How bad will it be? A discussion of damage forecasts.

In a previous article I discussed the problems of measuring wind speeds (link). But even if you know what wind speed someone is telling you in technical terms (units, averaging times, etc) that’s probably not helping you to understand what conditions will be like and what damage to expect. This is another generic article (summary says five minutes to read) that uses Helene as an example, but hopefully folks will read it to help understand what kinds of damage to expect when someone says “sustained winds of X” or “gusts to Y” and how to avoid confusing the two.

Before discussing the relationship between wind speeds and what to expect, it is important to understand gusts. In the discussion that follows (and in fact this is part of the technical definitions of wind speeds), we are talking about two minute average winds speeds – the kind that are measured and reported at your local airport, and the winds I discuss here on the blog. But recall that a two minute average consists of forty (40) three second observations (the 3 second gust). So if the forecast is for winds of 30 mph, that means in a typical environment you would expect at least one of those 3 second gusts to be 33% higher than the average – in other words, a 40 mph gust, and in convective (thunderstorm) conditions, or looking at a large area, you might see as much as 50% higher (45 mph).

This is why I get a bit animated about media reporting “hurricane force gusts” and scaring people in to thinking that means hurricane like damage across the entire area. A 50 mph tropical storm will probably have gusts up to 75 mph, but that is included in the definition of a tropical storm wind. And as you will see below, that’s very different in impacts from a sustained 75 mph hurricane wind (which includes the assumption of gusts over 100 mph). The main thing is to be consistent, and make sure the listener understands which measure you are using and how it relates to damage. I might mention gusts, but normally I only talk about sustained or average winds because the gusts are assumed in how winds are measured and used – and what we care about anyway is damage and impacts.

There are a number of different scales to try to relate conditions (damage) to wind speeds. The one I use in my impact maps is a version of the Extended Beaufort Scale (link to Wikipedia). Note that unlike the suburb of Frogmore known locally as Beaufort (pronounced “BeU fort”), the scale is mispronounced as “Boh fert”). The scale was devised in 1805 by Sir Francis Beaufort (who, despite being a really smart guy, also mispronounced his own name as “Boh fert”). He refined it over time and it was adopted by the Royal Navy in 1830, who began requiring ships at sea to report conditions using it, and interestingly was first used on the HMS Beagle while carrying Darwin on his first voyage in 1831-1836. The original scale only went up to hurricane force winds (“Force 12”), but was extended in the 1940’s to provide more detail at higher wind speeds encountered in tropical cyclones. The scale is nice in that it relates easily observed phenomena to the wind speed – and vice versa.

I only plot the highest nine levels of the extended scale on my TAOS(tm) maps to avoid coloring in areas that won’t see significant impacts from the storms. Here is the table used by the TAOS(tm) model for mapping, in MPH, with the typical and max gusts associated with each wind speed:

	Wind in MPH
Sustained	Typical Gust	Max Gust	Effects	SS Category
25	33	38	Small Trees Sway
33	44	50	Large Trees Sway
40	53	60	Branches Breaking	TS
50	67	75	Trees Down; some power loss	TS
60	80	90	Minor Damage; power out	TS
75	100	113	Moderate Damage 5% of value	Low 1
90	120	135	Widespread Damage	High 1
100	133	150	Severe Damage	2
125	166	188	Catastrophic Damage	3

You’ll notice the table only goes up to Saffir Simpson Category 3, or 125 mph sustained winds. Anything over that is, to be brutal, “making the rubble bounce” from a damage perspective and if you stick around in those kinds of conditions you lack common sense and are truly taking your life in your hands. So everything above 125 mph gets lumped in the “catastrophic damage” category since more detail doesn’t really matter.

Here is the map posted here on the blog from the Thursday afternoon Hurricane Helene forecast by NHC (about 12 hours before landfall) – notice the table and color codes in the upper right hand corner of the map:

Note that the Savannah area was mostly in the 50mph wind swaths, going up to 60 or so inland. Therefore the forecast impacts were for there to be trees down, power out, some minor structure damage. So how does that match up to wind reports and damage (again note we are just discussing wind here; the water is a whole different discussion)? Here is the preliminary Charleston WSFO map (their web page has rain and tornado reports on it as well); note these are gusts:

Especially considering the forecast map is a damage/impact/conditions estimate based on a forecast, that’s not too bad. I think most people along the coast experienced these conditions – power out, big branches and a few trees down, minor structural damage (except the unlucky who got hit by one of the trees, who of course saw more damage), although of course it was worse inland. The post-storm map, based on the actual storm track and intensity, fits even better (but that’s hindsight and only verifies the impact models, not what you were told to expect in advance).

Hopefully the above explains a bit more about the relationship between winds, what you see after a storm, and specifically what is shown here on the blog.

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Since it’s Saturday, here is some classical storm music (REO Speedwagon, “Ridin’ the storm out”:

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