What is a Monsoon and How Do They Affect the US?

The summer phase of the North American Monsoon is in full swing. But what, you may wonder, is a monsoon and how do they affect the United States?

While most people associate a monsoon with rain, that is only half the story. It is actually a wind system. More specifically, according to NOAA, a monsoon is “a thermally driven wind arising from differential heating between a land mass and the adjacent ocean that reverses its direction seasonally.” In fact, the word monsoon is derived from the Arabic word “mausim”, meaning seasons or wind shift.

In general, a monsoon is like a large-scale sea breeze.  During the summer months, the sun heats both the land and sea, but the surface temperature of the land rises more quickly. As a result, an area of low pressure develops over the land and an area of relatively higher pressure sits over the ocean. This causes moisture-laden sea air to flow inland. As it rises and cools, it releases precipitation. In winter, this situation reverses and a dry season takes hold.

Monsoon wind systems exist in many different parts of the world. In the US, we have the North American Monsoon that impacts states across the southwest. Summer temperatures in the region – mostly desert – can be extremely hot. Readings in the triple digits are not uncommon. This intense heat generates a thermal low near the surface and draws in moist air from the nearby Gulf of California. In addition, an area of high pressure aloft, known as the subtropical ridge, typically moves northward over the southern U.S. in summer. Its clockwise circulation shifts the winds from a southwesterly to a southeasterly direction and ushers in moisture from the Gulf of Mexico. This combination of heat and moisture rich air produces thunderstorms and heavy rainfall across the region. In fact, summer monsoon rains are reported to supply nearly 50% of the area’s annual precipitation.

Replenishing reservoirs and nourishing agriculture, these seasonal rains are a vital source of water in the typically arid southwest. Conversely, they can also cause a number of hazards such as flash flooding, damaging winds and hail, as well as frequent lightning.

Monsoon season in the American southwest typically runs from mid-June to the end of September.

The North American Monsoon pulls moist air (green arrows) inland over the typically arid southwest region of the US. Credit: NOAA/NWS

Powerful Thunderstorm Batters NYC

The first severe thunderstorm of the season barreled through the New York City metro area early Tuesday evening. Strong winds and heavy rain were reported across the region.

After an unseasonably hot and humid day, a fast moving cold front moved in from the west and triggered the violent storm.  According to the NWS, 0.58 inches of rain was reported in Central Park and wind gusts at JFK airport reached 55 mph. It is also interesting to note that the temperature in the city dropped from 88°F to 68°F in less than one hour.

The powerful and fast moving storm knocked down trees and caused power outages throughout the area. Rolling through the city a little after 5PM, the storm also wreaked havoc on the evening commute. All MetroNorth Railroad lines out of Grand Central Terminal were suspended because of downed trees on the tracks. Additionally, the city’s three airports reported significant delays.

Outside of the city, the storm turned deadly claiming the lives of at least five people. That number includes an eleven year old girl who was crushed by a tree while sitting in a car in Newburgh, NY. No injuries or fatalities were reported in the five boroughs.

A powerful thunderstorm moves across NYC. Credit: southerlysweet/Instagram

Weather Lingo: Downburst

Thunderstorms pose a number of familiar hazards, such as lightning and hail. The lesser-known downburst, however, is also a serious threat to life and property.

A downburst is a strong downward current of air that causes damaging winds on or near the ground. They initiate high up in the atmosphere, where relatively dry air is entrained inside of an intense thunderstorm. The dry air evaporates some of the storm’s raindrops, which has a cooling effect. Since this cooler air is denser than the warm air that surrounds it, it sinks rapidly toward the surface. When it hits the ground, it spreads out radially – in straight lines in all directions. Reaching speeds in excess of 100mph, a downburst will knock down trees and other obstacles leaving a trail of debris all facing the same direction.

These straight-line wind events, according to the NWS, can vary in size and duration. When they cover an area less than 2.5 miles, they are referred to as microbursts. These typically last between 5 and 15 minutes. Larger events, known as macrobursts, affect an area greater than 2.5 miles and last from 5 to 30 minutes.

