Public Information Statement
Issued by NWS Upton, NY

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Public Information Statement
National Weather Service New York NY
600 AM EDT Thu Oct 31 2024

...THIS IS WINTER WEATHER AWARENESS WEEK...

Please join the National Weather Service in promoting winter weather
education including safety.

The topic for today is winter floods.

Coastal storms can produce heavy rain, high winds, and storm surge
that can last for dozens of hours or even a few days. Strong onshore
winds will cause an abnormal rise of water above astronomical tides,
called storm surge. If the storm surge coincides with high
astronomical tides, it will likely produce coastal flooding. The
onshore winds are typically produced by a combination of high
pressure to the north and low pressure to the south. The strength,
duration, fetch of the onshore winds, the height of astronomical
tides, and coastal geography and topography are the primary
determinants of the magnitude and areal extent of flooding in
specific location.

A recent high impact coastal flooding event occurred on Dec 22nd
into 23rd of 2022, southeast to south wind gusts of 55 to 65 mph
developed ahead of a rapidly deepening low pressure over the Great
Lakes region that slowly tracked into southeast Canada. These high
winds developed a storm surge of 3 to locally 4 ft, coinciding with
an astronomically high tide (Perigean spring tide) on the morning of
Dec 23rd. This resulted in widespread moderate to major coastal
flooding along our coast, with bayfront communities in southern
Nassau and southern Queens reaching water level experienced during
Tropical Storm Irene.

Persistent onshore winds and waves over several tidal cycles can
prevent waters from receding, particularly in bays areas, after high
tide. This is referred to as tidal piling, making tidal flooding
worse with each passing high tide. A slow moving storm and strong
high pressure to the north can amplify these effects. The historic
December 1992 noreaster was a prime example of this, causing
widespread moderate to major coastal flooding on 4 consecutive tidal
cycles, peaking with with widespread major inundation of 3 to 5 ft
for many coastal communities.

Coastal flooding can even occur for our most vulnerable coastal and
shoreline communities during otherwise quiet weather conditions due
to high astronomical tides. The highest astronomical tides, called
Perigean Spring tides, occur when there is a new or full moon
coincident with it being at its closest elliptical orbit to the
earth (Perigee). The quiet weather flooding can be exacerbated by a
persistent offshore low pressure system sending waves and piling
water along the coast. This occurred during the Halloween Nor`easter
of 1991 that was stalled well north and east of the area, but still
caused widespread significant tidal flooding under partly sunny
skies.

Heavy rainfall can compound tidal flooding impacts. The winter of
2023-2024 featured a few intense December and January storms that
tracked west of the region, bringing compound flooding from both
heavy rainfall and storm surge. Heavy rainfall caused widespread
moderate to major flooding of rivers, streams, and adjacent low
lying areas during these events, compounded by high astronomical
tides and 3 to 4 ft of storm surge, bringing widespread moderate to
major flooding for coastal communities and along tidal rivers.

Heavy rain and snow melt also pose a compound flood risk. Following
the blizzard of January 7th and 8th of 1996, strong south winds
brought in mild air causing rapid snow melt, which in combination
with heavy rain caused widespread severe urban, river and small
stream flooding, and many roof collapses.

Finally, for riverine adajcent locations north and west of New York
City, flooding can be caused by ice jams. As river flow increases,
water levels rise. Since ice that covers the rivers is lighter than
water, it will tend to float. Under the pressure of river currents,
this floating ice can then break up and move downstream until it
runs into an obstruction such as a bend, island, or wide shallow
area. When this happens the ice will often pile up into an ice jam,
blocking the flow of the river and possibly resulting in water
overflowing the banks and flooding areas adjacent to the river. The
pressure of rising waters can also break the ice jam and release a
sudden surge of water and ice downstream. While ice jams are not
unusual, it is nearly impossible to predict exactly when or where a
jam will form, or if and when one will break.

The next statement around 6 am Friday will cover watches, warnings
and advisories.

$$