One of the first questions that follows a severe weather event (like the one Thursday) is: what happened? And how strong was it? Often, the first thing that comes to people’s minds is a tornado, but as we all know, not all severe weather damage is caused by a tornado.
It falls upon National Weather Service storm survey teams to make the decision. As a severe weather event is coming to an end, NWS forecasters and managers begin to determine strategies for surveying significant storm damage, and assemble teams to examine the damage first-hand. The surveyors are equipped with GPS devices and high-resolution maps, digital cameras, and reference material. The GPS and mapping work is done so that the scope of the event can be understood and catalogued for future research, and the reference material allows survey teams to compare what they see at the time to past surveys or idealized estimates. Radar data and existing reports are also key components of the survey, providing survey teams an estimate of what to expect. (Was there rotation? Was it a line of thunderstorms? How are the damage reports arranged?)
How do we decide if it’s a tornado, a microburst, or straight-line wind damage? It’s tough to describe it in a blog post, but like with many things in meteorology and the National Weather Service, training is required. Meteorologists must take special training modules to be permitted to survey thunderstorm damage, and must have a good idea of the inner workings of the thunderstorm to be able to visualize what may have occurred. Survey teams will often chart what they observe on maps to help with this visualization process and to compare with radar data. However, in general, straight-line winds sweep across an area, affecting multiple locations at the same time; tornado damage is usually more concentrated, possibly appearing somewhat random or occurring on a long, thin path; and microburst damage is a combination of the two, with concentrated damage occurring over a large swath where the storm collapsed and winds spread outward.
Determining the intensity of a storm has become more scientific with the implementation of the Enhanced Fujita Scale in February 2007. The “EF” scale was calculated by engineers and scientists based on extensive testing. It divides what surveyors may see into damage indicators (DI’s for short), which are then divided into degrees of damage (DOD’s). For example, if multiple large branches were snapped in an area of oak trees, that would fall under DI #27, “Tree-hardwood”, and the corresponding DOD would be 2 (Large branches broken, 1-3” in diameter). Based on testing, that yielded wind speeds ranging from 61 mph to 88 mph. If the trees were snapped at the trunk, wind speeds increase to 76 to 118 mph. Surveyors assign an EF-scale rating to tornadoes based on observations of these damage indicators and the degrees of damage.
Depending on the scope of the event, storm surveys can take hours or even days, as the survey team drives around to find damage, and stops to examine the patterns more closely. For example, in the case of the February 6, 2008 outbreak where two EF-4 tornadoes occurred, surveys occurred over a period of several days, including multiple trips to the damaged areas, aerial surveys to determine the extent of the damage, and consulting with experts with decades of experience. Conversely, weaker, shorter-lived tornadoes like the ones that occurred on April 2, 2009, required the work of two teams (due to the locations of the storms) over roughly half a day.
We hope to have more “inside the NWS” posts in the future to clear up some of the mysteries of NWS actions and operations. In the meantime, here’s hoping we spend less time on this particular duty over the coming weeks.