Editor’s Note: Today, May 9, is Day 4 of National Hurricane Preparedness Week. Today’s theme is “Get an Insurance Checkup,” and two hurricane experts talk in this blog post about why they decided to get flood insurance for their homes. For more information on flood insurance, you can visit http://www.floodsmart.gov or re-read our previous blog post on securing an insurance checkup.
By now, billion-dollar flood disasters in the U.S. are something of an overlooked seasonal rite-of-passage. The Midwest flooding of April 2017 feels like a distant memory, as does the California flooding two months before it, Hurricane Matthew’s historic flooding the fall before that, the devastating floods in Louisiana the summer before that, and Texas and Louisiana — again — the spring before that. Some 500,000 homes damaged or destroyed at a cost of more than $150 billion in two years from those events alone. No one expected that kind of flooding to affect them. They never do.
For Hurricane Harvey, its Category 4 winds at landfall were just a prelude of things to come. Harvey wasn’t even a hurricane by the time its heaviest rains reached Houston. Though the tropical storm’s still-high winds hampered rescue efforts, the winds were not at the forefront of the minds’ of residents living through the unimaginable. The magnitude of the flood nightmare caught even Houston, no stranger to big floods, by surprise. Three to five feet of rainwater poured down from the skies above in what would become the worst freshwater flood in United States history. The residences of nearly one-in-three people in America’s fourth-largest city were under water.
In Harvey’s wake lay a dizzying disbelief of devastation. More than 120,000 homes in Harris County, where Houston is located, were damaged by floodwaters. What’s more, an estimated 70 percent of those homes were uninsured for floods., That meant the majority of Houstonians, not the insurance companies, were on the hook for the bill. The best-case scenario for uninsured survivors was Federal Emergency Management Agency (FEMA) disaster assistance (typically ranging $3,000–$8,000) or a U.S. Small Business Administration (SBA) low-interest federal loan (up to $200,000 for home repair). In reality, covering the cost of repairs for most came from a combination of federal assistance and personal finances, which for many meant adding to or incurring new consumer debt.
It was never intended for the federal government to bail out the uninsured after a disaster— in fact, quite the opposite. When the National Flood Insurance Program (NFIP) was established 50 years ago, its goal was to help insure the uninsured before a disaster. Flooding is the most common and expensive type of disaster, and insuring high-risk flood areas often demands an astronomical price tag. Back in the 1960s, private market flood insurance simply wasn’t available. This is where the NFIP came in. Through the NFIP, the federal government began offering largely affordable policies to the residents of participating communities who adopted and enforced floodplain management ordinances in high-risk flood areas to reduce future flood risk. In theory, securing and insuring high-risk communities reduces the reliance on federal post-disaster assistance and saves the government (and U.S. taxpayers) money, which is a good thing.
Flood insurance, once voluntary, is today required for all properties with federally-backed mortgages in high-risk flood areas. To define these high-risk flood areas, FEMA routinely conducts flood hazard analyses to identify land areas at risk of being inundated by a flood that has a 1 percent chance of being equaled or exceeded in any given year. Since 1 percent is interchangeable with 1-in-100, these high-risk floods have become known as 1-in-100 year (or 100-year) floods. FEMA designates these so-called 100-year floodplains as Special Flood Hazard Areas (SFHAs).
Which brings us back to Houston. When Harvey’s floodwaters receded, some 2-in-3 survivors had no insurance to cover their flood losses.1,2 The vast majority of flooding during Harvey happened outside of the SFHA. The storm widened rivers and reservoirs to a point where roads became riverbeds. When everything’s flooded, flood zones feel a little meaningless. But one of the lessons Harvey taught is that flood zones, and our understanding of them, do matter — more so today than ever.
The so-called 100-year floodplain can be understandably confusing. A 1 percent chance of anything happening in a given year feels remote, but most of us don’t live in our houses for a single year. Consider, for example, a 30-year mortgage. The odds of one of those 100-year floods happening over the period of a 30-year mortgage is about 1 in 4. A 25 percent chance of a devastating flood over the period of your mortgage are higher than the odds of a devastating house fire, and you probably wouldn’t go 30 years without installing smoke detectors in your home.
