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

Heavy Rain Drenches NYC and Its Subways

An intense rainstorm swept through New York City on Monday. With bands of torrential downpours, it unleashed more than half a month’s worth of rain in just a few hours.

According to the NWS, 2.82 inches of rain was measured in Central Park. While that is an impressive total, it did not break the daily rainfall record for the date. That honor belongs to April 16, 1983 when 3.29 inches of rain was reported. New York City, on average, gets 4.50 inches of rain for the entire month of April.

The heavy rain caused flash flooding and disrupted travel across the city. Torrents of water poured into several subway stations through leaks in the ceiling and down the entrance/exit steps. During the morning commute, the MTA announced that several stops, including the 145th St station on the Number 1 line and the 42nd St-Bryant Park stop on the F and M lines, would be bypassed because of “excess water”.  Significant delays and cancellations were also reported at the area’s airports.

This type of heavy rain event, according to NOAA, is expected to become more common in the northeast as global temperatures rise and precipitation patterns change.

Heavy rain sends water cascading down the steps of the 145th St Station of the No. 1 train in NYC. Credit: Josh Guild/Twitter.

Do April Showers Really Bring May Flowers?

The phrase, “April showers bring May flowers “ has been around for centuries. It is derived from a poem written in the 1500s by Thomas Tusser – an English poet and farmer. This old adage, however, does not hold true in the northeastern United States.

Coming on the heels of the snowy months of winter, April typically produces more rain than snow. Many people, therefore, consider it a rainy month. Since water is necessary for the overall survival of plants, they also associate it with the bloom of flowers in May. Nevertheless, according to botanists, perennials – the plants that go dormant in winter and re-grow in the spring – are more dependent on the soil moisture derived from winter snowmelt and the long-term local precipitation pattern.

In the end, though, temperature is the most significant factor in determining when a flower will bloom. As soon as the weather becomes more spring-like, flowers will start to blossom, regardless of how much it rained in April or whatever the prior month was. That said, a “false spring” – a warm spell that triggers flowering but is followed by a hard frost – can kill the fragile blooms.

It is also worth noting that April is not typically the wettest month of the year for most places in the US. In New York City, July, on average, takes that honor because of the downpours associated with its strong summer thunderstorms.

Peonies in bloom. Credit: Melissa Fleming

How Rainbows Form

St. Patrick’s Day is a holiday often associated with images of rainbows promising a path to a leprechaun’s pot of gold. For most people, however, just spotting a rainbow is enough to brighten a day.

These amazing displays of nature form when raindrops, which act like prisms, scatter sunlight. To see one, an observer must be facing a moisture source like rain or mist with the sun at their back. The sun also needs to be at a low angle in the sky, less than 42° above the horizon. The lower the sun angle, the more of a rainbow’s arc will be visible.

Refraction and reflection inside a raindrop. Credit: Met Office

Passing from the air into a denser raindrop, the light slows and refracts. Since the different wavelengths of light bend by different amounts, the white light is dispersed into the colors of the visible spectrum: red, orange, yellow, green, blue, indigo, and violet. Red, which has a long wavelength, is refracted the least and is always on the top of a single rainbow. Violet, with a shorter wavelength, is refracted the most and is always on the bottom.

The light also needs to reflect off the back wall of the raindrop towards the viewer at the critical angle of 48° before it refracts again when it re-enters the air. A lesser angle will let the light pass through the raindrop and a larger angle will allow the light to reflect straight back out of the drop.

A double rainbow is seen when the light reflects twice inside the raindrop. Since each reflection weakens the intensity of the light, the second bow appears dimmer. The order of the colors is also reversed, with blue on top and red on the bottom.

Rainbow and faint second rainbow form after a rainstorm in Bermuda. Credit: Melissa Fleming

Powerful Nor’easter Slams NYC

A powerful nor’easter slammed the northeastern United States on Friday. Heavy precipitation, strong winds, and coastal flooding were reported across the region.

LGA airport. Credit: Chris Rudnick/Instagram

Here in New York City, 2.24 inches of rain fell in Central Park and wind gusts as high as 67mph were reported at JFK airport. These powerful winds canceled hundreds of flights, knocked down trees, and caused power outages in four of the city’s five boroughs. They also tore off a section of the roof of the American Airlines hangar at La Guardia airport and caused two tractor-trailers to flip over on the Verrazano Bridge.

Starting as an area of low pressure moving in from the west, this storm developed into a nor’easter over the Atlantic and then rapidly intensified. It underwent a process known as bombogenesis, the threshold for which is a drop in pressure of 24mb in 24 hours.This storm dropped 26mb in only 21 hours, producing its damaging winds.

Nor’easter of March 2, 2018. Credit: NOAA

From Snow to Ice, Winter Precipitation Can Take Several Forms

The winter season can produce various types of precipitation – rain, freezing rain, sleet, or snow. It depends on the temperature profile of the lower atmosphere.

All precipitation starts out as snow up in the clouds.  But, as it falls toward the Earth, it can pass through one or more layers of air with different temperatures.  When the snow passes through a thick layer of warm air – above 32°F – it melts into rain.  If the warm air layer extends all the way to the ground, rain will fall at the surface.  However, if there is a thin layer of cold air – below 32°F – near the ground, the rain becomes super-cooled and freezes upon impact with anything that has a temperature at or below 32°F.  This is known as freezing rain.  It is one of the most dangerous types of winter precipitation, as it forms a glaze of ice on almost everything it encounters, including roads, tree branches, and power lines.

Sleet is a frozen type precipitation that takes the form of ice-pellets. Passing through a thick layer of sub-freezing air near the surface, liquid raindrops are given enough time to re-freeze before reaching the ground. Sleet often bounces when it hits a surface, but does not stick to anything.  It can, however, accumulate.

