Akron's Hail Storm 15 Years Later

The strong upper level divergence strengthened frontogenetic forcing, and this led to strong lift ahead of the cold front. The surface analysis at 18Z (2 PM EDT) showed strengthening low pressure over northern Michigan with the cold front draped from Lake Huron through northwest Ohio and southern Indiana with an outflow boundary ahead of the front over southern Ontario. This outflow boundary was leftover from the previous day's convection over the Upper Midwest and acted as a pre-frontal trough to initiate new convection ahead of the cold front across northern Ohio. The strong upper divergence, strong lift ahead of the cold front and pre-frontal trough, and the frontal passage during peak heating created a very favorable set-up for a widespread severe weather outbreak across our region.

The enhanced southwesterly flow ahead of the front led to strong warm/moist advection and moisture pooling ahead of the cold front/pre-frontal trough. Temperatures at 17Z (1 PM EDT) were peaking in the upper 80s to near 90 degrees F in northern Ohio and northwest Pennsylvania as seen by the red temperature contours. Dew points, shown in the shaded colors, were pooling in the upper 60s to low 70s, so the convection had plenty of "fuel" in this warm and muggy airmass.

Another way to see the moisture pooling and lift along the cold front/pre-frontal trough is through the moisture convergence analysis. At 17Z (1 PM EDT), strong moisture convergence extended across much of northwest and north central Ohio. This is a great way to anticipate where convection may initiate.

Instability values were strong across southern and eastern Ohio on June 8, 2007. MLCAPE, a measure of instability, depicts values of 2000-2500+ J/KG in the area at the time of the hail event. While severe weather occasionally occurs with lower amounts of MLCAPE, most classic severe weather events feature over 1000 J/KG. We had plenty more than that to work with.

Wind shear, or changes in wind speed and direction with height, is a key ingredient in severe thunderstorms. Supercell thunderstorms, the type of thunderstorm that produces the vast majority of very large hail events, begin becoming possible when deep-layer shear reaches 30-40 knots. The more, the better for supercells. While deep-layer shear values weren’t off-the-charts on June 8, 2007, they did increase to 30-40 knots near Akron around the time of the hail event. This is enough for supercell development.

While the amount of instability was strong, it wasn’t off the charts. The same can be said about the deep-layer shear. However, the combination of the two was favorable for potentially significant severe weather. The Craven-Brooks significant severe parameter shows where the combination of instability and deep-layer shear is supportive of significant severe weather. This means, thunderstorm wind gusts of greater than 75 MPH, hailstones larger than 2” in diameter, or EF-2 or stronger tornadoes. Most of these significant severe events occur when this parameter is higher than 20. This day featured values of near 40 in the Akron, OH area.

Lapse rates are how quickly temperatures cool with height. Stronger, or steeper lapse rates point to more instability. When mid-level lapse rates are steeper, it tends to suggest greater instability in the layer of the atmosphere where hail develops in thunderstorms. Oftentimes, our mid-level lapse rates are weaker than 6C/km in summertime thunderstorm set-ups. On this day, they were near 7C/km, which is steep enough to begin pointing to increased potential for large hail, especially in the eastern U.S.

This plot shows the instability in the level of the atmosphere where hail forms in thunderstorms, along with the amount of deep-layer wind shear and the freezing level. While freezing levels were fairly high this day at greater than 4000m, instability neared 900 J/KG of CAPE in the hail growth layer, which is a substantial amount. Deep-layer shear increasing to 30-40 knots indicated just enough shear for supercell potential.

The Supercell Composite Parameter looks for combinations of instability and shear that can be favorable for supercells. Most supercells occur with values higher than 1. Values of between 2 and 4 were present near Akron, OH. While this isn’t extremely high, it is sufficient to support supercell thunderstorms.

