Harvest Moon and high tide bring coastal flooding to the Tri-State

Flood alerts are in effect in parts of the Tri-State into the weekend, but it's not because of the risk for heavy rainfall. Instead, the main culprit is the impending Full Moon, called the Harvest Moon because of its early Fall timing. As a result, the flood water will mostly be coming up from the ocean, not down from the sky. This type of coastal flooding is sometimes called sunny-day flooding, and it's becoming more common.

Sunny-day flooding can happen when an event other than rainfall causes water levels to rise. It's most common when the Moon's phase is full, as it will be early on Friday morning. You're probably familiar with the tide cycles and their alignment with the Moon's orbit around Earth. The Moon's gravitational pull is strong enough to pull ocean water toward it, so if you're at the beach and the tide is coming in, it's likely that the Moon is rising at your location. 

This graphic from NASA Science shows an exaggerated view of the 

Moon's gravitational pull on our oceans.

And during Full Moon, the Sun is in alignment with Moon's orbit, which further amplified the difference at high tide. But high tides at Full Moon didn't used to cause "nuisance flooding" on Tri-State streets on a regular basis. An updated report from Climate Central, released this week, shows that coastal flood days are on the rise as the sea level inches upward. The graphic below shows the average rise in sea level at gauges in the US since its lowest recorded point in 1925, and the corresponding rise in days with minor coastal flooding. 

In the US, sea levels were lowest nearly 100 years ago, and have been on a fairly consistent rise ever since. Climate Central used information from the National Ocean Service to develop this report.

But, the sea level has not risen evenly at every coastal city in the US. For example, the increase in Charleston, SC is about double what we have seen at New York's Battery Park: 

Graphics and information provided by Climate Central

The rise in sea level varies for two main reasons: local topography, and local latitude. Yes, you read that right: local latitude is a factor- and the science behind it is fascinating. 

The two main contributors to sea level rise are a process called thermal expansion and the melting of polar ice sheets. Sea ice, while frequently reported on as a signal of climate change, is not a long-term contributing factor to global sea level rise. This is because sea ice is cyclical; it melts during polar summer, but at least some of it re-freezes during polar winter. The graphics below show the latest sea ice maximum and minimum from this past (Northern Hemisphere) summer: 

Graphics and information from NOAA

But the loss of multi-year ice, which is found on glaciers and ice sheets, does contribute to sea level rise. The ice sheets at both poles are massive- literally miles thick at their highest points. This makes them big enough to have their own gravitational pull, and ocean water is gravitationally attracted to the poles. 

Graphic depicting the impact of ice sheet loss on the sheet's gravitational pull 

from the Virginia Institute of Marine Science

But as the ice sheets melt, they lose some of this attractive power, and the ocean's water gets more evenly distributed at higher and lower latitudes. This, in turn, contributes to bigger sea level increases near the Equator, and in polar regions, the sea level may actually be lower than a century ago. This article has a fantastic, well-researched description of this phenomenon. 

This year, rain will fall under the Harvest Moon. But if you live along the coast, don't assume that the water on the street is from rain! Get the weekend forecast from the NYC National Weather Service office here. 

Weather alerts from KOKX on the morning of Thursday, September 28th

Rules of the (tropical) road: how hurricanes chart their course

The Atlantic hurricane season is now past its climatological peak. As such, we should start to see fewer new disturbances in the Atlantic turning into tropical storms and hurricanes. Historically speaking, by October 1st, the number of named storms is about half what it was at the peak just a few short weeks prior. 

Hurricane and tropical storm climatology, from the National Hurricane Center

In early September, Hurricane Lee was forecast to make a beeline toward the coast of the Carolinas, then turn north without making landfall in the US. A couple of my relatives asked me at our football tailgate (Go Penn State!) why the storm would change course like that. It seems like the kind of question that would lend itself to this blog! So, without further ado, here are the peculiar paths that tropical storms follow- and the rules governing their movement.

Rule #1: Tropical systems follow the path of least resistance. When it comes to tropical storms and hurricanes, this is the path with the warmest water and the least wind. You can see this when looking at historical storm track maps in comparison to persistent ocean currents. For example, Hurricane Idalia's movement closely followed the Loop Current in the Gulf of Mexico.

Above: the wind history of Hurricane Idalia, August 26- Sept 2,  2023. 

Below: satellite imagery with color coded temperature data in the Gulf of Mexico, showing the Loop Current.