Snow is another type of frozen precipitation.  It takes the shape of six-sided ice crystals, often called flakes.  Snow will fall at the surface when the air temperature is below freezing all the way from the cloud-level down to the ground.  In order for the snow to stick and accumulate, surface temperatures must also be at or below freezing.

When two or more of these precipitation types fall during a single storm, it is called a wintry mix.

Precipitation type depends on the temperature profile of the atmosphere. Credit: NOAA

Harvey Sets New Rainfall Record for Continental US

Battering southeast Texas for days, Tropical Storm Harvey has produced more rainfall than any other single storm on record in the contiguous United States.

According to the NWS, 51.88 inches of rain was reported at a gauge near Mont Belvieu at Cedar Bayou, about 40 miles east of Houston. This crushes the previous record of 48 inches that was set by Tropical Storm Amelia which hit Medina, TX in 1978.

The record for the entire US, however, still stands. It was set in 1950 when Hurricane Hiki dropped 52 inches of rain on Hawaii.

Source: NWS

Harvey Unleashes Catastrophic Flooding in Houston

Days after making landfall as a category-4 hurricane, Harvey, now a tropical storm, is still battering southeast Texas. Relentless rain has unleashed catastrophic flooding across the region, including Houston – this country’s fourth largest city.

While this devastating event is still unfolding, rainfall totals in some areas have already exceeded two feet. Dayton, TX, northeast of Houston, has reported a staggering 39.72 inches of rain since Thursday. Houston Intercontinental Airport (IAH) has posted 25.66 inches of rain so far and reported its wettest calendar day on record on Sunday with 16.07 inches.

This intense rainfall is causing the many rivers, creeks, and bayous in the metro Houston area to overflow their banks and flood homes, businesses, and major roadways – effectively paralyzing the area. Rainfall rates reached as high as 3 inches per hour on Sunday in some areas, prompting the NWS to issue a flash flood emergency – the highest level of a flood alert.

No evacuation order was issued ahead of the storm in Houston and residents were told to shelter in place. Now, thousands of high water rescues are taking place via boat and helicopter to save people trapped in their homes. The situation has many people comparing it to scenes that played out in the aftermath of Hurricane Katrina in New Orleans. The mayor of Houston, Sylvester Turner, has opened the massive George R. Brown Convention Center to be used as a shelter. As of Monday, eight storm-related deaths have been reported. But sadly, local authorities expect this number to increase as the water continues to rise.

Southeast Texas is no stranger to flooding. However, officials in Houston say this event is the worst they have seen since Tropical Storm Allison in 2001. That storm dumped more than 35 inches of rain on the metro area over the course of five days and caused $5 billion worth of damage.

Harvey, wedged between two areas of high pressure, is expected to linger over the region for several more days and dump an additional 15 to 25 inches of rain. Forecasters at the NWS say the rainfall total for this storm could reach an unprecedented 50 inches in some spots.

Eighteen counties in Texas, according to Governor Greg Abbot, have been declared federal disaster areas because of this storm.

TS Harvey caused unprecedented flooding in Houston, TX. Credit: Aryan Midas

Hurricane Harvey Hammers Texas

Hurricane Harvey, the eighth named storm of the 2017 Atlantic Hurricane Season, made landfall in Rockport, TX late Friday night. It was the strongest storm to hit the United States in twelve years.

Coming ashore with winds measured up to 130mph, Harvey was classified as a Category-4 hurricane on the Saffir-Simpson Scale. These powerful winds along with storm surge flooding and torrential rain caused catastrophic property damage across the Texas Coastal Bend region. It also downed trees and left hundreds of thousands of people without power. To date, according to local officials, only two storm-related deaths have been reported.

Moving inland, toward Houston, Harvey weakened to a Tropical Storm on Saturday afternoon. However, despite this downgrade in wind speed, its rain bands are still drawing moisture from the Gulf of Mexico and unleashing massive amounts of precipitation. Wedged between two areas of high pressure, the storm is essentially stalled over the region. Therefore, even more flooding rain is expected over the next several days.

Harvey was the first major hurricane – category 3 or higher – to make landfall in the Lone Star State since Hurricane Carla in September 1961.

Satellite imagery from NOAA shows Hurricane Harvey along the Texas Coast. Credit: NOAA

Weather Lingo: The Brown Ocean Effect

Tropical cyclones are fueled by warm ocean water and typically peter out over land. Sometimes, however, their lives are extended by something called the “brown ocean effect”.

This is a phenomenon where a storm derives energy from the evaporation of abundant soil moisture deposited by previous rainfall. Essentially, the saturated soil mimics the role of the ocean allowing a tropical cyclone to maintain its strength or even intensify after making landfall.

For the brown ocean effect to occur, according to a NASA funded study by Theresa Andersen and Marshall Shepherd of the University of Georgia, three criteria need to be met:

  • The soil needs to contain copious amounts of moisture.
  • Atmospheric conditions near the ground must have tropical characteristics with minimal variation in temperature.
  • Evaporation rates must be high enough to provide the storm with sufficient latent heat that it uses for fuel, at least 70 watts averaged per square meter.

Although this process supplies less energy than the ocean, it is enough to sustain a storm for a longer period than normal over land. It was first noticed in 2007 after Tropical Storm Erin made landfall in Texas and then intensified as it traveled inland. It formed an eye over Oklahoma and unleashed a massive amount of rainfall.

Storms that are impacted by the brown ocean effect maintain a warm-core and are known as Inland Tropical Cyclone Maintenance and Intensification events (TCMIs). While rare, they are most common in the US, China, and Australia.

Credit: NBC/TWC