September 2018: Unusually Warm And Wet in NYC

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September 2018 was another temperature roller coaster in New York City. Highs ranged from an unseasonably cool 62°F to a sizzling 93°F. However, with fifteen days posting above average temperatures, including three with readings in the 90s, the heat won out in the end. This warm finish was also aided by the unusually balmy overnight lows that were seen throughout most of the month. In fact, on September 5, the low only dropped to 77°F. That tied the record high minimum temperature for the date, which was set in 1985. In the end, the city’s mean temperature for the month was 70.7F°, which is 2.7°F above average.

In terms of precipitation, September was a soggy month in the Big Apple. In all, 6.19 inches of rain was measured in Central Park. Of this impressive total, 51% fell on just two days, each of which saw flash flooding around the five-boros. The city, on average, gets 4.28 inches of rain for the entire month.

This Year’s “Climate Week NYC” Will Focus on Accelerating Climate Action

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Climate Week NYC begins on Monday. This annual global summit takes place alongside the UN General Assembly and brings together leaders from a variety of sectors, including government, business, and non-profit organizations, to discuss solutions to climate change.

Organized by The Climate Group since 2009, the goal of the conference is to keep this pressing issue high on the global agenda. This year, which marks the halfway point between the passage of the Paris Climate Agreement in 2015 and the 2020 target for countries to ratchet up their greenhouse gas cutting commitments, the event will focus on ways to accelerate climate action. The ultimate goal of the non-binding Paris Accord is to limit global warming to 2°C (3.6°F) above pre-industrial levels

Public events to raise awareness and support of the summit’s mission are scheduled all week around the city, from September 24-30. They range in style from panel discussions and seminars to concerts and exhibitions. For the full program of events, go to the Climate Week website.

The Weather Gamut Celebrates Seventh Anniversary

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It is hard to believe, but today marks the seventh anniversary of The Weather Gamut.

Initially begun as a way to deepen and share my knowledge about weather and climate change, this blog has allowed me to expand on my interests and concerns in ways that I never thought possible. This past year, I gave a presentation on creative climate communication at the Annual Meeting of the AMS and was invited to speak at a variety of other venues.

Through writing this blog, I have also met many wonderful people working in this fascinating field. I am grateful for all their support and encouragement.

As always, thank you for reading!

What Causes the Autumnal Equinox

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Today is the Autumnal Equinox, the first day of fall in the northern hemisphere. The new season officially begins at 9:54 PM Eastern Daylight Time.

The astronomical seasons, as opposed to the meteorological seasons,  are a product of Earth’s axial tilt – a 23.5° angle – and the movement of the planet around the sun. During the autumn months, the Earth’s axis is tilted neither toward nor away from the sun. This position distributes the sun’s energy equally between the northern and southern hemispheres.

Since the summer solstice in June, the arc of the sun’s apparent daily passage across the sky has been sinking and daylight hours have been decreasing. Today, the sun appears directly overhead at the equator and we have approximately equal hours of day and night. The word “equinox” is derived from Latin and means “equal night”.

Transitioning from summer to winter, autumn is also a season of falling temperatures. According to NOAA, the average high temperature in most US cities drops about 10°F between September and October.

Earth’s solstices and equinoxes. Image Credit: NASA

August 2018: Earth’s Fifth Warmest August on Record

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Our global temperature continued its upward trend last month. August 2018 marked not only the fifth warmest August on record, but also closed out the planet’s fifth warmest June to August season – a period known as meteorological summer in the northern hemisphere.

According to the State of the Climate report by NOAA’s National Centers for Environmental Information, Earth’s combined average temperature for August – over both land and sea surfaces – was 61.43°F, which is 1.33°F above the 20th-century average. This August also marked the 404th consecutive month with a global temperature above its long-term norm. That means the last time any month posted a below average reading was December 1984.

Globally, the collective period of June, July, and August was also unusually warm. NOAA reports that Earth’s average temperature for the season was 1.33°F above the 20th century average of 60.1°F. That makes it the fifth warmest such period on record.