Storm relative helicity measures the potential for cyclonic updraft rotation in right-moving supercells. More simply, it measures low-level wind shear in relation to a potential thunderstorm’s movement. Higher helicity values are considered more favorable for tornadoes. Values of 50-100 m2/s2 were in place on this day. This doesn’t necessarily prohibit tornadoes, but values higher than 100 m2/s2 begin signaling increased potential for tornadic supercells. Supercells with lower values are more often non-tornadic.

There were no tornadoes this day, despite a supercell (rotating) thunderstorm that produced very large hail in the Akron area. Most notable tornadoes occur with a Significant Tornado Parameter of greater than 1. Values this day were generally around 0.5. While not necessarily prohibitive of tornadoes, it suggests some factors (likely, a lack of stronger wind direction change in the low levels of the atmosphere, and somewhat high cloud bases) weren’t especially favorable for tornado development.

DCAPE, or downdraft CAPE, represents how much negative energy the descending cold air in a thunderstorm, or downdraft, has to work with. Stronger downdrafts suggest greater potential for damaging straight-line winds. DCAPE values over 1000 J/KG seem to begin to really increase damaging wind potential in thunderstorms; values of 1000-1200 J/KG were in place of the storm that produced the softball sized hail in Akron. Numerous reports of wind damage were received from thunderstorms across the area on this day.

Another way to diagnose the state of the atmosphere is by launching a weather balloon. The data from a weather balloon is called a sounding and is displayed as a Skew-T Log P diagram as shown here. The National Weather Service Cleveland office does not launch balloon soundings, but the Wilmington office in southwest Ohio launches them. These balloons are released twice per day at 8 AM EDT (12Z) and 8 PM EDT (00Z).

The 8 AM EDT observed sounding from Wilmington shows an increasingly favorable environment for severe thunderstorms. While this sounding is not the best representation of the environment in northeast Ohio later that afternoon, it does show CAPE values already building to several hundred joules. A capping inversion can be seen around 800 mb as indicated by the temperature line briefly rising with height instead of cooling with height. A capping inversion or "cap" is like putting a lid on a boiling pot of water. The atmosphere is most unstable when the air cools with height, so a layer that warms with height caps the unstable air below it, allowing the instability to build up. This cap allowed CAPE to continuously build through the early afternoon before the cap broke and finally allowed the built-up instability to be released, leading to explosive thunderstorm development. The shear profile, as seen on the wind barbs, is pretty unidirectional with height meaning that there is not much veering with height. This is not surprising since the helicity as discussed earlier was only 50-100 m2/s2, so any tornado threat would be isolated. However, the winds do increase with height, with values of 35-40 knots around 850 mb. This led to deep layer shear of 30-40 knots which could support some rotating supercells, increasing the potential for large hail and perhaps an isolated tornado within the rotating mesocyclones. The sharp decrease in temperature above the capping inversion also shows the steep mid-level lapse rates that were discussed earlier, and this also implies a favorable environment for large hail.

Another piece of data that comes from a balloon sounding or a wind velocity profile in Doppler radar is a hodograph. This essentially plots the wind barbs in a way that easily shows the amount of veering/turning with height. Hodographs that are curved in a looping shape suggest a large amount of veering with height, and thus high amounts of helicity to support rotating supercells with tornadoes. In this case, the hodograph was pretty straight and elongated which supports the unidirectional shear profile that was discussed previously, and this is why helicity values were only 50-100 m2/s2. There was enough curvature to support isolated supercells, but not widespread supercells and tornadoes which makes the Summit and Portage Counties supercell all the more impressive.

The loop on the right shows the radar imagery from approximately 2:00 to 3:30 PM EDT across the Northeast Ohio region. Base reflectivity at the lowest tilt (0.5°) is shown on the left while velocity at the lowest tilt (0.5°) is shown on the right. Note the "storm of the day", which really started to take shape just west of Medina, moving in an east-southeast direction as it approached the Akron area. Severe thunderstorm warnings are displayed with yellow polygons while tornado warnings are displayed with red polygons.

