensation Level (LCL) (compared to the prior day). This drier low level airmass is not typically associated with tornadic activity. Local Analysis and Prediction System Sounding (LAPS) over southern Cattaraugus County at 600 PM showing good speed shear but little directional shear.
Local Analysis and Prediction System Sounding (LAPS) over southern Cattaraugus County at 600 PM showing good speed shear but little directional shear.
 

Instability is important, but it's wind shear, and in particular directional shear (winds that change direction with height) that's needed to get a rotating storm and the possibility for tornadoes.  The immediate storm environment where the tornado formed clearly had directional shear, as the storm was definitely rotating at about 636 PM when it crossed into Southwest Cattaraugus County. The storm did not look very impressive from a reflectivity standpoint, although there was an inflow notch. The presence of the directional shear may have been due to the fact that in the afternoon prior to the tornadic storm, a line of thunderstorms had developed on a lake breeze off of Lake Erie and moved east across New York's Southern Tier and Finger Lakes region. The westerly flow behind the lake breeze, combined with weak outflow from the thunderstorms, together with the approaching cold front may have created a low level directional wind shear capable of producing a rotating storm.

Forecasters can look at the velocity data from a storm in a “storm relative” sense.  To understand storm relative winds, follow this example:  If there is a 10 mph out of the north and you ride a bicycle 10 mph to the south, the wind would be at your back and it would feel like there's no wind at all.  However, a person watching you ride by would still be experiencing a 10 mph wind out of the north.  In this case the storm is analogous to the bicycle rider.  With an average wind estimated to be out of the southwest at around 30 mph, the storm relative winds from this storm show some rotation at 636 PM.

Reflectivity at 636 PM EDT showing weak storm but with inflow notch
Reflectivity at 636 PM EDT showing weak storm but with inflow notch
Storm relative winds showing rotation at 636 PM over southwest corner of Cattaraugus County
Storm relative winds showing rotation at 636 PM over southwest corner of Cattaraugus County
At 650 PM, the reflectivity showed a classic hook echo, often an indicator of strong rotation.  A tornado warning was issued at 659 PM after the storm took on more characteristics associated with tornadic activity. 
Reflectivity showing hook echo near Onoville Marina at 650 PM
Reflectivity showing hook echo near Onoville Marina at 650 PM
Tornado warning issued at 659 PM
Tornado warning issued at 659 PM

 

With the exception of the period during which a hook echo was noted, the reflectivity of the storm did not look overly persuasive.  However the storm relative winds showed strong rotation during the same time frame.
Reflectivity loop from 600 PM through 800 PM
Reflectivity loop from 600 PM through 800 PM
Storm Relative loop from 600 PM through 800 PM
Storm Relative loop from 600 PM through 800 PM
   

By 745 PM the storm showed little rotation and no evidence of a hook.  The forecast office canceled the tornado warning and issued a severe thunderstorm warning in its place. 

 

Loop showing warnings issued by NWS BUF during the Cattaraugus County Tornado
Loop showing warnings issued by NWS BUF during the Cattaraugus County Tornado

 

The storm continued into Allegany county and strengthened again after 800 PM, prompting another tornado warning at 815 PM. However, no verification was received.


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 the storm survey of the Cattaraugus County tornado.

Select NWS storm survey images of damage resulting from the Cattaraugus County tornado

 

Entrance to Cain Hollow Campground
Entrance to Cain Hollow Campground - note downed trees facing different directions
Large pine near Cain Hollow
Large tree near Cain Hollow.
Trees in playground at Cain Hollow Campground
Trees in playground at Cain Hollow Campground
Parking entrance to see the remnants of earlier tornado damage from an early 1990s tornado in Allegany State Park. This site was very close to the July 26 2009 tornado track.
Parking entrance to see the remnants of earlier tornado damage from an early 1990s tornado in Allegany State Park. This site was very close to the July 26 2009 tornado track.
Portable Bathroom at Onoville Marina
Portable Bathroom at Onoville Marina. Most of the trees at the marina were cleaned up before the storm survey. Similarly, overturned boats and trailers had been moved back to their proper positions.