It’s easy to see the need for flood insurance when it’s required; it’s not as clear when it isn’t. So what about those living in high-risk areas without a federally-backed mortgage? Or what about those living outside the high-risk, 100-year floodplain? After all, a moderate chance of flooding hardly implies you’re safe. In fact, FEMA estimates that nearly 1 in 4 of all federal flood claims occur outside of high-risk areas. As every billion-dollar flood disaster shows, floods can be some of the most egregious rule breakers.
Financial decisions, including whether to insure your home and belongings from a flood, are deeply personal family issues. They often aren’t easy decisions, even for those most familiar with the threat. Below, two of the nation’s leading hurricane experts discuss their own experiences living in places where water is a stark reality. Though the aim of their livelihoods is piecing together the clues of Mother Nature’s next step, they’ve each lived through the unpredictable moments. The billion-dollar floods don’t get any easier, and as coastal populations soar, neither will the decisions that shield us from Mother Nature’s most unpredictable moments.
Bill Read, Former Director, National Hurricane Center
I think a big problem people have is differentiating between “requiring” and “needing” flood insurance. My first exposure to the issue of “needing” flood insurance occurred when we moved from Maryland to League City, Texas, in 1992. The house we decided to purchase was (barely) outside the floodplain indicating the 100-year elevation. However, I was concerned about the risk mainly due to storm surge, as data indicated a Category 3 hurricane or above would bring water levels above the level of our home’s foundation. Our realtor and our lender were both adamant in telling us we didn’t “need” flood insurance. I asked our insurance agent at USAA and he was spot on in differentiating between “required” and “needed”. We chose to follow our insurer’s advice. When we decided to move to a new house in 2005, I found a development on the lowest flood risk land in League City, a parcel that sat outside the 500-year elevation. While no longer in a storm surge risk area, I was concerned about flooding from an “off the charts” rain event. Interestingly, my realtor for this move was savvy about flooding along the Gulf coast and advised us to keep flood insurance, as did USAA, which I had already decided I would do based on the many floods I had witnessed outside the 100-year risk areas in Houston since 1992. Along came Harvey, and although we did not flood from the 45 inches of rain we received, the water reached our porch and was one inch from entering the house. Six of my friends were not as fortunate, and two of them did not have flood insurance. Needless to say, when my policy came up for renewal in February, I quickly did so!
Jamie Rhome, Storm Surge Specialist, National Hurricane Center
I recently purchased a home in South Florida, and while going through the mortgage approval process, was informed that the home was outside of the high-risk area (aka the 100-year flood zone) and thus flood insurance wasn’t “required.” I’m also far enough inland to prevent storm surge (saltwater) inundation. However, the home is situated near a freshwater lake, and South Florida often experiences very heavy rains, sometimes exceeding 10 inches in a day. One can easily envision a scenario where debris, from heavy rains or winds, clogs the storm water drains and water pools in the street, eventually coming up the driveway and ultimately wetting the bottom floor of my home. Indeed, South Floridians are very accustomed to this very issue as it frequently occurs during our rainy season. Without flood insurance, I as the homeowner would be responsible for all the damage, which can easily climb into the thousands of dollars. Imagine replacing floors, walls, furniture, possessions, etc., and then taking steps to prevent mold. Given the small cost of flood insurance, the decision was an easy one and I determined that flood insurance was “needed” even if it wasn’t “required.” I was also lucky enough to have a realtor who was well-informed on flood insurance and overall flood risk. He encouraged the purchase of flood insurance citing his experience living in Florida and personal experience with freshwater flooding. Not all home buyers benefit from such well-informed or well-intentioned realtors and home buyers often navigate these complicated waters on their own. If in doubt, it’s better to be safe than sorry.
