Public Information Statement
Issued by NWS Upton, NY
Issued by NWS Upton, NY
682 NOUS41 KOKX 311000 PNSOKX 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. $$