July 24, 2010 Southern Tier Tornadoes

OVERVIEW

Between 4 and 6 PM EDT on Saturday July 24, 2010, four tornadoes touched down in the Southern Tier of Western NY (Figure 1 and table below).  The first went through Mayville at around 440 PM, crossed the Chautauqua Lake, and moved through Dewittville, while a second touched down and went through Randolph around 525 PM.  These two tornadoes caused significant damage and were rated EF2.  A third but weaker EF1 tornado touched down in Great Valley at around 525 PM.  The fourth tornado was rated an EF1 and moved through the town of Carrollton and into the village of Allegany at around 550 PM.
	Figure 1: Path of four tornadoes moving across Chautauqua and Cattaraugus Counties	Disclaimer

METEOROLOGICAL BACKGROUND

Western NY was under what is commonly referred to as the warm sector, the area between a warm and cold front (Figure 2).  The airmass over the region was unusually moist, with dewpoints in the lower to mid 70s.  In fact, as we moved into the warm sector on the previous day, the precipitable water rose to 2.26 inches, one of the highest readings over a 50 year period.   By Saturday morning, a significant area of dry and warmer air aloft had worked its way into the region, dropping the precipitable water down to a more tolerable 1.41 inches. The airmass near the ground retained a high level of moisture. Even with the warmer air in place aloft, we were still unstable at the start of the day.    A significant amount of cloud cover prevented storms from quickly forming though and temperatures, already in the mid 70s in the morning, were slow to rise.  By mid afternoon, a bit of clearing allowed temperatures to rise into the lower 80s, increasing instability. 

Meteorologists often refer to three key “ingredients” needed for thunderstorm development:  moisture, instability, and lift.  For tornadic storms, you need an additional ingredient called directional wind shear.  It is clear that there was plenty of moisture as noted above with high dewpoints in the area.  We also had plenty of instability.  The lifting mechanism came in the form of a dying Mesoscale Convective System (MCS) over Southern MI, that left what is known as a Mesoscale Convective Vortex (MCV) spinning quietly away as it crossed into Lake Erie (Figure 3).  The origins of the MCS and eventual MCV can be traced back to Iowa Friday night.  The MCV provided both the lift and directional shear needed for a tornadic event. 

Figure 2: RUC analysis of mean sea level pressure and HPC surface analysis showing warm and cold fronts for the Eastern Great Lakes Region at 12Z (7 AM EDT)	Figure 3: Infrared Imagery combined with KCLE WSR-88D radar showing MCV near Detroit MI at 1640Z (1240 PM EDT). Five minute lightning (white plus and minus marks) also shown. Green line and circle shows track of MCV from IA east across southern MI and over Lake Erie. Click on the image for a loop showing the MCV moving eastward from Southern MI into central Lake Erie.

The MCV adds rotation into the air mass as seen by helicity, which is a measure of rotation in the world of fluid dynamics.  (The word “vortex” also refers to a spinning air mass, so if you have a vortex moving by, you should expect to be able to measure helicity.)  While far from a "silver bullet" for forecasting tornadoes (due to many cases where helicity is present without any tornadoes), there was a clear indication of helicity, with storm relative values exceeding 200 m2/s2 (Figure 4).  Generally, values in excess of 150 m2/s2 are important if other atmospheric conditions are conducive to tornadic storm development.

Figure 4: 0-1KM Storm Relative Helicity at 21Z (5PM EDT)

Between 12 and 3 PM, the vortex with a broad area of rotation in the atmosphere (Figure 5) can be seen clearly on both the Detroit and Cleveland NWS WSR-88D radars. There was little thunderstorm development with this feature until after 3 PM, when a line of thunderstorms developed on the back side of the MCV. This line continued to rapidly intensify as it moved across the lake while other storms formed out ahead  of the main line.  These storms quickly began rotating, that is, they developed into supercells.  Individual supercells commonly form out ahead of a squall line in the Midwest and are sometimes tornado producers.  As the line approached these storms, their rotation increased, as seen in the storm relative motion depicted by NWS BUF WSR-88D.  While the radar cannot directly pick out a tornado, a trained meteorologist can infer strong rotating updrafts and storm structures from the radar that typically produce tornadoes.  In this case, the forecaster first issued a Severe Thunderstorm Warning at 341 PM, then upgraded the warning to a Tornado Warning at 408 PM.  (This resulted in a lead time of about 30 minutes (Figure 6).  Typical lead times across the NWS are about 15 minutes.) 

Figure 5: MCV seen on KCLE radar at 1640Z (1240PM EDT). The cyclonically curved yellow arrow shows the location of the MCV. Five minute lightning is displayed as minus marks. Click on the image for a loop showing the MCV moving eastward from Southern MI into central Lake Erie.	Figure 6:2008Z (408PM EDT). Storm Relative Motion showing velocity couplet between Ripley and Mayville. The yellow box indicates a severe thunderstorm warning issued at 341 PM. The red box indicates the tornado warning area issued at 408 PM. The first tornado touched down near Mayville at 440 PM.

At one point there were three prominent areas of rotation over Chautauqua County, although only one of those radar signatures produced a tornado in that county (Figure 7).  The first area of rotation didn't produce a tornado until it moved over Great Valley, in Cattaraugus County.  The second area of rotation never produced a tornado.  The trailing area of rotation was responsible for three tornadoes, with the first touchdown near Mayville and Dewittville, then again over Randolph, and finally over the Village of Allegany.  The radar showed several other areas of rotation over both NY and PA, although only one other tornado was reported (in Galeton, PA (Potter County)).

Figure 7: Same as Figure 6 but at 2036Z (436 PM EDT). Click on the image for a loop showing the progression of the storms and NWS warnings between about 4 and 6 PM EDT.

 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 surveys of the areas affected by the tornadoes on July 24, 2010 (Figures 8-20).

Figure 8: NWS storm survey - Mayville - Roof damage to a warehouse near Chautauqua Lake...then the debris field follows along the path of the tornado out to/across Chautauqua Lake.	Figure 9: NWS storm survey - Mayville - Areal shot of residential damage. Numerous trees down and roof/siding damage.
Figure 10: NWS storm survey - Mayville - Residential damage...downtown Mayville.	Figure 11: NWS storm survey - Mayville - Another angle of the damage path heading out across Chautauqua Lake.
Figure 12: NWS storm survey - Mayville - Debris thrown more than a mile away litters this field.	Figure 13: NWS storm survey - Mayville - Major EF2 damage to these structures in Dewittville...across Chautauqua Lake from Mayville.
Figure 14: NWS storm survey - Mayville - Almost complete destruction to this building in Dewittville.	Figure 15: NWS storm survey - Mayville - Condos on the Chautauqua Lake shore line that lost the majority of its roof. Not visible in this photo, but a kayak from the lake was thrown into one of these condo units.
Figure 16: NWS storm survey - Mayville - Multiple trees down in a row with topped trees in the background.	Figure 17: NWS storm survey - Randolph - Tree on care with nearby wires down.
Figure 18: NWS storm survey - Randolph - Damage to area homes from fallen trees.	Figure 19: NWS storm survey - Randolph - Barn roof lifted off with multiple trees downed or topped in the area.
Figure 20: NWS storm survey - Randolph - Barn roof lifted off.