While short-lived, these powerful winds can pose a threat to property on the ground as well as airplanes in the process of taking off or landing.

Credit: NWS

Weather Lingo: Venturi Effect

March, a transitional month between winter and spring, is well known for its winds. In large cities like New York, however, the wind can be accelerated by something called the Venturi Effect.

Tall buildings and straight, grid-like streets essentially create man-made canyons that affect how the wind moves through a city. Funneled through the buildings, the wind is constricted and forced to speed up. The same process is seen when you put your thumb over the mouth of a hose to create a choke point and make the water flow faster.

The Venturi Effect is named for Giovanni Battista Venturi, an 18th-century Italian physicist.

Credit: Currents

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

How the Santa Ana Winds Help Wildfires Spread

The Santa Ana winds are notorious for exacerbating wildfires in southern California.

These strong winds blow warm, dry air across the region at different times of the year, but mainly occur in the late autumn. They form when a large pressure difference builds up between the Great Basin – a desert that covers most of Nevada and parts of Utah – and the coastal region around LA. This pressure gradient funnels air downhill and through the passes of the San Gabriel and San Bernardino mountains toward the Pacific. According to the NWS, the Santa Ana winds can easily exceed 40 mph.

Originating in the high desert, the air starts off cool and dry. But as it travels downslope, the air compresses and warms. In fact, it warms about 5°F for every 1000 feet it descends. This dries out the region’s vegetation, leaving it susceptible to any type of spark. The fast-moving winds then fan the flames of any wildfires that ignite.

The Santa Ana winds are named for Santa Ana Canyon in Orange County, CA.

Credit: NWS

Energy Flow: A Wind Powered Public Art Installation in Pittsburgh

On a recent visit to Pittsburgh, PA I had the opportunity to see “Energy Flow”, a wind-powered art installation on Rachel Carson Bridge. It was created in 2016 through a collaboration between environmental artist, Andrea Polli, and WindStax, a Pittsburgh-based wind turbine manufacturer.

Using sixteen small vertical axis wind turbines to power 27,000 LED lights, the piece produces a rainbow of colors up and down the bridge’s vertical supports. The light show visualizes the local wind speed and direction in real time as detected by an onsite weather station. The project also has battery storage to power the lights for up to twelve hours when the wind is not blowing. In essence, this artwork turned the bridge into a micro-grid – a single location where power is produced, stored and consumed.

As a site-specific art installation, it honors the environmental legacy of Rachel Carson (1907-1964). She was a native of the Pittsburgh area who went on to become a marine biologist and famous environmental writer. Her books, such as “Silent Spring”, are widely credited with inspiring the environmental movement of the 1960s that eventually brought about federal laws like the Clean Air Act.

The project also highlights Pittsburgh’s new focus on technological innovation and sustainability. Once known as the “Smokey City”, because of all the pollution that billowed from its plethora of mills and factories, Pittsburgh has refocused its economy and remade its image. Situated at the confluence of three rivers – the Allegheny, Monongahela, and Ohio – it is home to an astounding 446 bridges and is now often referred to as the “City of Bridges”. The Rachel Carson Bridge was originally, and rather simply, called the Ninth Street Bridge after it opened in 1926. It was renamed on Earth Day 2006.

“Energy Flow” was only expected to be up for a few months to celebrate Pittsburgh’s Bicentennial. But due to popular demand, its run has been extended through the end of 2018.

“Energy Flow” Wind/Light Installation on Rachel Carson Bridge, Pittsburgh, PA. Credit: Covestro LLC

Weather and the NYC Macy’s Thanksgiving Day Parade

The Macy’s Thanksgiving Day Parade is a long-standing holiday tradition in New York City.  For 91 years, it has marched rain or shine. Nevertheless, the weather has been a factor in the event several times over the years.

