What is the North American Monsoon?

The summer phase of the North American Monsoon is underway. But what, you may wonder, are monsoons and how do they impact 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 landmass 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, with the most famous one setting up over India and Bangladesh. In the US, we have the North American Monsoon that impacts states across the southwest. Summer temperatures in the region, which is 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 Plains in summer. Spinning clockwise, this shifts the winds in the area 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. Monsoon rains reportedly supply 50-70% 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, dust storms, hail, and frequent lightning.

The wet phase of North American Monsoon typically runs from mid-June to the end of September.

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

Swamped by Barry, Arkansas Sets New State Tropical Rainfall Record

Hurricane Barry made headlines for making landfall in Louisiana over the weekend, but its torrential rain made history Arkansas this week.

According to the NWS, the remnants of the moisture-laden storm dumped 16.59 inches of rain in Dierks, Arkansas, setting a new tropical rainfall record for the state. The previous record of 13.91 was set in Portland, AR during Tropical Storm Allison in 1989.

Arkansas is the fifth state to set a new tropical rainfall record in the last two years. The other four include Texas, Hawaii, North Carolina, and South Carolina.

These types of extreme rain events have been happening more frequently in recent years and most experts see a link to climate change. As the atmosphere and oceans warm, storms are able to carry more moisture, and therefore drop more rain.

Credit: The Weather Gamut

June 2019: A Soggy Start to Summer in NYC

June 2019 was another month of wild temperature swings in New York City. Highs ranged from an unseasonably cool 65°F to a balmy 91°F. In the end, however, the cold and warmth balanced each other out. The city’s mean temperature for the month was 71.7°F, which is only 0.3°F above average.

In terms of precipitation, the city was unusually wet. Overall, 13 out 30 days posted measurable rainfall that added up to 5.46 inches for the month. While that is a soggy statistic, it was not the wettest June the city has seen. That dubious honor belongs to June 2003 when 10.26 inches of rain was measured in Central Park. The city, on average, gets 4.41 inches for the month.

Credit: The Weather Gamut

May 2019: Second Wettest Month on Record for US

When it rains, it pours! This old adage was proven to be true last month. With national weather data dating back to 1895, May 2019 was the second wettest month ever recorded in the contiguous United States. Only May 2015 was wetter.

According to NOAA, an average of 4.41 inches of rain fell across the lower 48 states, which is 1.50 inches above average. Heavy precipitation was reported from the West Coast through the Mid-West, and into parts of the Northeast. Kansas, Nebraska, and Missouri each experienced their wettest May on record.

This relentless rain caused deadly and destructive flooding in several states along the Arkansas, Missouri, and Mississippi Rivers.

While there were a number of factors that contributed to May’s excessive rainfall, including El Nino, climate change also likely played a part.  As the National Climate Assessment points out, heavy precipitation events are increasing in both frequency and intensity across the United States as the atmosphere warms.

As greenhouse gases heat 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%. In other words, warmer temperatures lead to more evaporation from oceans, rivers, and lakes, and therefore more water vapor is available to condense and fall as precipitation.

Sadly, with our global temperature continuing to rise, experts say we should expect to see more extreme rain and flooding events in the future.

Credit: NOAA

May 2019: A Soggy Month for NYC

May was another month of wild temperature swings in New York City. Producing several cases of weather whiplash, highs ranged from a chilly 48°F to an unseasonably balmy 86°F. In the end, however, these extremes balanced each other out. The city’s mean temperature for the month was 62.2°F, which is only 0.2°F below average.

On the precipitation side of things, May was unusually wet. The month brought the city a relatively rare spring nor’easter and several impressive thunderstorms. One of which produced golf ball sized hail on Staten Island, one the city’s five boroughs. Overall, 19 out 31 days posted measureable rainfall that added up to 6.82 inches for the month. While that is a soggy statistic, it was not the wettest May the city has seen. That dubious honor belongs to May 1989 when 10.24 inches of rain was measured in Central Park. The city, on average, gets 4.19 inches for the month.

Credit: The Weather Gamut

Summer Preview Brings NYC First 80° Day of the Year

It felt more like June than April in New York City on Friday. The temperature in Central Park soared to 80°F, marking the city’s first 80-degree day of the year.

Topping out at 17°F above average, the day was unseasonably warm. However, it was not a record breaker. That honor belongs to April 19, 1976, when the mercury soared to 92°F. The low temperature was 58°F, which is also warmer than normal for the date.

This spring heat was the result of a ridge in the jet-stream that allowed warm southern air to move further north than it normally would at this time of year.  These balmy conditions did not last long, though. A heavy rainstorm moved through the region over night and brought temperatures back to more seasonable levels.

Summer does not officially begin until June 21.

Credit: The Weather Gamut

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.

Spring Peonies. Credit: Melissa Fleming

Weather Safety: Flooding and the Power of Water

It is no secret that heavy rain can cause flooding. However, it can be surprising to learn how little water is required to create significant impacts.

As anyone who carries a water bottle knows, water is heavy. In fact, just one cubic foot of fresh water weighs 62.4lbs (28.3kg).  Multiplied many times over, raging floodwater can carry away or destroy most things in its path. Moving at just 4-mph, water has enough force to cause structural damage to an average home.

Flowing floodwaters can also pose a danger when hiking or driving. According to NOAA, it only takes six inches of fast moving water to knock a person off their feet. Twelve inches of water can sweep a small car off the road and eighteen to twenty-four inches can float most large vans and SUVs.

Since it is impossible to know how deep water is just by looking at it, it is best to err on the side of caution. As the saying goes, “Turn around, don’t drown!”

Credit: NWS/NOAA

A Look at Rainbows and their Legendary Pots of Gold on this St Patrick’s Day

According to Irish folklore, a pot of gold can be found at the end of a rainbow. In reality, however, it is impossible to locate the terminus of this optical phenomenon.

Refraction and reflection inside a raindrop. Credit: Met Office

For a rainbow to form, rain has to be falling in one part of the sky while the sun is out in another. The water droplets in the air act like prisms that refract and reflect the sunlight, revealing the colors of the visible spectrum. Red is refracted the least and is always on the top of a single bow while blue is on the bottom. Since we only see one color from each drop, it takes a countless number to produce a rainbow.

A double rainbow is seen when the light reflects twice inside the raindrops. 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.

That said, these colorful arcs are not physical entities that can be approached. No matter how close they appear to be, they are always tantalizingly out of reach. Nevertheless, most people consider seeing one to be a treasure with no gold required.

With a little luck, you can spot a rainbow if you face a moisture source – rain or mist from a waterfall – while the sun is at your back.

Happy Saint Patrick’s Day!

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

From Snow to Freezing Rain, Winter Precipitation Can Take Several Forms

The winter season can produce various types of precipitation – rain, freezing rain, sleet, or snow. The form we see at the surface 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