Severe thunderstorms, excessive rainfall, critical fire weather are all in the outlook today and into this last weekend of April. Very large hail, severe wind gusts, and a couple strong tornadoes will all be possible across the central and southern Plains today. Flash flooding also possible for portions of the Plains and Mississippi Valley. Critical fire weather conditions for Southern High Plains Read More >
Severe thunderstorms smack portions of the area July 26, 2005
Several rounds of thunderstorms occurred across the area on the 26th. Some of these storms reached severe levels with some significant damage noted, especially during the evening hours. One F2 tornado occurred during this event in extreme eastern Cass county and extreme western Miami county in north central Indiana.
Synoptic Conditions
An unstable atmosphere was in place across the forecast area much of the day. Surface temperatures in the 80s and dewpoints in the 70s allowed for lifted indices (LI's) to drop into the -5 to -7 C range, with surface based CAPE running from 2000 J/kg to as much as 4000 J/kg. Moisture convergence along the pre-frontal trough was allowing for a large pool of 75 to 77 degree dewpoints on the trough. A mid level cap was in place through much of the afternoon, but was able to weaken sufficiently to allow for convection to fire during the afternoon hours. Mid level winds of greater than 60 to 80 knots were observed for the storms to transport to the surface, with low level winds in the 30 to 40 knots range, allowing for sufficient deep layer shear to develop and sustain organized severe thunderstorms. Freezing levels during the event were very high, around 15,000 feet, which would only allow for hail with the strongest updrafts. Conditions evolved into a somewhat more favorable setup for severe weather into the evening hours.
During the evening, while much of the northern half of the forecast area had seen a large amount of thunderstorm activity and had much of the potential energy used already, locations in the south, especially southwest parts of the forecast area, remained unstable with a Lifted Index in the -3 to -5 C range. A shortwave entering the region, enhanced the lift and allowed for the cap to weaken much more than during the afternoon event. Numerous outflow boundaries were left over from the afternoon convection. I addition, winds near the surface backed to the southeast just ahead of an approaching line of thunderstorms that developed in Illinois. The outflow boundaries and the southeast surface wind were critical elements of the development of the F2 tornado.
Radar Observations and Interpretations
Once convection initiated and tapped the available energy, storms quickly intensified with many reaching maximum heights of 45,000 to as much as 55,000 feet. Strong updrafts allowed for cores of higher reflectivites to exist and maintain themselves above the freezing level, with velocity data indicating the potential for winds in excess of 70 mph (mid level winds of 60-80 knots with low level winds of 30 to 40 knots). Additional storms continued to develop along the pre-frontal trough as well as from interactions with numerous outflow boundaries from past storms.
During the late afternoon and early evening hours, a line of strong to severe storms developed on the main cold front in response to increasing large scale lift from the approaching mid level shortwave. One part of the line in particular, caused several reports of 60 mph wind gusts and some damage with several mid level mesocyclones being noted as it moved east into Indiana during the evening hours. A Line Echo Wave Pattern, or LEWP, evolved out of this line, which is a bulge in a thunderstorm line producing a wave-shaped "kink" in the line (Fig. 4). The potential for strong outflow and damaging straight-line winds increases near the bulge, which often resembles a bow echo. LEWP can also produce tornadoes if a sufficient source of low level horizontal vorticity, or spin, is present. In this case, the outflow boundaries from previous thunderstorms as well as the surface winds backed to the southeast provided that low level source of horizontal vorticity. Several radar images from the Chicago, Lincoln and Indianapolis NWS radars showed mesocyclones persisting along its track. Damage was noted just outside the CWA prior to the damage in the evening, with a few tornadoes being confirmed from the same complex of storms in Clinton County Indiana.
A strong mid level circulation was noted at 807 pm EST on the 1.5 degree elevation of the KIWX radar Storm Relative Motion (SRM) data as the leading edge of the storms moved into Carroll county and towards Cass and Miami county. This circulation, along with the history of the line and the presence of the remnant outflow boundaries, prompted the upgrade of the Severe Thunderstorm Warning for eastern Cass county and southern Miami county (issued at 749 pm EST) to a Tornado Warning at 812 pm EST. The initial touchdown occurred at 825 pm EST, giving a 13 minute lead time from the issuance of the Tornado Warning and a 38 minute lead time from the Severe Thunderstorm Warning. By 827 pm, the circulation is visible on the 0.5 degree SRM with gate to gate shear observed (an excellent indicator of an increasing potential for tornadic development). Spotter reports confirmed a tornado was on the ground at this time, producing damage. The associated 0.5 degree reflectivity image at the same time also showed a cell merger in progress between the line and another area of storms that had formed just ahead of the line. In the 832 pm EST 0.5 degree velocity image, a feature called the Rear Flank Downdraft (RFD) is observed on the velocity data as an area of significantly higher wind speeds. As the RFD moved quickly east it cut off the inflow of air into the tornado from the southeast and caused a rapid demise of the tornado. This feature was still visible at 837 pm. This area of wind subsequently began to weaken but not before quickly moving through parts of Wabash and Huntington counties causing damage as well. This line of storms continued rapidly east into northwestern Ohio.
Damage Information and other Noteworthy Items
During the afternoon thunderstorms, some damage was reported by trained spotters and county officials. A listing of this preliminary damage can be found by viewing the Local Storm Report for this event. A firefighter in Decatur who was observing the storms after hearing of the warning was indirectly struck by lightning near his house. He was transported to the hospital as a precaution but luckily he suffered no significant injuries. A house next door was also affected with 2 boys that were playing video games receiving a shock through their game controllers. No injuries were reported.
During the evening severe weather damage was more widespread, especially over the southwest portion of the area which was impacted by the aforementioned LEWP and F2 tornado. Trees and power lines were knocked down from White county east into Grant County Indiana from straight line winds. Amboy in southern Miami county received the worst straight line wind damage with many large trees down and a large building severely damaged. This area likely experienced 70 to 80 mph straight line winds as the Rear Flank Downdraft (RFD) wrapped around the south side of the storm producing the tornado farther north.
By far the worst damage was from the F2 tornado. The tornado formed over extreme eastern Cass county just south of Orland, and continued for about 4 miles into western Miami county a few miles northwest of Bunker Hill. Click HERE for a map of the tornado path. The actual tornado path is in yellow, with the light gray shaded area depicting where straight line wind damage occurred. For a detailed description of the tornado damage see the NWS Storm Survey. For a complete preliminary summary of all storm damage from the evening severe weather click HERE.
The following are pictures taken from the Storm Survey conduced by the National Weather Service in Northern Indiana. Hover your mouse over the image for a description. Click on the thumbnails for a larger image.
7/31/05...LWF/JPH/SLL
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