Rule #2: Tropical systems go where the (trade)winds take them. Without the Earth's rotation, there wouldn't be hurricanes. As thunderstorms form in the tropics, the Earth's rotation imparts the Coriolis effect on these clusters, allowing them to organize and spin around each other. Since tropical storms form over the ocean, there's nothing below them to steer their direction. 

This map from the National Hurricane Center shows the point of origin (red dot) and track (grey line) of tropical storms 

that have formed in late August. Notice that the lines emanating from the dots on the right side of the map, 

near the west coast of Africa, primarily move due East.

This is why almost every tropical storm that forms off the Cape Verde islands initially moves almost due east. Storms that form off the western coast of Africa move from east to west with the trade winds, unless or until another weather feature takes over. Speaking of which: 

Rule #3: Large-scale weather systems easily steer tropical storms. It may be surprising, since the average hurricane expends more energy than that in the entire nuclear arsenal on Earth, but these storms are pretty easy to move. If you've ever seen the sport of curling, it may be helpful to envision the curling stone- which weighs over 40lbs!- sliding effortlessly on the low-friction surface of ice. Tropical storms have a similarly frictionless surface below them, which means even "low energy" weather features like a dome of high pressure will change a hurricane's path. When a Bermuda High is strong, for example, you can almost guarantee that the small island nation will not take a direct hit from a tropical storm; instead, the storm track will curve near the East coast, sometimes making landfall in a spot that juts out into the ocean, such as the Outer Banks of North Carolina. 

Bermuda highs often steer tropical storms nearly parallel to the Eastern seaboard of the United States. 

Full animation available at NASA Scientific Visualization Studio.

Rule #4: Tropical systems can get caught up in each other. When two storms get close to each other over the ocean, they can rotate around each other, creating what's known as the Fujiwhara effect. Eventually, the dueling vortices will go their separate ways, or if one storm is smaller, it can get absorbed into the bigger one. 

GIF of Hurricane Irwin and Hurricane Hilary interacting to create the Fujiwhara effect.
Source: National Weather Service "News Around NOAA" site

For more on tropical storms and hurricanes, check out this post about Hurricane Idalia from August, and my thoughts back in May about the start of the tropical season in the Atlantic Basin.

Desert drenching: how the Southwest got its water back

In August, Hurricane Hilary brought drenching rain to much of the desert Southwest. The map below shows radar-estimated rainfall totals in southern California; the spots of pink and white show mountainous areas where 4"-6" of rain fell, with even more at the highest elevations. It was a rare occurrence for a part of the country that averages less than 10" of rain in an entire year to get so much rain in just a 3-day period.

Radar estimated rain from Hilary, which was a Tropical Storm at landfall in southern California.

Map imagery from the National Weather Service

However, Hilary was just the latest storm to bring record rainfall to the West. From December 2022 through March, a series of atmospheric rivers carved through communities along California's coast, breaking rainfall records from San Luis Obispo to Los Angeles. In San Diego, more rain has fallen so far this year than in all of 2021 and 2022 combined- and the "wet season" hasn't even started yet. While these staggering statistics have brought devastating impacts to millions of Californians, the long-term impact to the Golden State's water supply is the silver lining. 2023's rain has essentially ended the drought in both California and Nevada, and even arid Arizona has seen big improvements.  

The latest drought conditions for the United States and US territories.

An interactive version of this graphic is available here. 

The drought relief has been most profound in California and Nevada. A year ago, nearly every square mile of these states was experiencing at least Moderate Drought conditions. Today, drought conditions are nearly nonexistent there. 

The latest drought conditions for the western region of the US.

The Drought Monitor is updated on Thursdays. Graphic courtesy of the US Drought Monitor

The 2023 deluge has also been a boon for California's reservoirs. California is by far the most populous state in the country, and it is also the USA's #1 supplier of produce. Maintaining this water supply is absolutely crucial, and the situation was bleak at this time last year.

Reservoir levels in California on August 28, 2022.

An interactive version of this graphic is available here.

Now, the major reservoirs are brimming with fresh water, with some even coming close to capacity:

Reservoir levels in California on September 10th, 2023.

An interactive version of this graphic is available here.

As mentioned earlier, a series of atmospheric river events at the beginning of the year brought catastrophic flooding to California, especially its central coast. But, the deluge fell in the form of snow in the highest elevations of the Sierra Range, providing a slow-deploying source of fresh water for the state's reservoirs. 

Snow is by far a better moisture source for the water supply, not only because it can slowly melt into reservoirs for months, but also because liquid rain is more likely to run off ending up in the ocean where it can't be drank or used for irrigation. So, what's responsible for this stunning change in fortune- and how long will it stay wet out West?

