Flood Science

What to Expect This Hurricane Season

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June 5, 2020

Hurricane Season 2020 is upon us. We face an unprecedented season this year. Scientific consensus points toward an active season, while society is trying to pull out of a global pandemic and economic crisis.

LOOKING  BACK

The first step in looking ahead is to look back. Understanding long-term historic risk can help us put the present in context.

Hurricanes throw three hazards at us—strong wind, rainfall flooding, and storm surge flooding. The risk of these hazards varies considerably with geography.

HISTORIC HURRICANE WIND RISK

The map below shows return periods of hurricane-force winds along the U.S. Gulf and Atlantic coasts, according to NOAA. Return periods are an indication of how frequently something happens. The map depicts the most vulnerable areas in red and orange.

Map of hurricane return periods from NOAA.
Source: https://www.nhc.noaa.gov/climo/

The three most vulnerable areas to hurricane force winds are the Carolinas, South Florida, and Southeast Louisiana. The Outer Banks of North Carolina is the location with the most frequent hurricane strikes, averaging a strike every five years. While these areas are most vulnerable to hurricane strikes, all cities along the U.S. Gulf and Atlantic coasts should prepare for damaging winds this season.

HISTORIC STORM SURGE RISK

Hurricane winds drive saltwater storm surge to inundate the coastline, but storm surge risk has a different pattern than hurricane wind risk. In addition to the frequency of hurricane strikes, the coastal water depth, called bathymetry, and the coastline shape greatly influence storm surge heights.

Storm surge return periods from U-Surge (www.u-surge.net)

The map above shows storm surge return periods for 26 locations along the U.S. Gulf Coast. The red dots represent historic observations and the blue bars represent return periods, with the top of the blue bars approximating the 500-, 200-, 100-, 50-, 25- and 10-year storm surge levels. In summary, the higher the blue bars the higher the risk. These data are provided by the U-Surge Project, which provides the first data-driven coastal flood risk from Texas to Maine.

Note that coastal Mississippi stands out as the most vulnerable location along the Gulf Coast. The shallow bathymetry and the sharp angle of the Mississippi River Delta cause storm surge to efficiently pile up in this region. Interestingly, Mississippi does not stand out on the hurricane return period map as the most vulnerable area for wind risk. We realize that wind and flood risk levels do not necessarily overlap when we compare these two maps.

Along the U.S. Atlantic Coast, the concave-shaped coastline from north Florida to southern South Carolina, including all of Georgia, has a heightened storm surge risk because of the shallow offshore water and coastal shape. However, this section of coast does not stand out on the hurricane return period map as the riskiest for wind damage.

INCREASING TREND FOR STALLED TROPICAL SYSTEMS AND HEAVY RAIN

Finally, we come into this hurricane season on the heels of five consecutive years when tropical systems stalled, dumping 20 or more inches of rain on coastal and inland communities. Here is a list of these storms.

October 2015: Hurricane Joaquin stalled in the Bahamas and a plume of moisture fed into South Carolina for days, dumping 20 or more inches of rain in many areas.

August 2016: A hybrid tropical system with no name dumped a widespread swath of rain exceeding 20” in much of South Louisiana. Some areas received more than 30” of rain.

A flooded fire station in Denham Springs, Louisiana, just east of Baton Rouge. The area received more than 25” of rain in August 2016. Source: Firefighter named Matt gave pic to Hal Needham.

August 2017: Hurricane Harvey stalled along the Texas Coast, dumping 40” or more in a broad area of Southeast Texas, including metro Houston and Beaumont/Port Arthur. Isolated locations near Port Arthur exceeded 60” of rain.

September 2018: Hurricane Florence stalled near the South Carolina/North Carolina border, dumping more than 30” of rain in North Carolina.

September 2019: Hurricane Dorian stalled as a cat-5 hurricane in the Bahamas. In addition to catastrophic wind and storm surge damage, areas of the Bahamas observed more than 20” of rain.

Rainfall map for Hurricane Dorian (2019). The United States narrowly avoided destructive rainfall damage. Source: NOAA/ National Hurricane Center

September 2019: Tropical Storm Imelda stalled near the Houston/Galveston corridor, dumping more than 30” of rain across Southeast Texas. Isolated locations received more than 40”. Some homes inundated from Harvey flooded for the second time in three years.

If you sense a pattern to these stalled-out rain storms, you are right. Climate science supports the prediction that more storms should stall because they’re guided by weakening atmospheric steering currents. These currents are becoming weaker at times, as the temperature and energy gradient, or difference, between the poles and the tropics is reducing because the poles are heating faster than the tropics as the climate warms.


