Idalia and Savannah: What is wind?

TLDR: going to be a rough afternoon/evening in coastal Georgia, however, it’s not likely there will be hurricane force winds inland north of Hazlehurt-Jesup-Darien, except right on the coast. Coastal flooding is going to be an issue, but not Matthew levels. You know the power is going to go out, the usual streets in town will flood, and there will be a mess with limbs down, etc. Here’s more:

In terms of impacts, the picture really hasn’t changed too much despite the fact the area is now under a hurricane warning. The storm is projected to be moving a bit faster after landfall so impacts are a couple hours earlier than forecast yesterday. NHC raised the hurricane warning because winds right on the coast might reach hurricane force, and offshore in squalls will likely do so.

For Coastal Georgia and South Carolina, don’t be duped by the fact that landfall will be in Florida mid-morning Wednesday – given the size of the storm, by then conditions will be deteriorating across South Georgia. Brunswick/Glynn and Camden Counties will see things deteriorating around sunrise, in Savannah a bit later by the 10am to noon time frame, and Beaufort area a couple hours later (noon to 2pm). The worse of it should again progress south to north – peaking in Brunswick around 4 pm, Savannah 6 pm, and Beaufort a couple hours after that.

The inland track and “wobbles” will matter a lot. The discussion below is based on the National Hurricane Center scenario, which is what you should use for planning purposes. We might not get so much if the storm decays faster after landfall (which I think might happen), but while we can hope for that, hope isn’t a good plan …

On the most likely track, the storm center will pass just inland of the coastal counties. As you can see in the impact swath map below, that’s not great, as the worst impacts extend from about 20 miles left of the track to 60 or 70 miles to the right of the track. So, using the NHC track as the worst case, and given how storms decay, Savannah will get tropical storm conditions – 50mph sustained winds in open areas, gusts probably to the 70mph range inland (higher on the water). That’s enough to cause big limbs to come down, widespread power outages, that sort of thing. A couple of trees are likely to topple over. Heavy rain, 6-10″, are also possible on that track. Idalia should be fast moving, so while there will be flooding in the usual places in town, it should be in the “bad but not catastrophic” category.

So in short, in coastal GA/SC Idalia will be hazardous, you don’t want to be out in it, but not dangerous if you are in a typical residential structure or commercial-residential (apartment complex). Mobile homes are a bit iffy – this is at the threshold where you should consider moving to a sturdier shelter, especially if tornadoes start to crop up since even a weak one is bad news for manufactured housing. And speaking of tornadoes …

Tornadoes: landfalling storms in the southeast do produce tornadoes as they decay. They tend to be on the weaker side – F0 to F1 – so these are not the Kansas, Wizard of Oz type tornadoes that sweep homes off their foundations. But they do cause damage, and be alert for warnings. This is why I strongly recommend having a battery powered weather radio handy – cell phone and networks often go down in storms, and it’s the only RELIABLE and timely way to get warnings. There are already tornado warnings up in south Georgia (Pierce/Brantly County) this morning at 6:30am.

Flooding: The latest forecast shows the peak water levels at the Fort Pulaski tide gauge at about 10.5 ft MLLW (remember, that’s only about 3 feet above our normal high tides), so that’s the threshold of “major coastal flooding”. The GFS based tide forecast shows 10.5ft tonight at 9pm (high tide). According to the NWS/NOAA guidance …

• At 9.5 ft MLLW, minor coastal flooding occurs. Flooding will begin to impact Shipyard Road to Burnside Island. Parts of Ft Pulaski National Monument will begin to flood, including several trails. Flooding will also begin to impact Tybee Island including Catalina Dr and Lewis Ave. In Bryan County, water could breach docks near Ft McAllister and flooding will impact portions of Mill Hill Rd. In Liberty County, flooding impacts the Halfmoon Landing area and Cattle Hammock Rd near Bermuda Bluff subdivision.
• At 10.0 ft MLLW, moderate coastal flooding occurs. Shipyard Rd will be impassable, isolating residents on Burnside Island. Water will start to encroach on HW-80 and as the tide gets closer to 10.5 ft MLLW, could begin to cover portions of the roadway. Flooding will expand on Tybee Island and Catalina Dr and Lewis Ave will be impassable. Flooding will also impact Wilmington Island, the Coffee Bluff community, Ossabaw Island, Sapelo Island, and portions of HW-17 south of Darien.
• At 10.5 ft MLLW, major coastal flooding occurs. Damaging flooding is expected, expanding along the entire southeast Georgia coast. Flooding will likely cause the closure of HW-80, isolating residents on Tybee Island. Several other island communities will also likely become isolated due to flooded and impassable roadways. On Tybee Island, widespread significant flooding is expected with numerous properties impacted.

So if we get ~10.5 feet at the gauge, that is a couple feet below the Irma/Matthew levels.

