Public Information Statement
Issued by NWS Upton, NY

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NOUS41 KOKX 030904

Public Information Statement
National Weather Service New York NY
504 AM EDT Thu Nov 3 2016


Please join the National Weather Service and the New York State
Office of Emergency Management in promoting winter weather
education...including safety.

The topic for today is winter floods.

Coastal storms can produce heavy rain and high winds that can last
more than 12 hours.

Heavy rain can result in flooding of rivers...streams...and low
lying and poor drainage areas. 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.

High winds combined with high astronomical tides can produce coastal
or tidal flooding. Prolonged...steady onshore or east to northeast
winds over a long distance or fetch can cause water to pile along
the coast...a process known as tidal piling. The onshore winds are
usually produced by a combination of high pressure to the
north...and low pressure to the south along the coast. The duration
and strength of the onshore winds...the rate and magnitude of
pressure falls associated with the approaching low...and the height
of astronomical tides determine the magnitude and areal extent of
flooding. On March 13th of 2010...northeast winds of 50 to 70 mph
developed between low pressure tracking to the south...and strong
high pressure over southeastern Canada. These strong winds produced
a storm surge of 3 to 5 feet...causing widespread moderate coastal

Tidal piling can also prevent waters from receding after high tide.
This water will still be along the coast for the next high
tide...making tidal flooding worse with each passing high tide. A
slow moving storm and strong high pressure can combine to produce a
prolonged onshore wind that can affect several tidal cycles. During
the December 1992 Noreaster...significant flooding occurred on 4
consecutive days.

Although rainfall can compound tidal flooding is not a
direct cause of tidal flooding. During the Halloween Noreaster of
1991...widespread significant tidal flooding occurred under partly
sunny skies.

Another winter problem for locations north and west of New York City
is flooding caused by ice jams in rivers.

As river flow increases...water levels rise. Since ice that covers
the rivers is lighter than 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 is nearly impossible to predict
exactly when or where a jam will form...or if and when one will

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

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