The PBS documentary “How a Warming World May Have Caused Hurricane Florence to Stall” discusses the science behind stalling tropical weather systems. I worked with PBS on this documentary to provide climate data behind the science.

https://www.pbs.org/newshour/show/how-a-warming-world-may-have-caused-hurricane-florence-to-stall

Image from the PBS documentary “How a Warming World May Have Caused Hurricane Florence to Stall”

Going into this hurricane season, we should be on guard against more rainfall flooding from stalled tropical systems. The great danger of these systems is that they can extend flooding to within several hundred miles of warm ocean waters, putting cities like Houston, Atlanta, and Orlando in the crosshairs of extensive inundation.

SEASONAL HURRICANE FORECAST FOR 2020

Looking ahead to the forecast for this hurricane season, we rely on teleconnections, or long-distance relationships in atmospheric pressure patterns. Three teleconnections are particularly useful: 1) Atlantic water temps; 2) Equatorial Pacific water temps; 3) The North Atlantic Oscillation, which gives insight into potential storm tracks.

TROPICAL ATLANTIC WATER TEMPERATURES

Climate scientists look at the Main Development Region in the Atlantic as an indication of tropical cyclone activity. This area covers the southern Caribbean and tropical Atlantic. As of early June, water temps in this region are running warmer than normal, indicating increased tropical activity is likely.

Water temps in the Main Development Region of the Caribbean and tropical Atlantic are running warmer than normal, making increased tropical activity likely. Base map: Source: https://www.ospo.noaa.gov/Products/ocean/sst/anomaly/index.html

Note that the Gulf of Mexico is not included in this box. Many climate scientists explain that the Gulf is warm enough to support hurricane activity every year, so they do not include the Gulf temperatures in seasonal forecasts.

While this is true, a warm Gulf of Mexico early in the season can support increased tropical activity in the month of June. Consider the fact that Tropical Storm Cristobal strengthened considerably while tracking over warmer than normal water in the Bay of Campeche this week.

I did some original research this week on hurricane strike frequency along the U.S. Gulf Coast during the month of June from 1851–2019. The graph below shows that 15 of the 17 hurricane landfalls in June occurred in the second half of the month. Note that 16 of these 17 hurricanes made landfall as category 1 (yellow dots) or category 2 (orange dots) hurricanes. Only one storm made landfall in June as a major hurricane (category 3 or higher) along the U.S. Gulf Coast. This storm was Hurricane Audrey, on June 27, 1957—almost in the month of July.

Consider that June is a sensitive month on the “shoulder” of the main hurricane season. Warmer-than-normal water temperatures in this month can bring hurricanes to the Gulf, but unusually cool water temperatures will certainly suppress tropical activity so early in the season.

Graph of historic hurricane landfalls in June along the U.S. Gulf Coast.
Data Source: HURDAT

EQUATORIAL PACIFIC WATER TEMPERATURES

Equatorial Pacific water temperatures influence the amount of thunderstorm activity in the extreme eastern Pacific Ocean and near Central America. These storms affect the amount of wind shear, or upper level winds, that suppress hurricane activity in the Atlantic.

When water temperatures of the equatorial Pacific are warmer than normal we call the pattern El Niño. When the temperatures are unusually cold we call it La Niña. These words are the Spanish equivalent of “The Child,” and are used because these unusual water temperatures were often noticed around Christmas, the time of the Christ child.

During El Niño patterns, wind shear tends to increase in the Atlantic and hurricane activity diminishes. The opposite is true during the La Niña phase.

The region in which water temperatures best define this teleconnection is called Niño 3.4 and is depicted on the map below. The map clearly shows blue shading, representing cooler than normal water temperatures, in the region as of June 1, 2020.

Sea surface temperature anomaly map, depicting the Niño 3.4 area.
Base Map Source: https://www.ospo.noaa.gov/Products/ocean/sst/anomaly/index.html

Often the “El Niño” teleconnection is given the acronym ENSO, which stands for El Niño and Southern Oscillation. The National Oceanic and Atmospheric Administration (NOAA) and National Centers for Environmental Prediction (NCEP) released an updated seasonal ENSO forecast on May 14, 2020, which indicates a high likelihood that this teleconnection will move from ENSO-neutral conditions towards La Niña this hurricane season.

The graphic below provides the seasonal forecast in 3-month increments. The time period on the far left of the x-axis is labeled “AMJ,” standing for April-May-June. ENSO-neutral conditions dominated this season, as we see from the high grey bar. As we move to the right on this chart, time is moving forward and ENSO neutral (grey bars) are phasing out while La Niña (blue bars) are phasing in. By late in hurricane season, we see the likeliness of La Niña nearly match that of ENSO-neutral.

The combination of warm Atlantic water temperatures and a building La Niña in the equatorial Pacific leads seasonal forecasts to predict an active hurricane season for the Atlantic. As of May 2020, NOAA and the National Hurricane Center (NHC) predicted a 60% chance that the 2020 Atlantic Hurricane Season would be more active than normal, a 30% chance that it would be near normal and a 10% chance that it would be less active than normal.

