Hurricane Michael has made landfall near Mexico Beach, Florida with sustained winds of 155 mph and a minimum central pressure of 919 mb — making it the strongest storm to hit the US since 1969.
Michael's wind speed puts it just shy of a Category 5 storm (the cut off is 156 mph), but the wind is not the main threat to people and property. Instead, it's storm surge, which will likely cause serious flooding in the low-lying coastal regions of Florida.
The National Hurricane Center defines a storm surge as an abnormal rise of water over and above the predicted tide. In short, it's when a large storm pushes water up over the high tide line.
"Life-threatening storm surge is expected along more than 325 miles of coastline," the National Weather Service said on Twitter on Tuesday.
The coastal area of Florida from the Tyndall Air Force Base to the Aucilla River is expected to see the highest storm surge, with 9 to 14 feet predicted.
Here's why storm surge happens and why it can be so dangerous.
How storm surges form
Hurricanes are large low-pressure systems that create a cyclonic wind effect. Those winds force ocean water to spin down into the water column. In deep water, this spinning motion has little effect. But when the storm approaches the coast, the rotating water mass has nowhere to go — except onto land.
Strong winds associated with the storm also create large waves that travel ahead of the storm.
Additionally, hurricanes and tropical storms cause a "dome effect" that pulls water levels up under the low air pressure in the storm's center, though the NHC notes that the effect of low pressure is slight when compared to wind.
The strongest surges occur when a storm's winds are blowing directly toward the shore and the tide is high. In such cases, storm surges can force water levels to rise as rapidly as a few feet per minute, according to the Coastal Emergency Risks Assessment.
There are multiple factors that can affect the size of a storm surge. Storms that approach land perpendicular to the coast produce more powerful surges. And the shape of a coastline, as well as local features like sandbars and barrier islands, can affect how the water propagates over land.
Storm surges aren't included in the Saffir-Simpson scale for measuring hurricane intensity. That scale includes only wind speed.
Storm surge projections for Michael
In Florida, storm-surge warnings are in effect from the Okaloosa/Walton County Line to the Anclote River. That means those areas are in "danger of life-threatening inundation from rising water moving inland from the coastline," according to the National Hurricane Center (NHC).
Storm-surge watches are in effect from the Anclote River to Anna Maria Island — an area that includes Tampa — and for the coast of North Carolina from Ocracoke Inlet to Duck.
At 1 p.m. ET, the NHC reported that water levels were rising quickly along the coast of the Florida Panhandle. A station run by the National Ocean Service at Apalachicola, Florida had already reported over 6.5 feet of water above ground level.
The compounding effects of rainfall on storm surges
When storms bring heavy rain, runoff from land and rivers combines with the storm surge in coastal inlets, exacerbating flood problems.
That was especially true of Hurricane Florence, which drenched the Carolinas last month, and Hurricane Harvey, which flooded parts of Texas last year.
Hurricane Michael's rainfall total is not expected to be as high as either of those storms — the forecast indicates that parts of Florida, Alabama, and Georgia could see 4 to 8 inches, with isolated areas seeing up to 12 inches.
But that's still enough rain to cause life-threatening flash floods, according to the NHC.
In recent years, hurricanes have started to become more sluggish, which allows them to drop more rain and cause flooding. Research from the National Oceanic and Atmospheric Administration found that storms slowed by an average of 10% between 1949 and 2016, making storm surges and the associated flooding more costly and disastrous.