— Michael Lowry (UCAR Visiting Scientist), Jamie Rhome and Robbie Berg (NHC)
 Preliminary analysis of Hurricane Harvey flooding in Harris County, Texas. California WaterBlog. Available at https://californiawaterblog.com/2017/09/01/preliminary-analysis-of-hurricane-harvey-flooding-in-harris-county-texas/
 Hurricane Harvey: 70% of home damage costs aren’t covered by insurance. CNN Money. Available at http://money.cnn.com/2017/09/01/news/hurricane-harvey-cost-damage-homes-flood/index.html
The State of Hurricane Forecasting is . . .
The National Hurricane Center (NHC) has the responsibility for issuing advisories and U.S. watches/warnings for tropical cyclones (TCs), which includes tropical depressions, tropical storms, and hurricanes, for the Atlantic and east Pacific basins. NHC has a long history of issuing advisories for TCs, with the first known recorded forecast being in 1954, when 24-hour predictions of a TC’s track were made. Since then, we’ve expanded our forecasts out in time and added predictions of TC intensity, size, and associated hazards, such as wind, storm surge, and rainfall. In addition, the lead times of tropical storm and hurricane watches and warnings have increased to give the public additional time to prepare for these potentially devastating events. Since we’re at the time of year when the U.S. President and state governors have just given their “State of the Union” or “State of the State” speeches, we thought this might be a good time to give our own “State of Hurricane Forecasting” speech. This blog entry takes a look at the accuracy of NHC’s forecasts and quantifies how much more accurate they are today compared to decades ago.
Track Forecasting (a.k.a., Where the Storm Will Go)
We are usually more confident in predicting the path of TCs as compared to predicting the strength or size of a TC. The primary reason for this is because the track of a TC is governed by forces larger than the tropical system itself, since the surrounding steering currents cover a much larger area than the hurricane. Because these nearby weather patterns are big, we can usually “see” them easily, and the global weather models do a fairly good job in predicting how these steering features might evolve over the course of a few days.
The figure below shows the average NHC track forecast errors for tropical storms and hurricanes by decade beginning in the 1960s. You can see that there has been a steady reduction in the track errors over time, with the average errors in the current decade about 30-40% smaller than they were in the 2000s and about half of the size (or even smaller) than they were in the 1990s.
If that doesn’t seem impressive, let’s look at another example. The next graphic shows two circles centered on a point near Pensacola, Florida, with the blue one representing the average 48-hour track error in 1990 and the red one showing the average 48-hour error today. What it shows is that if NHC had made a forecast for a storm to be over Pensacola in 48 hours back in 1990, the TC would have ended up, on average, not exactly over Pensacola but somewhere on the blue circle. If NHC makes the same forecast today, now the storm ends up, on average, somewhere on the red circle. You can easily see that the NHC forecasts for the path of a TC today are much more accurate, on average, than they were decades ago, and these more accurate forecasts have helped narrow the warning areas, save lives, and make for more efficient and less costly evacuations.
So, you might be wondering why the track forecasts are more accurate today than in the past. Well, the primary reason is the advancements in technology, specifically the improvements in the observing platforms (satellites, for example) and the various modeling systems we use to make forecasts. The amount and quality of data available to the models so they can paint an initial picture of the atmosphere have increased dramatically in the last 20 to 30 years. Also, the resolution and physics in the models we use today are far superior to what forecasters had available in the 1990s or prior decades, in part due to the tremendous improvements in computational capabilities. In addition, NHC has found ways to even beat the individual dynamical models by using a balance of statistical approaches and experience.
We often hear a lot of questions asking which model is the best one. Although some models are usually better than others, no model is perfect, and their performance varies from season to season and from storm to storm. Two of the most well-known models for weather forecasting are the U.S. National Weather Service’s Global Forecast System (GFS) and the European Centre for Medium-Range Weather Forecasts (ECMWF). The figure below shows a comparison of the NHC forecasts (OFCL, black) and forecasts from the GFS (GFSI, blue) and ECMWF (EMXI, red) models for Hurricanes Harvey, Irma, Maria, and Nate in 2017. In all of these cases, except for Hurricane Irma, OFCL performed as well as or better than GFSI and EMXI. Among the two models, EMXI beat GFSI for Harvey, Irma, and Nate, but GFSI beat EMXI for Maria.