While heat dominated most of the planet during this three-month stretch, some places were particularly warm, including much of Europe, central Asia, and the southwestern United States. For the contiguous US as a whole, the season tied with 1934 as the fourth warmest summer on record.

These soaring temperatures are largely attributed to the long-term trend of human-caused climate change. ENSO-neutral conditions prevailed in August, which means there was neither a warm El Niño nor a cool La Niña in the Pacific to influence global weather patterns.

Year to date, the first eight months of 2018 were the fourth warmest such period of any year on record. Global temperature records date back to 1880.

Credit: NOAA

Remnants of Hurricane Florence Soak NYC and Its Subways

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The remnants of Hurricane Florence swept through New York City on Tuesday. While significantly weaker when compared to its time over the Carolinas, the storm still made its presence known with strong thunderstorms and bands of torrential downpours.

According to the NWS, 1.19 inches of rain was measured in Central Park. As impressive as that total is, it did not break the daily rainfall record for the date. That honor belongs to September 18, 1936 when 3.92 inches of rain was reported. New York City, on average, gets 4.28 inches of rain for the entire month of September.

The heavy rain caused flash flooding on roadways and disrupted travel across the city. Impacts were also felt underground as torrents of water poured into several subway stations through leaks in the ceiling and down the entrance/exit steps. Even the city’s infamous rats were seen sheltering from the rushing water.

This type of heavy rain event, according to NOAA, is expected to become more common as global temperatures continue to rise.

Heavy rain created underground waterfalls at several NYC subway stations on Tuesday. Credit: R.Mondshein/Twitter

 

Hurricane Florence Swamps the Carolinas

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Hurricane Florence, the 6th named storm of the Atlantic Hurricane season, slammed North and South Carolina this weekend.

Steered across the Atlantic by a strong area of high pressure, Florence made landfall near Wrightsville Beach, NC on Friday morning as a category-1 storm.  It peaked at category-4 strength while still over the ocean, but weakened as it moved closer to the US coast.

Despite this downgrade, Florence still packed a powerful punch. Its strong winds, flooding rains, and storm surge forced people to evacuate their homes and caused significant property damage as well as widespread power outages across the region. In the hard hit city of New Bern, NC, at the mouth of the Neuse River, a storm surge of more than ten feet was reported. Local officials there say upward of 4000 homes and businesses were damaged or destroyed.

Moving as slowly as 2 mph at one point, Florence essentially stalled out over the region, allowing it to unleash massive amounts of precipitation. Preliminary reports show that the storm set new state records for rainfall from a single tropical cyclone in both North and South Carolina. In Elizabethtown, NC, 35.93 inches was reported, crushing the previous record of 24.06 inches set by Hurricane Floyd in 1999. In South Carolina, the town of Loris, about 25 miles north of Myrtle Beach, reported 23.63 inches of rain, eclipsing the old record of 17.45 inches set by Tropical Storm Beryl in 1994.

If these numbers are confirmed by the NWS, that would mean four state tropical cyclone rainfall records were broken in the last thirteen months. The other two being Texas with 60.58 inches of rain from Hurricane Harvey in August 2017 and Hawaii with 52.02 inches from Hurricane Lane just last month.

Measuring 400 miles wide, Florence’s successive bands of heavy rain also caused catastrophic inland flooding as several rivers in the region overflowed their banks and inundated communities. In Fayetteville, NC – nearly 100 miles from the coast – more than 15 inches of rain was reported as of Monday. The Cape Fear River, which runs through the city, is forecast to crest at 61.8 feet on Tuesday, which is more than 25 feet above flood stage.

The death toll from this storm currently stands at 20, with most fatalities being water related. Sadly, as the rivers across the area continue to rise, that number is expected to increase in the coming days.

Hurricane Florence off the coast of the Carolinas. Credit: NOAA

How Hurricanes are Classified

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Hurricanes are one of nature’s most powerful storms. When formed in the Atlantic Ocean or North-Eastern Pacific, they are rated according to the Saffir-Simpson Scale.

