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