Over the past decade, the average track errors of GFSI and EMXI models have been quite close, so even though EMXI was the best-performing model most of the time in 2017, it does not mean that it will always be the best for every storm. The models that typically have the lowest errors are consensus aids, which blend several models together. Forecasters construct their own forecasts of how the storm will evolve, aided by model simulations and their knowledge of model strengthens and weaknesses.
Even though our track forecasts are much more accurate today – in fact preliminary estimates are that the 2017 Atlantic track forecasts set record low errors at all time periods – typical track errors currently start off at 37 n mi at 24 hours and then increase by about 35 n mi (40 mi ) per day of the forecast. This means that our 5-day track error is on average around 180 n mi (210 mi). So, keep that in mind and be sure to account for forecast uncertainty when using NHC forecasts next hurricane season.
Intensity Forecasting (a.k.a., How Strong the Storm Will Get)
Predicting the intensity of a tropical storm or hurricane is usually more challenging than forecasting its track. This is because the intensity of these weather systems is affected by factors that are both big and small. On the large scale, vertical wind shear (the change of wind speed and direction with height) and the amount of moisture in the atmosphere greatly affect the amount or organization of the thunderstorm activity that the TC can produce. Ocean temperatures also affect the system’s intensity, with temperatures below 80° F usually being too cool to sustain significant thunderstorm activity. However, smaller-scale features can also be at play. One of the more complex phenomena that affects a TC’s intensity is an eyewall replacement cycle. Initially, when two eyewalls, one inside the other, are present, the hurricane’s wind field will begin to expand, and as the inner eyewall dies, the hurricane’s peak winds start to weaken. However, if the second eyewall contracts, the hurricane can often re-intensify. The radar image below of Hurricane Irma (2017) was taken at the beginning of an eyewall replacement cycle, when the hurricane had a double eyewall structure.
Given these complex factors and the fact that errors in the track can also affect the TC’s future intensity, we have not made as much progress in this area as we have for track forecasting. The next graphic (below) shows NHC average intensity errors for Atlantic tropical storms and hurricanes by decade starting in the 1970s. Note that only small improvements were made in the intensity predictions from the 1970s through the 2000s. A much more significant reduction in error has occurred in the current decade, which could mean that the recent investment in new models and techniques is beginning to pay off. Today’s intensity errors are close to 15 kt (17 mph) from 72 to 120 h. This number is on the order of one Saffir-Simpson category, so we often encourage those who could be affected by a TC to prepare for a storm one category stronger (on the Saffir-Simpson Hurricane Wind Scale) than what we are forecasting.
Although the GFS and ECMWF models are skillful for track forecasting and help us understand the environment around the TC, did you know that these models are typically inadequate to predict how strong a TC might become? Both the GFS and ECMWF are global models, and they cannot “see” sufficient detail within the storm to represent and predict the core winds in the hurricane’s eyewall. Therefore, we use different models to predict intensity, some that are run at high resolution specifically for TCs (e.g., Hurricane Weather Research and Forecasting [HWRF] model, Hurricanes in a Multi-scale Ocean-coupled Non-hydrostatic [HMON] model) and some that are statistical in nature (e.g., Statistical Hurricane Intensity Prediction Scheme [SHIPS], Logistic Growth Equation Model [LGEM]). The statistical models tell the forecaster what typically occurs for a TC in a specific location and environment based on past storm behavior. Even though the intensity models are improving, the gains in these models are much smaller than what has occurred in the models we use for track forecasting.
If you want more information on the models, please visit the following page for details: http://www.nhc.noaa.gov/modelsummary.shtml
Will the errors keep decreasing?
The short answer is they likely won’t forever. At some point the forecasts made by NHC and other forecasting centers will likely reach the limits of predictability. No one knows for sure what those limits are or when they will be reached, but researchers are still providing great information that is helping NHC make steady advancements as discussed above.
For more information on the NHC and model verification please visit the following page: http://www.nhc.noaa.gov/verification/