JACKSONVILLE, Fla. – A survey of storm damage performed by the National Weather Service of Jacksonville allowed investigators to determine an EF-0 tornado tore down tree limbs and damaged homes on Monday at 4:17 p.m.
The NWS released a summary of the damage it found:
A discrete supercell well ahead of an approaching cold front and downstream of a warm front north of the storm moved across the Suwannee Valley paralleling Florida State Road 247. A tornado developed from this supercell northwest of Lake City, east of Lake Wilson, and north of Lake Loma in Columbia County in North Florida in the late afternoon. Sporadic wind damage occurred with the tornado including fencing, roof and house damage and downed tree limbs.NWS Jacksonville
The NWS estimated the tornado had peak winds of 80 mph and made a path that was 60-yards wide for just over a third of a mile.
When the NWS sees indication of rotation on the radar, it issues a tornado warning and later goes out to survey the damage to see if it was in fact a tornado. It looks for signs of rotating winds within the damage, like trees knocked down in differing directions.
According to the NWS’ website, a survey team’s mission is to gather data in order to reconstruct a tornado’s life cycle, including where it occurred, when and where it initially touched down and lifted (path length), its width and its magnitude. It should also be mentioned that survey teams are occasionally tasked with determining whether damage may have been caused by straight line winds or a tornado and assessing the magnitude of straight line winds. With respect to tornado damage surveys, one of the most difficult tasks is assigning a rating to a tornado.
To determine the magnitude of the tornado, the survey team will attempt to find the worst damage since this is how the tornado will ultimately be rated. Once the worst damage is identified, the survey team will assign a damage indicator to the structure or object.
There are 28 damage indicators, including one- or two-family residences, manufactured homes, motels, warehouses, schools, small retail buildings (e.g. fast food restaurants), and even trees. Each one of the damage indicators has a description of the typical construction for that category of indicator. For example, typical construction for one- and two-family residences includes asphalt shingles, tile, slate or metal roofing, attached single car garage, and brick veneer, wood panels, stucco, vinyl or metal siding.
The degree of damage has several different categories, and each category has an expected wind speed and a lower and upper bound wind speed. For one- and two-family residences, if a tornado breaks glass in windows and doors, the expected wind speed is 96 mph, the lower bound wind speed is 79 mph, and the upper bound wind speed is 114 mph.
If a tornado produces damage that results in the collapse of all interior and exterior walls, the expected wind speed is 170 mph, the lower bound wind speed is 142 mph, and the upper bound wind speed is 198 mph. This is where the job becomes difficult for the survey team because the team must know some basics about construction. If the quality of construction meets strict building code, the survey team will likely assign an expected wind speed to the damage.
If the construction fails to meet code, a lower bound wind speed may be assigned, but if the construction exceeds code and/or is well-engineered, it may be assigned an upper bound wind speed. Once the expected, lower bound, or upper bound wind speed is determined, it is applied to the EF Scale to assign a rating.