A hurricane is a weather phenomenon that can only survive over warm water. Hurricane Matthew’s direct effects devastated Haiti and were felt along the coast from Florida to the Carolinas. But Matthew’s moisture also interacted with a feature of the mid-latitude weather to create historic floods in North Carolina.
The Destructive Forces In A Hurricane
Everyone knows that a hurricane can produce furious winds — up to 200 miles per hour. Most structures cannot withstand the onslaught of such a wind. But water is the cause of most deaths from hurricanes.
It is generally understood that hurricanes produce a lot of rain. The mechanism by which a hurricane gets its power is conversion of latent heat of evaporation to wind. A water molecule acquires energy when it evaporates. When it condenses, the energy is released into its surroundings and is eventually converted to energy of wind.
In a hurricane, water is constantly evaporating from the ocean’s surface, then condensing to water droplets as the air rises. The result is rain — often a lot of rain.
As it turns out, wind and rain get the headlines. But it is the storm surge that is most often a cause of loss of life. Yes, the surge is caused by wind, and sometimes the surge is exacerbated by rain. But the surge is dependent on many other factors: the direction of the wind; the length of time the wind blows; the distance over which the wind acts (the fetch); and the central pressure of the hurricane.
Matthew Hammers Haiti
Haiti is a poor country, trying to recover from a devastating earthquake. The southwest portion of the country took a direct hit from Matthew; over 1,000 died and most structures were destroyed.
Florida Disaster Averted As Matthew Stays Offshore, Then Weakens
The center of Matthew stayed just far enough offshore the Florida coast so that the worst possible effects never materialized. Most of the 2 million evacuees from coastal homes in Florida, Georgia, and the Carolinas could return home within a day. But as the storm moved out to sea, its moisture was diverted inland, eventually flooding most of eastern North Carolina.
The North Carolina Flooding
In an atmosphere that has disparate interconnected parts, interactions between the tropics and the middle latitudes can create deluges.
Flooding that is related to, but not directly caused by, a hurricane can occur hundreds of miles from the coast. As Hurricane Matthew meandered out to sea, its moisture became entwined in the mid-latitude flow. Normally this flow is from west to east, but a dip in the jet stream turned the flow to south to north over the Carolinas as Matthew approached. While Matthew’s low-level circulation continued out to sea, its moisture higher in the atmosphere was drawn north over the Carolinas.
In North Carolina, an old frontal system had produced rainfall that left the ground saturated when Matthew arrived.
The cold air behind the lingering cold front was very stubborn. Cold air is heavy and sluggish and difficult to dislodge once in place. Matthew’s moisture rode over this cold surface air and dumped copious amounts of rain in the same place for more than a day.
All that rain falling on ground saturated by the previous rains left the water nowhere to go but into rivers, which rapidly rose to flood stage.
So far, 20 people have died in North Carolin and hundreds of thousands are without power — and probably will be for weeks.
And it isn’t over. Rivers will crest closer to the coast later this week and new evacuations have been ordered.
An Interesting Feedback Mechanism
A hurricane stirs up water from as deep as 100 feet. This water is colder than the surface water. When a hurricane stalls, it cannot continue to feed off the surface water indefinitely, just as a cow cannot graze the same small patch of pasture forever. Matthew stalled in the central Caribbean, not long enough to weaken it significantly, but long enough to create a cool spot on the sea surface.
So What Did We Learn?
Preventing loss of life is the number one goal when a hurricane threatens. This necessity puts forecasters and those who issue warnings and evacuation notices in a terrible spot. What is the criterion for telling someone to evacuate? That his/her chance of drowning is 50%? 20%? 5%? 1%? One chance in a hundred thousand?
The lower the probability threshold, the more times nothing will happen.
The dilemma is that if the forecaster doesn’t issue an alert and the storm hits, he’s to blame for loss of life. But if he insists on evacuation and the storm doesn’t hit, no one will listen next time — like with little boys who warn of wolves. Disturbing reports of large numbers of people ignoring evacuation notices have come in the wake of Matthew.
To make matters worse, since a hurricane is a rare occurrence even in the most susceptible places, most people have never experienced one. When the weather is tranquil most days, and at worst windy and rainy on a few days, it’s hard to imagine the destruction that category four winds and inches-per-hour rain can bring.
At a critical time, the National Hurricane Center (NHC) website went down for about one-third of users. For three hours while the center of Matthew was within 50 miles of the coast of south-central Florida, many people had no access to an updated forecast.
Much of the NHC problem derives from out-dated methods. A simple smartphone app would be a good start on modernizing NHC’a communications. But beyond that, the reports must be issued much more frequently when a storm threatens. As of now, a tropical storm thousands of miles from land and days form impact gets the same six-hourly update as a storm that’s about to inundate Charleston. Information should be disseminated as it arrives, not at some pre-determined time, when it’s already old news.
Global Warming & Hurricane Matthew
There is no definitive way to prove that the devastating effects of Matthew were caused by global warming. But everything about the storm is consistent with climate change: warmer ocean water makes stronger hurricanes; a warmer atmosphere can hold more water and thus produce more rain.
We ignore the message of Mathew at our peril.