Last winter, the La Niña phase of ENSO was still firmly in place. While El Niño is much better known for delivering rain to Southern California, the entire ENSO cycle is only capable of slightly nudging the primary storm track of the West Coast. So even in La Niña, the opposite phase of El Niño, it's still possible for a storm track to impact California's coastline, which extends over 3,000 miles. 

A snapshot of SST anomalies during the 1997-1998 El Niño, which helped lead to major storms in southern California. 
Image from NASA

And now, southern California's "wet season" is on the horizon. Nearly all of Los Angeles's average annual rainfall is recorded from October to April. And, El Niño conditions are expected to continue in the equatorial Pacific through the winter. This warm phase of ENSO is favorable for more wet weather in the region. If you're old enough to remember the 1997-1998 El Niño episode, which brought mudslide-inducing rainfall and coastal erosion that forever changed the California coastline, then the thought of another ENSO warm phase may be alarming. But it's important to remember that no two ENSO events are exactly alike, and the impacts can vary. Wasn't it just last winter when the La Niña phase was supposed to keep California bone-dry? Well, that didn't happen. Another example: the "Super El Niño" of 2015-2016 was even longer and stronger than the 1997-1998 event, but rainfall was below average in southern California during this time. For more on ENSO and its varied impacts, including the affects that climate change may be having on this oscillation, check out this blog post. And check out my post from May on the potential impacts of El Niño in the Tri-State area.

NYC's first heat wave of 2023 happens in... September?

This week, the Tri-State dealt with its worst hot stretch of the season. On both Tuesday and Wednesday, high temperatures reached the low 90s in Central Park, and on Thursday, the preliminary data shows a high temperature of 93°, the hottest it's been in the city since July 5th. Further, with 3 days in a row of 90+ degrees, this will be the first heat wave of 2023 in New York City. That means our first heat wave will have taken place during Meteorological Fall! 

Heat index forecast for Friday, September 8th, which will be the coolest day since Sunday.
Graphic courtesy of the National Weather Service office in Upton, NY

We got close a couple of times; in early July, when we reached 93°, we just barely missed heat wave criteria as the temperature failed to reach 90° for three consecutive days. Overall, this summer is part of a larger trend in the Tri-State, as recent summers have not even come close to some of the scorchers of the later 20th Century. 

Our local Weather Service office keeps record of every heat wave at the official NYC observation station in Central Park. Below are the stats on the longest heat waves the city has seen: 

Chart of longest heat waves in NYC from the National Weather Service office in Upton, NY

Other than a particularly nasty stretch in 2002 and a long stretch of heat in 2013, this century has been devoid of long-lasting periods of intense heat. It's even easier to see the change in 90° frequency on this chart, also from NWS New York: 

Chart of 90°+ days in NYC from the National Weather Service office in Upton, NY

In the 20th Century, it was fairly common for the city to deal with 90s for basically an entire month of the summer. But here's the kicker: despite the lack of heat waves, our summers haven't gotten any cooler, because the average temperature has actually gone up. 

The chart below shows the average temperatures during Meteorological Summer at Central Park's observation station, dating back to the 1880s. The average temperature is computed using the daily high and daily low from every day in June, July, and August. In the 1880s, the average was a bit above 72°, and so far in the 2020s, the average is about 4 degrees warmer- and the upward tick has been pretty consistent from one decade to the next. 

The upward trend can be attributed to two main factors: fewer cooler than average days (such as highs in the 60s), and warmer overnight temperatures. However, the warmer nighttime lows are the bigger contributor of the two. According to Climate Central, "summer minimum temperatures across the US have warmed on average at a rate ... nearly twice as fast as the warming rate observed for US summer daytime highs over the same period". Higher humidity is largely to blame. As the bodies of water surrounding the Tri-State get warmer, the evaporation rate increases, introducing more moisture into the air. Humid air heats and cools more slowly than dry air, meaning that it's harder to cool down at night. But it's also harder to get really hot. Take a look at the change in 100° days in NYC. After peaking in the mid 20th Century, we've largely failed to reach the triple digits ever since. 

So, even when we're not breaking record highs, we're still feeling the summer heat in the Tri-State, and that's largely due to higher humidity. For this reason, some researchers suggest that heat waves should be declared based on heat index, not just the number on the thermometer. For more on nighttime heat, check out my post from June, and you can get tips on beating the heat from the National Weather Service here.