Why Heavy Rain Events are Becoming More Common

Torrential rain events and the flooding they cause are nothing new. Global warming, however, is helping to make them more likely.

Heavy rainfall trends in NYC. Credit: Climate Central

According to the most recent National Climate Assessment, heavy precipitation events have increased in both frequency and intensity across the United States. While there are seasonal variations with different regions, the greatest increases have been observed in the northeast.

Climate scientists attribute this increase in heavy precipitation to our warming world. As greenhouse gases warm the atmosphere, the air is able to hold more water vapor. More specifically, according to the Clausius–Clapeyron relation, for every increase of 1°F, the saturation level of the atmosphere increases by about 4%. That means there is more evaporation from oceans, rivers, and lakes, and therefore more water vapor available to condense and fall as precipitation.

Heavy rain events have a number of consequences, including an increased risk of both flash floods and river floods. This, in turn, is a threat to life and property. Over the long-term, it also affects insurance rates and property values. According to NOAA, individual billion-dollar flooding events (excluding tropical cyclones) in the U.S. have added up to $39 billion in losses since 2010.

As our global temperature continues to rise, experts say we should expect to see more extreme rain events, even in areas where overall precipitation is projected to decrease. In other words, when it rains, it will likely pour.

Downpours have been getting more frequent and intense across the US. Credit: Climate Central and NCA4

Report Finds Hurricane Harvey’s Record Rainfall Linked to Climate Change

Hurricane Harvey – one of the big names of the 2017 Atlantic hurricane season – unleashed catastrophic flooding in southeast Texas at the end of August. Now, after months of reviewing the data, scientists say the storm was exacerbated by climate change.

According to a peer-reviewed report by World Weather Attribution (WWA), an international coalition of scientists, human-caused climate change made Harvey’s devastating rainfall three times more likely to occur and fifteen percent more intense. Using historical rainfall data and high-resolution climate models to compare conditions in a pre-warming world to those at the time of the storm, the WWA team was able to separate the climate signal from natural variability. They found that the deluge caused by Harvey would have been a 1-in-2400-year event in the absence of global warming, but is now a 1-in-800-year event and becoming more likely.

Heavy rainfall events, in general, are becoming more frequent in many different places, because as the atmosphere warms it can hold more moisture. In fact, it can hold four percent more moisture for every 1°F of warming. This means there is more water vapor available in the air that can fall as precipitation.

After rapidly intensifying in the Gulf of Mexico, Hurricane Harvey made landfall as a category-4 storm in the Texas Coastal Bend region on August 25. It then stalled over the area for several days, unleashing massive amounts of rainfall. Cedar Bayou, outside of Houston, reported a staggering 51.88 inches of rain, setting a new record for the continental US. The storm claimed the lives of 80 people and more than 120,000 residents across the area had to be rescued from their homes. The economic impacts of the deluge are still being tallied, but it is expected to be one of the most expensive in US history.

The WWA study only analyzed the impact of climate change on Harvey’s rainfall, not its role in the storm’s formation or strength.  Those connections remain an active area of research.

Climate change made Hurricane Harvey’s rainfall worse. Credit: Climate Central. (World Weather Attribution is led by Climate Central, a non-profit research group.)

Extremely Active 2017 Atlantic Hurricane Season Comes to a Close

The 2017 Atlantic Hurricane Season officially ends today.  Not only was it above average, as predicted, it was the basin’s fifth most active season on record.

According to NOAA, there were seventeen named storms this season. Of these, ten developed into hurricanes and six were major hurricanes with ratings of category 3 or higher on the Saffir-Simpson Scale. An average season produces twelve named storms, six hurricanes, and three major hurricanes. It is also interesting to note that this year’s ten hurricanes developed consecutively over the course of ten weeks, marking the largest number of hurricanes to form in a row in the satellite era.

This season’s Accumulated Cyclone Energy (ACE), which measures the intensity and duration of storms, was also exceptionally high. On average, a season will post an ACE of 104 in the Atlantic. This year, according to researchers at Colorado State University, it was 226 – the seventh highest in the historical record.

