Winter Weather PNS 5

Public Information Statement 
National Weather Service New York NY 
600 AM EST Thu Nov 2 2023

...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 and high winds that can last
more for dozens of hours or even a few days. 

Prolonged 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. 

Strong onshore winds will cause an abnormal is of water above 
astronomical tides, called storm surge. If the storm surge coincides 
with high astronomical tides, it can produce coastal flooding. 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 
strength, duration, fetch of the onshore winds, the height of 
astronomical tides, and coastal geography, topography, and 
development are the primary determinants of the magnitude and areal 
extent of flooding in specific location.

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.

On March 13th of 2010, northeast wind gusts 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. 

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. 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 produces these effects. 

On March 3rd-4th 2018, widespread moderate coastal flooding occurred 
for 3 consecutive tidal cycles, while during the historic December 
1992 noreaster, widespread moderate to major coastal flooding 
occurred on 4 consecutive tidal cycles. 

Although rainfall can compound tidal flooding impacts, it is not a 
direct cause of tidal flooding. During the Halloween Nor'easter 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. 

$$