Check local media for closures and emergency manger guidance. Bridges will be closing this morning – Sidney Lanir at 10am, the Talmage bridge in Savannah at 2pm. Most of the schools will be closed Wednesday and likely Thursday for cleanup. Given the labor day weekend, some may well say forget it and not bother to restart until Tuesday depending on how extensive the power outages and cleanup is. My expectation is things in GA/SC will be back to normal by the weekend except in isolated cases where there was damage. Florida and far south Georgia are is looking at weeks of cleanup and recovery – hopefully we can pay attention long enough to help them out.

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So what about this whole hurricane winds over Savannah business? As a storm moves inland it begins to decay because it is cut off from its energy source, warm water. How fast that happens is a complex mix of terrain and environment. Here is a map of the winds using the Saffir Simpson scale, based on the NHC forecast at 5am:

And here is my impact map:

So, why does NHC show Idalia as a hurricane, but I’m showing less than that? No, I’m not actually contradicting the National Hurricane Center, but in fact there are lots of different ways of measuring and reporting wind, and when trying to calculate the impact of wind, it is vital you use a common reference. The wind the NHC uses in their reports isn’t the wind measured at the airport! To understand this we have to take a deeper dive in to the world of measuring wind …

Unfortunately, even in meteorology there are half a dozen kinds of “wind.”

For historical reasons, NHC wind is reported as a “one minute sustained wind.” In the old days, a meteorological technician would stare at the mechanical wind gauge for one minute, and the sustained wind was the wind below which the wind did not drop. The offshore buoys report 10 minute averages becuase that is typically used in oceanography to calculate waves. In fact, the World Meteorological standards that define hurricanes use the 10 minute wind. Well, except some buoys report an 8 minute wind. When the automated networks (ASOS, AWOS) were installed at airports in the 1990’s, they were mostly funded by the FAA and designed to support aviation. So they reported a two minute average of three second gusts, and these days we have a dilemma: nothing actually measures the one minute sustained wind that NHC uses in their reports! So there are lots of tables and conversion equations and factors to try to convert between these different standards.

Engineers typically use gusts when designing structures. What kind of gusts? Um, ok, we’re not any better than the meteorologists. Some use 2 second, some 5 second, some 3 second. So, more tables and conversion equations and factors.

The bottom line is that the wind in the NHC reports is about 8 to 10 percent higher than the same wind that will be measured at your local airport. That’s why typically when NHC says “winds of 100mph” and you look at weather reports, the wind at the airport will only read 90 or 92mph.

But … it’s worse than that. NHC reports “wind at 10 meters over open terrain or water.” Well, now things get really complicated because there isn’t a lot of open terrain inland; there are trees, buildings, etc. And the wind reaching you depends on the terrian several miles upwind from you – so, in a typical location, the wind is now only 80 to 85% of what the wind shown in the NHC reports. You can see some of this complexity in my maps, which take in to account terrain and as you can see above are not smooth except offshore.

Dr. Peter Sparks, Professor of Engineering at Clemson, used to joke that the guys at the hurricane center must be paid based on miles per hour 😛 .. but in fairness, their job is watch and warning, not damage estimation. However it is very confusing to the professional, much less the average person or “journalist”.

Why does all this matter? If you are trying to figure out what the impacts of the storm are going to be, which is my job, you are interested in the dynamic wind pressure. That’s the force the wind pushes on you, trees, buildings, and causes them to fall down or break. Take Downtown Pooler as an example. Let’s say the NHC wind is given as 80mph – minimal hurricane. The dynamic wind pressure is related to the square of the wind speed, so leaving out the other complicated parts of the equation, has a scale of 6400. But … when you account for averaging times that wind is an ASOS (airport wind gauge) value of 74mph, and terrain, likely only 68mph – tropical storm. So the wind pressure is only 72% of what you might get if you just plugged in the numbers.

The World Meteorological Organization has tried to bring some order to this mess with a report outlining conversion standards, but even there they resort to some hand waving. Even more unfortunately, the vast majority of media/news people don’t understand or appreciate these nuances, much less try to explain them. They often go for the biggest number they can, and do irritating things like say “gusts to hurricane force.” That’s nonsense – hurricane winds are measured by sustained or average winds. A 75mph sustained wind will produce gusts to maybe 85 or 90mph – but a 75mph gust is NOT a hurricane force wind. To say so is both unscientific and scare mongering.

So what’s the bottom line? All these debates over winds, etc. are sort of beside the point. The difference between a 73 mph “tropical storm” and a 74mph “hurricane” isn’t really that important (assuming you’ve done your math properly!). These are complex storms with complex behavior that isn’t captured by one number. This is a bad storm and will be a catastrophe for the small towns in the Big Bend of Florida, and generate significant damage across far south Georgia. For coastal Georgia, there will be a mess to clean up, some damage but hopefully nothing too serious, probably a lot of inconvenience, but if you aren’t in one of the areas likely to flood (right on the coast) or very unlucky with a tree, then by this weekend things should be back to normal.