Officially running from June 1 to November 30, the season got off to an early start this year. Tropical Storm Arlene was a rare pre-season storm that developed in April. Another interesting outlier was Hurricane Ophelia in October. It was the easternmost major hurricane ever observed in the Atlantic Basin and the strongest storm on record to hit the Republic of Ireland. The biggest names of the season, however, were Harvey, Irma, and Maria.

In August, Harvey made landfall in Texas as category-4 hurricane. It was the first major hurricane to hit the United States since Wilma in 2005. It was also the wettest storm on record, dumping more than 50 inches of rain in southeastern Texas. This extreme precipitation caused catastrophic flooding in the Houston area.

Hurricane Irma maintained category-5 strength winds for 37 hours before making landfall as a category-4 storm in the Florida Keys. Measuring about 425 miles in diameter, Irma was wider than the Florida peninsula and its effects were felt across the entire Sunshine state in early September.

Just two weeks later, Hurricane Maria devastated Puerto Rico. It was the third category-four hurricane to hit the US, or one of its territories, in less than a month. In 166 years of record keeping, that never happened before.

Causing so much destruction, Harvey, Irma, and Maria will likely be retired from the World Meteorological Organization’s list of storm names.

This active hurricane season was largely the result of above-average sea surface temperatures in the Atlantic and ENSO neutral to cool La Niña conditions in the Pacific. With warm water to fuel storms coupled with reduced wind shear across the Gulf of Mexico, tropical development in the Atlantic basin was essentially unhindered.

In terms of economic impact, ENKI Operations, a private research firm, estimates the property damage, clean up costs, and lost business productivity from this year’s storms to be $206 billion. That would make 2017 the costliest hurricane season on record for the US. The official tally from NOAA will not be available until early 2018.

Data: NOAA

May 2017: A Month of Weather Extremes in NYC

May was a month of weather extremes is New York City this year. Highs ranged from a cool 53°F to a record warm 92°F. May also produced the city’s first heat wave of the year. However, with 24 out of 31 days posting below average readings, the chill won out in the end. The city’s mean temperature for the month was 61.1°F, which is 1.3°F below average.

In terms of precipitation, May was unusually wet. In all, 6.38 inches of rain were measured in Central Park. Of this impressive total, 3.02 inches (47% of the monthly total) fell in just a few hours on May 5, setting a new rainfall record for the date. Another 1.61 inches came down during an unseasonable nor’easter on May 13. On average, the Big Apple gets 4.19 inches of rain for the entire month of May.

Credit: The Weather Gamut

The Active Atlantic Hurricane Season of 2016 Comes to a Close

The 2016 Atlantic Hurricane Season officially ends today.  Not only was it above average, as predicted, it was the basin’s most active season since 2012.

According to NOAA, there were fifteen named storms this season. Of these, seven developed into hurricanes and three – Gaston, Nicole, and Matthew – were major hurricanes with ratings of category 3 or higher on the Saffir-Simpson Scale. An average season produces twelve named storms, six hurricanes, and three major hurricanes.

This season’s Accumulated Cyclone Energy (ACE), which measures the intensity and duration of storms, was also above normal. On average, a season will post an ACE of 104 in the Atlantic. This year, according to hurricane researchers at Colorado State University, it was 134.

Officially running from June 1 to November 30, the 2016 season got off to an unusually early start. Hurricane Alex developed in January and made landfall in the Azores. It was the first Atlantic hurricane to occur in January since Hurricane Alice in 1955.

Of the season’s 15 named storms, five made landfall in the US –  Bonnie, Colin, Julia,  Hermine, and Matthew. Hermine was the first hurricane to make landfall in Florida in 11 years, ending the Sunshine State’s so called “hurricane drought”. The biggest headliner of the season, however, was Hurricane Matthew.

Matthew was the first storm to reach category-5 strength in the Atlantic in nine years. It weakened as it moved northward parallel to the US coast, but unleashed powerful winds and a damaging storm surge in Florida, Georgia, and South Carolina. The storm officially made landfall in South Carolina as a Category-1 hurricane, but its rain bands reached well inland and caused catastrophic river flooding in both North and South Carolina. In Fayetteville, NC – 100 miles from the coast – 14.82 inches of rain was reported.