The seasonal forecast predicts 13-19 named storms, 6-10 hurricanes, and 3-6 major hurricanes. Normal values are 12-6-3 for these three categories. Considering that we have already named the “C” storm by June 3, we are well on our way to observing more tropical activity than normal.

Forecast for seasonal hurricane activity from NOAA/ National Hurricane Center

INDICATION OF STORM TRACKING – THE NORTH ATLANTIC OSCILLATION

While predicting the amount of tropical activity is important, most impacts are felt if storms make landfall. For an indication of storm tracking during hurricane season, we look to a teleconnection called the North Atlantic Oscillation (NAO).

This teleconnection is related to the positioning of a massive area of high pressure called the Bermuda High and a persistent area of low pressure called the Icelandic Low. The NAO index has positive and negative phases depending on the positions of these pressure regions, with the index values for May and June correlating best with hurricane tracks.

North Atlantic Oscillation Index Values from Feb–June 2020.
Source: https://www.cpc.ncep.noaa.gov/data/teledoc/nao.shtml

While the May–June index values will not be known until early July, it appears that May averaged slightly negative and the first half of June is also forecast to be negative. If the forecast holds and late June is not highly positive, the index will average negative for May and June.

A negative phase of the NAO teleconnection usually means the Bermuda high is weaker and positioned farther southwest in the Atlantic Ocean. This tends to guide storms to track farther west, impacting the Caribbean and United States. In the positive phase, the Bermuda high tends to be stronger and positioned farther to the northeast, enabling more storms to track over the open Atlantic.

The images below show generalized pressure patterns and storm tracks during positive and negative NAO phases. As the negative is a phase most likely for this May and June, we should expect more tropical cyclones than normal to track towards the U.S. and Caribbean.

North Atlantic Oscillation Positive Phase. Graphic: Hal Needham
North Atlantic Oscillation Negative Phase. Graphic: Hal Needham

PREPARING FOR HURRICANE SEASON FOLLOWING A PANDEMIC AND ECONOMIC CRISIS

The recent global pandemic and economic crisis add a twist to this hurricane season. While much remains uncertain about the impacts of these compounding hazards, three trends seem likely.

In numerous coastal cities, I’ve heard predictions that fewer people than normal will be willing to evacuate this year. The combination of economic burdens and less willingness to stay in a shelter or with extended family because of health concerns may mean that more people shelter-in-place and are vulnerable to wind and flood impacts. Therefore, direct storm impacts on more people is the first likely trend we will see this season.

Pandemic masks placed on the Storm Survivors Monument in Galveston are a reminder that Gulf Coast communities are facing multiple hazards this hurricane season. Photo: Hal Needham.

The second likely trend relates to more indirect storm impacts. Disaster science has consistently revealed that more people die from the indirect effects of hurricanes than direct effects. Indirect effects include heat stroke from trying to live in hot houses that lost electricity, carbon monoxide poisoning from improperly vented generators, snake bites, electrocutions from live wires in water, and chainsaw accidents from tree removal. We can expect more indirect health impacts this season.

The final trend that is likely related to the inability of some homeowners to pay insurance premiums or deductibles related to insurance claims following a disaster. This may mean more people lose important safety nets that were previously in place to help them recover from a storm.

FOLLOW FORECASTED HURRICANE IMPACTS WITH A NEW COOL TOOL

This hurricane season you can follow forecasted hurricane impacts with a new cool tool. The Hazard Area Likeliness (HAL) Index maps the likeliness of the three major hurricane hazards — wind damage, rainfall flooding and storm surge flooding, for specific locations along the coast and inland.

Hazard Area Likeliness (HAL) Index Map for Tropical Storm Cristobal

Hazards are classified according to the likeliness that they will exceed specific thresholds, ranging from likely (90%+) to probable (50-89%), possible (10-49%) and unlikely (< 10%).

The map above shows the likeliness that minor damage will occur during Tropical Storm Cristobal from tropical-storm-force winds (39 mph+), rainfall flooding (6”+ rain), and storm surge flooding (4’+ surge) for 22 locations. Each location is given a symbol that looks like a stoplight, depicting wind risk on top, rain risk in the middle, and storm surge on the bottom.

As of June 4 at 5:00PM CDT, the risk of minor wind damage was listed as possible from Galveston, Texas through Shell Beach, Louisiana. The risk of minor flood damage from rain was listed as possible from Cypremort Point, Louisiana to Destin, Florida, including Baton Rouge, Louisiana, which is inland. The likeliness of minor storm surge damage was listed as possible from Cameron, Louisiana to Shell Beach, Louisiana. These maps are frequently updated as the storm approaches the coast.

We’ve learned that we cannot stop storms from coming, but improved preparation and forecasting storm impacts can reduce the impacts of these storms in our communities. Have a safe hurricane season everyone and stay tuned for frequent blog and social media updates through CNC Catastrophe and National Claims.