Winter Weather PNS 5

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 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 Nor'easter 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 4 ft 
for many coastal communities. 

Coastal flooding can even occur for 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 fresh water 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 salt water 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. 

$$