This active hurricane season was largely the result of above average sea surface temperatures in the Atlantic and ENSO neutral to cool La Niña conditions in the Pacific. With warm water to fuel storms coupled with reduced wind shear across the Gulf of Mexico, tropical development in the Atlantic basin was essentially unhindered.

Despite this busy season, the US has luckily not been hit by a major hurricane since Wilma in 2005. With records dating back to 1851, it is the longest such stretch on NOAA’s books.

Source: NOAA

Source: NOAA

Drought Update: Autumn 2016

This autumn has been marked by heavy rain and catastrophic flooding in some parts of the United States.  Drought, however, continues to plague large sections of the country.

According to the latest report from the U.S. Drought Monitor, 49% of the nation is dealing with drought. While this number represents a slight improvement for parts of the west, both the southeast and northeast have been drying out.

Currently, 57% of the southeastern US is in some form of drought and 21% is suffering from conditions of extreme drought. These parched conditions, which have been building for months, are now fueling wildfires across the region. According to the US Forest Service, 59 of the 61 active large wildfires burning in this country are in the southeast.

Another hard hit area is the northeast, where 19% of the region is in severe or extreme drought. Water restrictions are in place in parts of Massachusetts and communities in New Jersey are asking residents to conserve water voluntarily.

On the other side of the country, California – now in its fifth year of drought – received some much-needed rainfall recently. However, most of it fell only in the northern counties. Overall, 88% of the Golden State remains in some form of drought with 21% in exceptional drought, the worst possible category.

The Drought Monitor is a weekly publication produced by a partnership of government agencies, including the National Drought Mitigation Center, the United States Department of Agriculture, and the National Oceanic and Atmospheric Administration.

Credit: US Drought Monitor

Credit: US Drought Monitor

Hurricane Matthew Slams the Southeastern US

Hurricane Matthew, the 13th named storm of the Atlantic Hurricane Season, hammered the southeastern United States this weekend from Florida to Virginia.

Tearing up the coast as it trekked northward, Matthew made landfall near McClellanville, SC on Saturday as a category-1 hurricane with 75mph winds. It had reached category-5 status in the Caribbean – the first storm to do so since Hurricane Felix in 2007- but weakened as it moved toward the US.

Despite this downgrade, Matthew still packed a powerful punch. Its strong winds, flooding rains, and storm surge caused significant property damage and widespread power outages throughout the region. The death toll from this storm currently stands at 26 people from across five states and is expected to increase in the coming days.

With successive bands of heavy rain, Matthew also caused catastrophic inland flooding. In Fayetteville, NC – 100 miles from the coast – 14.82 inches of rain was reported. As a result, several rivers in the region rose to record or near-record levels and overflowed their banks, inundating communities.

All told, Matthew dumped 13.6 trillion gallons of water on Florida, Georgia, South Carolina, North Carolina, and Virginia before heading out to sea as a post-tropical storm. That is enough water to fill over 20 million Olympic-size swimming pools. The highest rainfall total, 17.49 inches, was reported near Savannah, GA.

The damage caused by Matthew is currently estimated at $6 billion.

Hurricane Matthew batters the south eastern US. Credit: NOAA/NASA

Hurricane Matthew batters the southeastern US. Credit: NOAA/NASA

Hurricane Hermine Batters Florida’s Gulf Coast

Hurricane Hermine, the eighth named storm and fourth hurricane of the 2016 Atlantic Hurricane Season, made landfall in the Big Bend region of Florida early Friday morning. It slammed the Sunshine state’s west coast from Tampa to Tallahassee with heavy rain and winds measured up to 80 mph.

The category-1 hurricane generated a 9-foot storm surge in Cedar Key and dumped more than 22 inches of rain in parts of Pinellas County, flooding many communities. The storm also downed trees and knocked out power to over 250,000 people. Only one storm related death was reported.