Developed in the early 1970’s by Herbert Saffir, a civil engineer, and Dr. Robert Simpson of the National Hurricane Center, the scale classifies hurricanes into five categories based on the strength of their sustained winds. Each category is considered an estimate of the potential damage that a storm will cause if it makes landfall.  As conditions change within a storm, its category is re-assessed.

The different categories, 1 through 5, represent increasing wind speeds and escalating degrees of damage. Storms rated category 3 or higher are considered major hurricanes. The last category 5 storm to make landfall in the US was Hurricane Andrew in 1992.

While a useful tool, the Saffir-Simpson scale does not tell the whole story of the dangers to life and property posed by a hurricane. Regardless of category, these storms can produce dangerous storm surges in coastal areas and flooding rains further inland. Recent examples of these types of impacts were seen during Sandy and Harvey, respectively.

Wind Cave National Park and the Science Behind What Makes the Wind Blow

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I recently visited Wind Cave National Park in South Dakota, which protects a beautiful expanse of the Northern Great Plains as well as one of the largest and most complex cave systems in the world. While well known for its geology, the park’s namesake feature is also an excellent example of the science behind a basic weather phenomenon – wind.

Wind, which is air in motion, is the result of differences in atmospheric pressure. These pressure differences are caused by the temperature differences created by the uneven heating of the Earth’s surface by the Sun.  Several factors contribute to this unbalanced process, including cloud cover, large bodies of water, topography, and vegetation.

As the surface warms, air heats and rises, creating an area of low pressure. To fill that void, air from an area of relatively higher-pressure rushes in, creating a flow of air that we recognize as wind. The greater the pressure differences between these two areas, the stronger the breeze.

Atmospheric pressure conditions at the cave entrance during my visit. Credit: Melissa Fleming

At Wind Cave, given its vast size, the air pressure inside the cave is constantly working to equalize with that above ground. Therefore, when there is an area of high pressure at the surface, the wind will blow into the cave. If there is an area of low pressure on the surface, the wind will blow out of the cave. For this reason, the cave is described in the oral histories of the Lakota – a Native American tribe who consider it scared – as “the hole that breathes cool air”.

Park Ranger demonstrates the flow of air coming out of the small cave entrance with a ribbon. Credit:RVDreamLife

While other large cave systems can generate barometric winds, those at Wind Cave are more noticeable because of the small size of its entrance. As the Venturi Effect shows, when space is constricted, air will flow faster. Legend says that the first non-native settlers to discover the cave – two brother named Jesse and Tom Bingham – did so by accident when the wind from its entrance blew the hat off one of their heads in 1881.

According to the NPS, winds at the cave’s natural entrance have reached up to 25-mph.

Wind Cave National Park, SD. Credit: Melissa Fleming

The Continental Divide Determines Where Rain Goes After it Hits the Ground

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Most people are familiar with the various types of precipitation that falls from the sky. However, have you ever wondered where all that water goes after it falls or melts? The answer is largely dependent on what side of the continental divide it landed.

A continental divide is a natural boundary that separates the river systems of a continent. They are usually tall mountain ranges that direct the flow of rivers and streams to different oceans. Essentially, any precipitation that falls or melts on one side will flow into one ocean basin and anything that falls or melts on the other side of the divide will flow into another basin.

Sign in Rocky Mountain National Park marks the location of the Continental Divide in CO. Credit: Melissa Fleming

Every continent has at least one and some have multiple. In the United States, the main divide is the Rocky Mountains. It is part of the Great Continental Divide of the Americas, which runs from western Alaska through the Andes Mountains in South America. It separates water that runs into the Atlantic Ocean from water that flows into the Arctic or Pacific Oceans.

In some cases, water finds its way into an endorheic basin with no outlet to an ocean.  Utah’s Great Salt Lake and Oregon’s Crater Lake are well known examples.  Here, the water re-enters the water cycle via evaporation. A small percentage of precipitating water also seeps into the ground where it replenishes soil moisture and underground aquifers. That said, the vast majority of water returns to the world’s oceans where it will eventually be evaporated and fall as precipitation again somewhere on the planet.

North America has several drainage divides, but the Great Divide (red) is the largest. Credit: ContinentalDivide.net