The loop on the right shows a radar cross-section from 2:23 to 3:40 PM EDT, depicting the developing reflectivity structure for the Akron storm. The cross-section is drawn roughly on a northwest to southeast line from just west of Medina to just east of Akron. The y-axis shows the height in thousands of feet. Note the development of pink and even purple colors aloft as the storm entered the Akron area. Bright pink colors represent radar reflectivity values starting at 60 dBZ, with darker pinks the closer you get to values of 70 dBZ.

Looking at the storm from this time-height point of view helps meteorologists determine the strength of the thunderstorm, which can help determine the greatest threat at the surface. For example, note the rapid development of pink colors aloft (between 10 and 20,000 feet and reaching perhaps as high as 25 to 30,000 feet at times) as the storm approaches the Akron area from the west. We are likely sampling hail and the hail growth process since the melting level was around 13,000 feet as mentioned above. At some point, the hail (pink and purple colors) becomes too heavy for the storm's updraft to hold, and thus allows hail to descend to the surface, noted by some of these pink and purple colors reaching below 10,000 feet and eventually to the surface just east of the city of Akron.

As the thunderstorm cluster moved east out of Medina County, it began to evolve into a discrete supercell. A Severe Thunderstorm Warning was initially issued for Summit County at 2:46 PM EDT for penny size hail and damaging winds over 60 mph.

The left-slider image shows a cross-section of the radar reflectivity from around 2:46 PM EDT while the right-slider image shows both radar reflectivity aloft (left) and storm relative velocity (right) aloft. Pink colors represent reflectivity values of 60 to 69 dBZ with purples representing 70 and greater dBZ. In the velocity image, green/blue colors and pink/red colors just west of the Akron area suggest the presence of storm top divergence, which is in an indication of the strength of the storm and can be helpful in determining potential hail size. The storm top divergence in this image is measured at around 100 knots, suggesting a max hail size of 1.75 to 2.00 inches.

As the discrete supercell rapidly evolved over Summit and southern Portage Counties, a new Severe Thunderstorm Warning was issued for Portage County at 3:12 PM EDT for golf ball size hail and winds over 80 mph.

The left-slider image shows a cross-section of the radar reflectivity from around 3:12 PM EDT while the right-slider image shows both radar reflectivity aloft (left) and storm-relative velocity (right). In the right of the right-slider image, the storm top divergence was calculated to be around 120 knots, with research suggesting that a max hail size of 4 inches or greater is in the realm of possibility.

Just 4 minutes later, rotation within the supercell had intensified over Summit County, so the Summit County warning was upgraded to a Tornado Warning. Reports of hail stones between golf ball and softball size were flowing into the office at this time, so the warning mentioned that the storm was capable of softball size hail and destructive straight line winds in addition to a tornado.

The left-slider image shows a cross-section of the radar reflectivity from around 3:17 PM EDT while the right-slider image shows both radar reflectivity aloft (left) and storm-relative velocity (right). The storm top divergence was still measured around 120 knots, so max hail size of 4 inches or greater was still possible at this time.

Rotation remained consistently strong as the supercell continued moving east, so Portage County was upgraded to a Tornado Warning at 3:21 PM EDT, and the warning continued to mention a threat of softball size hail, destructive straight line winds, and a tornado.

The left-slider image shows a cross-section of the radar reflectivity from around 3:21 PM EDT while the right-slider image shows both radar reflectivity aloft (left) and storm-relative velocity (right). The storm top divergence had reduced to 110 knots, though hail of 4 inches or greater remained in the realm of possibility. In the left-slider image, you can visualize the hail core aloft, descending down to the surface, with pink colors evident.

The supercell remained impressive as it approached Mahoning County, and reports of major hail damage in Summit and Portage Counties were widespread, so a final Tornado Warning was issued for Mahoning County at 4:06 PM EDT. The warning continued to mention softball size hail and destructive straight line winds of 80 mph or greater in addition to a tornado.