Interesting Twitter Thread from Weather Service

After the tornado this week (or tornadoes) the National Weather Service in Birmingham tweeted a miltipart description of how they determine strength and damage of a tornado. I thought it was interesting.

@NWSBirmingham: 🌪🔍How the National Weather Service Conducts Post-storm Tornado Surveys.🌪🔍 (thread/1)

@NWSBirmingham: After a storm hits, such as the ones that produced the tornadoes across central Alabama the last couple of weeks, the National Weather Service is tasked with trying to determine the strength and extent of the tornado's damage. /2

@NWSBirmingham: NWS meteorologists don't simply look at damage (either on scene or in pictures) and just come up with a rating. /3

@NWSBirmingham: Since the Enhanced Fujita (EF) scale was implemented in 2007, the process of tornado damage assessment has become much more structured and systematic. It requires being equal parts meteorologist, engineer, and detective. /4

@NWSBirmingham: After the severe weather has ended, the NWS first identifies survey target areas. These might include areas with known tornado damage, damage of unknown cause (tornado or straight-line winds), or where there were radar indications of a tornado but no damage reports. /5

@NWSBirmingham: Survey teams of 2 to 3 individuals are then sent out to these areas to do the investigation. The tools of the trade include GPS receivers, laptop computers, internet connected tablets, as well as some lower tech items like binoculars, compasses, and paper road atlases. /6

@NWSBirmingham: Once an area of damage is identified, its position (using GPS) is digitally plotted on an electronic map. The surveyor then determines a wind speed estimate at that location using a multi-step process that was developed with the implementation of the EF-scale. /7

@NWSBirmingham: The first step is to determine which of 28 "Damage Indicators" ("DIs") the damage falls under. These DIs are things like hardwood/softwood tree, one- or two-family residence, single/double wide mobile home, motels, school, metal building system (warehouse), etc. /8

@NWSBirmingham: After choosing the appropriate damage indicator, the surveyor then determines the "Degree of Damage" ("DoD") associated with that DI. /9

@NWSBirmingham: For things like trees, DoDs can range from "small limbs broken," to "trees debarked with only stubs of largest branches remaining." /10

@NWSBirmingham: For buildings, such as homes and other larger structures, DoDs can range from minor roof damage, to complete destruction, and everything in between. Each damage indicator has its own unique DoD table. /11

@NWSBirmingham: Finally, using the DI-DoD table, a wind speed estimate is determined. /12

@NWSBirmingham: The surveyor(s) has the option to increase or decrease the expected wind speed estimate, based on the construction (of buildings) or condition (of trees). This is where the engineering aspect comes into play, especially where larger buildings experience significant damage. /13

@NWSBirmingham: For example, a "well-built" single family home that is completely flattened, with all walls collapsed, is expected to have experienced winds around 170 mph (EF4). /14

@NWSBirmingham: However, if a surveyor finds that the framing of a home was not sufficiently attached to the foundation, or other/additional structural deficiencies, he/she would then have reason to lower the speed estimate to as low as 145 mph (EF3). /15

@NWSBirmingham: Please keep this in mind when you see pictures of homes with tornado damage. "Well-built" does not necessarily mean "newly built." Brand new homes may have structural points of failure that may cause them to become more damaged than older homes that are structurally secure. /16

@NWSBirmingham: This final (adjusted) wind speed estimate determines the EF rating at that damage point. This information is then saved to a database, along with other details about it (GPS location, orientation of damage, width of damage path in that area, other notes). /17

@NWSBirmingham: Once all (or a representative sample) of the damage points are entered into the database, they can be viewed on a mapping application to determine the total damage swath/path. The tornado's final EF rating is determined by the highest EF rating at any point along its path. /18

@NWSBirmingham: This may seem like an unnecessarily complicated or cumbersome method of doing tornado damage surveys. But doing it this way ensures consistency, from one surveyor to the next (even in differing parts of the country). /19

@NWSBirmingham: It also explains why it often takes multiple days to complete tornado damage surveys, especially for larger outbreaks. End of thread/20