Famous for its giant character balloons, high winds are the main weather challenge for the parade. According to city guidelines, the multi-story balloons cannot fly if there are sustained winds in excess of 23 mph or gusts higher than 34 mph. These regulations were put in place following a 1997 incident where gusty winds sent the “Cat in the Hat” balloon careening into a light post, which caused debris to fall on spectators.

The only time the balloons were grounded for the entire parade was in 1971 when torrential rain swept across the city. In 1989, a snowstorm brought the Big Apple a white Thanksgiving with 4.7 inches of snow measured in Central Park. The parade marched on that year but without the “Snoopy” and “Bugs Bunny” balloons as they were damaged by high winds earlier that morning.

This year, the wind is not expected to be a problem. Temperatures, however, are forecast to be a bit chilly – mostly in the 30s during the parade hours. So, bundle up if you are planning to be outside along the route.

Marching from West 77th Street to West 34th Street in Manhattan, the 91st Annual Macy’s Thanksgiving Day Parade is scheduled to begin at 9 AM on Thursday morning.

Happy Thanksgiving!

Paddington Bear Balloon floats down 6th Ave in Macy’s Thanksgiving Day Parade. Credit: Macy’s

Wildfire Outbreak in Northern California

Northern California is ablaze with wildfires.

As of Thursday, according to CalFire, 22 wildfires are burning in the Golden State. They have scorched over 190,000 acres and forced the evacuation of entire neighborhoods. Officials say at least 26 people have been killed and more than 400 are missing. In terms of property damage, they conservatively estimate that 3,500 homes and structures have been destroyed.

The smoke and ash from these fires are also causing widespread air quality issues. In the Bay Area, well outside of the burn zone, air quality reached historically poor levels this week.

These huge fires are largely the result of climate whiplash. California has distinct wet and dry seasons, but they have been extreme recently. After years of drought, this winter brought the state record amounts of precipitation that spurred explosive new plant growth. Then during the summer, which was the state’s warmest on record and unusually dry, all that vegetation turned to tinder.

Making matters worse, the region’s seasonal winds known as the Diablo Winds began blowing over the weekend. They reportedly reached speeds as high as 70mph in some areas. Blowing east to west, these winds warm from compression as they flow downslope from the mountains towards the coast and dry out vegetation even further. They also fan the flames of any fire already burning and can cause it to spread very quickly.

The Diablo Winds are essentially the same type of air current as the famous Santa Ana winds in southern California. They only really differ in location and name. The Diablo Winds are named for Mount Diablo in the East Bay area and the Santa Ana Winds are the namesake of Santa Ana Canyon in Orange County.

Wildfires are not unusual in California, but this outbreak is considered one of the worst in state history. The exact cause of the fires is still unknown.

Wildfire burns near Glen Ellen, CA. Credit: The SF Chronicle

Hot Temperatures and Strong Winds Fuel Western Wildfires

Summer is wildfire season in the American West and this year it is off to an explosive start.

As of Wednesday, according to the National Interagency Fire Center, thirty large wildfires – defined as greater than 100 acres – are burning in ten western states.  These include Alaska, Arizona, California, Idaho, New Mexico, Nevada, Oregon, Texas, Utah, and Washington.

The largest is the Brian Head Fire in southern Utah. It has burned approximately 50,000 acres and forced the evacuation of nearly 1500 people. Ignited on June 17, the massive blaze is only 10% contained.

Another hard hit state is Arizona, where six large fires are currently burning. The governor, Doug Ducey, has declared a state of emergency in Yavapai County in response to the Goodwin Fire, which has burned more than 20,000 acres near the Prescott National Forest. Local officials have ordered the full evacuation of the town of Mayer, AZ.

These huge fires are being fueled by extremely hot and dry conditions that have left the region’s vegetation susceptible to any type of spark. Just a few days ago, excessive heat advisories were in effect for a large swath of the area as temperatures soared into the triple digits. Now, high winds are fanning the flames and helping the fires to spread.

Year to date, 2.7 million acres in the US have been charred. The country’s worst wildfire season on record was 2015 when more than ten million acres burned.

Brian Head Fire, UT. Credit: Desert News