Traveling across Florida, Hermine was downgraded to a tropical storm. It is now in the Atlantic moving north along the eastern seaboard. Impacts such as powerful winds, heavy rain, coastal flooding, and dangerous rip currents are expected to be felt from Georgia to Connecticut this holiday weekend.

Hermine was the first hurricane to make landfall in Florida in eleven years.

Hurricane Hermine makes landfall in Florida on September 2, 2016. Credit: NOAA

Hurricane Hermine makes landfall in Florida on September 2, 2016. Credit: NOAA

Historic Flooding in West Virginia

Relentless rain unleashed catastrophic flooding across West Virginia late last week. Officials say it was the state’s worst flood disaster in more than a century.

According to the NWS, the Mountain State received about 25% of its average annual rainfall in just a few hours. In Greenbrier County, more than 10 inches of rain fell between Thursday and Friday. This massive amount of precipitation in such a short period of time overwhelmed rivers and streams throughout the area. In Kanawha County, which includes the state capital of Charleston, the Elkview River crested at 33.37 feet – its highest crest in more than 125 years of record keeping.

The raging torrents of floodwater damaged or destroyed thousands of homes and businesses, as well as infrastructure across the state. To date, twenty-three weather-related deaths have been reported and more than 10,000 customers are still without power.

This type of rainfall is considered a one in one thousand year event in West Virginia. That does not mean it can only happen once every thousand years. It is the recurrence interval, a statistical calculation that means an event has a one in one thousand chance (0.1%) of happening in any given year in a given location.

The cause of this devastating flooding was twofold and involved a combination of weather and topography. First, “training” thunderstorms developed along a boundary between cooler air to the northeast and warm, moist air to the southeast. This is a meteorological phenomenon where strong storms flow continuously over the same area for a relatively short period of time – like train cars traveling along a track – dumping excessive amounts of rain.

The second major player in this deadly deluge was the state’s mountainous topography. When substantial rain falls in hilly terrain, it runs downslope very quickly and causes flash flooding in valleys, where most people tend to live. Moving with tremendous force, this type of fast flowing water can pick up and destroy almost anything in its path.

The Governor of West Virginia, Earl Ray Tomblin, has declared a state of emergency in 44 of the state’s 55 counties as a result of the flooding. Additionally, President Obama declared a major disaster in three of the hardest hit counties – Kanawha, Greenbrier, and Nicholas – which allows federal funds to supplement state and local emergency efforts.

Flooding In West Virginia. Credit: ABC11

Flooding in Richwood, Nicholas County, West Virginia. Credit: J. Rose/ABC11

El Niño Comes to an End

It’s official! The El Niño event of 2015-16 has ended.

According to NOAA, the sea surface temperatures in the so-called “Nino 3.4” region of the tropical Pacific have fallen below the El Niño threshold and ENSO-neutral conditions have returned.  But, this El Niño episode will not soon be forgotten.

Developing in the spring of 2015, this El Niño was one of the three strongest on record, along with the 1997-98 and 1982-83 events. Transferring heat from the ocean to the atmosphere, it helped make 2015 this planet’s warmest year on record. Coming on top of human-caused global warming, the new record was heads and shoulders above the previous one.

Influencing weather around the world, El Niño is probably best known in the US for bringing heavy rain (and the mudslides that go with it) to southern California. This time, however, the little rain that did come was not enough to end the multi-year drought plaguing the Golden State.

With exceptionally warm sea surface temperatures, the eastern Pacific hurricane season was extremely active. Eighteen named storms developed and thirteen became hurricanes. Hurricane Patricia, with winds measured at 200 mph, was the strongest hurricane ever recorded in the western hemisphere.

By contrast, the 2015 Atlantic Hurricane season was below average. It produced eleven named storms and four hurricanes. This relatively quiet season was largely the result of wind shear in the Gulf of Mexico, generated by El Niño, which helped to hinder most tropical development in the Atlantic basin.

Now that El Niño has dissipated, NOAA expects a La Niña episode – characterized by cooler than average Pacific sea surface temperatures – to develop in the autumn.

Data: NOAA/CPC

ENSO records date back to 1950. Data: NOAA/CPC