Laws, Kevin B; Crowe, Christina C

This presentation will give an overview of the tornado warnings issued by the Huntsville and Birmingham NWS Weather Forecast Offices on 27 April 2011, from the perspective of those who issued warnings on that day. Between the two offices, over 140 tornado warnings were issued during a 19 hour period, with approximately 60 verified tornado touchdowns. The early morning Quasi Linear Convective System produced several tornadoes, many of which were surveyed as stronger than EF-2, while an even greater number of EF-3 or stronger tornadoes were associated with high precipitation and classic supercells during the afternoon and evening hours. Several key warning decisions were made by forecasters that potentially saved numerous lives, including issuing warnings well downstream of the parent storm, along with frequent use of the tornado emergency wording within the warning text or in place of the tornado warning headline.

In order to provide adequate lead times for long-lived supercell storms, various warning philosophies were used at each office. The Birmingham office implemented the use of 60 minute warning polygons for situations where a tornado traversed several counties, including crossing county warning area boundaries, as in the case of the Hackleburg/Phil Campbell EF-5 tornado that was on the ground for over 100 miles. When the decision to use longer-fused or overlapping warnings was applied, it was imperative to provide the end users with frequent storm updates within the context of severe weather statements. In nearly all of the warnings that day, statements were issued at a 10 minute or shorter interval to provide key information such as tornado position and movement within the larger warning polygon. In contrast, the storm structure further north required the Huntsville office to use shorter fused warnings to prevent confusion where storms were occurring close to each other and passing over the same locations. In both situations, new warnings were often issued after storms had progressed half way through the original polygon to ensure adequate lead time for customers downstream of the supercell storms and tornados.

Another crucial warning decision was the use of the tornado emergency wording in severe weather statements and headlines of the tornado warnings issued when large tornadoes were interpreted from radar due to the appearance of a debris ball, or reports were being received of large tornadoes on the ground. Previously, this unique wording was mainly applied to destructive tornadoes that were impacting large metropolitan areas, where the potential loss of life and property damage was expected to be great. However, for this unprecedented outbreak, numerous tornado emergencies were issued that included many smaller communities in an attempt to emphasize the extremely dangerous nature of the situation and save as many lives as possible. In the case of the Tuscaloosa-Birmingham EF-4, tornado emergencies were broadcast for the communities of Pleasant Grove, Concord, and Hueytown, east of the city of Tuscaloosa where the tornado was surveyed to be nearly one and a half miles wide, with a wind speed of 190 mph. In addition, for a vast majority of the tornado emergencies issued for northern Alabama, many of the locations in the warning polygons were rural townships. The utter devastation seen during storm surveys in these smaller communities can only testify to the equal necessity of elevated wording in such events.