Public Information Statement
Issued by NWS Upton, NY

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077
NOUS41 KOKX 021102
PNSOKX
NYZ067>075-078>081-176>179-031000-

Public Information Statement
National Weather Service New York NY
702 AM EDT Thu Nov 2 2017

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

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
flooding. Prolonged, steady onshore winds over a long distance 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 southeast Canada. These high winds
produced a storm surge of 3 to 5 feet, causing widespread moderate
coastal flooding.

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 impacts, it 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 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.

$$



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