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Climate:
Snowfall Patterns on Long Island

By Edward Moran
Published on February 26, 2005

Climate articles:

There are distinct patterns of snowfall that can be observed across the different parts of Long Island. There are a number of factors that influence this and the interaction of these factors can be quite complex. This article explores some of these factors and how they influence the local winter weather. In a sense it is incomplete, as an effort produce a new map showing average winter snowfall across Long Island is ongoing. A follow-up article describing the snowfall zones on Long Island in more detail will be published on this site.

Long Island lies on the Atlantic coastal plain and thus its climate is heavily influenced by the proximity of the Atlantic Ocean. The location is northerly enough that heavy snowfalls are not uncommon, although during some winters snowfall may be quite light. Because the prevailing movement of weather systems is from west to east off of the continent, the influence of the ocean is less pronounced than in areas facing the ocean to the west, such as along coastal sections of western North America and western Europe. For this reason, snowfall on Long Island is considerably heavier than at many coastal locations that are farther north in those areas.

Because the ocean is such an abundant source of moisture, it can enhance the precipitation from winter storms and bring very heavy snowfalls to Long Island. This is especially true in our own special east coast breed of storm, the nor’easter. However, a more common consequence of the living next to the ocean is that relatively warm air flowing off of the ocean causes winter precipitation to fall in the form of rain, rather than snow.

Consequently, those parts of the island most removed from the maritime influence tend to experience the heaviest annual snowfalls. There is in fact a considerable variation in annual snowfall across Long Island and the presence of the ocean is probably the single biggest factor in that variation. However, it is not the sole influence and each of the factors discussed here can impact the amount of snow that falls in any one location. This article discusses the following:

  • Proximity to the Atlantic Ocean
  • The track taken by winter storms
  • Proximity to the New York City urban heat island
  • Effect related to the Long Island Sound
  • Elevation
  • Snowfall zones on Long Island

There are trade offs between these factors which vary from storm to storm, for example being farther away from the track of a coastal storm can mean that western Long Island is more likely to receive snow (instead of rain) than points farther east, but it can also mean that there will be less precipitation. Over the long run, the combination of these factors cause certain areas to average more snowfall than others.

In general, the highest annual average snowfalls occur along the north shore from roughly Port Washington in Nassau County eastward through Port Jefferson in Suffolk County. Not surprisingly, the lowest average annual totals occur along the south shore. However, in any given season the highest totals may occur in any part of the island, and likewise in a single storm any part of the island can be within the "bulls eye" for heaviest snowfall. Most of the images that follow in this page can be clicked to either display a larger image or additional related information.

Proximity to the Atlantic Ocean

The number one factor effecting local snowfall on the island is the most obvious one; the extent of maritime impact on the local weather. There are both north-south and east-west components of location which may have bearing over where it snows and where it rains on Long Island; frequently one or the other is a dominant factor determining where the most snow falls.

Other things being equal, with a marginally favorable storm track for snow, or with marginally cold enough boundary level temperatures for snow, the changeover to rain will occur last along the north shore if at all. Along the immediate north shore, there may be some warming from the sound as well, but this impact is subtle compared to the influence of the ocean.

It is unusual for the rain-snow line to setup and remain over the island (although it does happen infrequently in light to moderate snowfalls), so the more common result is for the north shore to experience higher snowfall totals due to a longer period of snow prior to a change to rain. A somewhat more common occurrence than finding the rain/ snow line stalled over the middle of island is the occasional mixing of snow with sleet or rain along the south shore while the north shore remains all snow. This is because of more warm air from the ocean being present in relatively low levels of the atmosphere at the immediate coast.

The same factors also result in a higher probability of a mix or change to rain over the east end of the island. During rare exceptions, particularly with weak storms, the east end may be closer to a source of cold air (for example, cold high pressure near the northern New England coast) and the precipitation will be more likely to fall as snow there than at points farther to the west. With such a setup, the cold high over northern New England is usually retreating into the Atlantic and these situations tend to be short lived.

The track taken by winter storms

While in some Long Island snowstorms, the heavier snowfalls are measured in the places that are closest to the storm, in others this is not necessarily the case.

During east coast snowstorms relatively warm air from the ocean is transported up and over the colder air that is present over land. As the warm air is forced to rise it cools and forms precipitation. Typically in these storms the heaviest snowfalls occur a good distance to northwest of the center, where the optimal combination (for snowfall) of rising warm air over a deep layer of cold air near the surface exists. In coastal storms close enough to spread heavy precipitation over the entire length of the island, the east end is closer to the path of the storm and is more prone to seeing a mix or change to rain because the warmer air from the ocean is closer to the surface and more of the air above the surface is above freezing.

Depending on the proximity to the storm, precipitation totals may also be lower in areas that are closer to the storm. This is often referred to as the "dry slot". In these situations the heaviest snowfall totals occur over the western half of Long Island. A noteworthy example occurred during the "millennium storm" on December 30, 2000. In that storm, as much as 16 inches of snow fell over western Long Island while the eastern half of the island received snow which turned to sleet and then rain. Eventually the east end was "dry slotted" while heavy snows continued to the west. Eastern Long Island dry slotted during a snowstorm, 12/30/2000 The storm passed over eastern Long Island. Over central Long Island (roughly east of the Sagtikos Parkway), sleet and rain mixed in for a time with increasing amounts of rain and decreasing snow amounts to the east. For more information on this storm and about dry slotting, click on the radar image to the left.

Storms that hug the coast, particularly stronger storms, will bring more warm air inland and typically cause rain over the entire length of the island, although if cold enough air is present the precipitation may start as snow and remain as snow for the longest amount of time over the western end.Satellite image from 2/25/99.  Bands of clouds and snow from the offshore storm were backing to the west at the time of this image.  Click to enlarge The same is true for storms that move northward on a mainly inland track farther to our west, although on Long Island the majority of these cause only rain to fall.

Storms that pass farther offshore, east or south east of the benchmark may still drop heavy snows over the east end while little or no snow may fall farther to the west. An interesting example of this occurred February 25, 1999, when a strong ocean storm backed far enough west to give heavy snows from the twin forks through Cape Cod. Storm totals on Long Island ranged from 13.5 inches at Montauk Point down to an inch or two in New York City. [Click here to view a chart of snowfall totals from the February 25, 1999 snowstorm]

Effect related to the Long Island Sound

Lake effect snow (LES) is a well known local phenomenon in areas downwind of the great lakes, and a very similar phenomenon, ocean effect snow (OES), is sometimes observed along the coast of the northeastern United States. "Sound Effect" is an infrequently used term in local meteorology that best describes the subtle (and sometimes not as subtle) contribution that the Long Island Sound makes to our annual snowfall.

Although true "lake effect" type events are very rare on Long Island, the Sound does contribute to the amount of snowfall on Long Island, with the effect being most pronounced along the north shore.

The amount of "Sound Effect" snow that falls on Long Island is difficult to quantify because it is usually mixed in with the larger amounts of snowfall that are deposited by coastal storms. Statistical tracking is also difficult because of the relative rarity of snowfall records from areas of Long Island where the effect may be most pronounced. However, it seems possible that "Sound Effect", or perhaps more appropriately "Sound Enhanced" snow contributes several inches per year to local snowfall totals, primarily along the north shore. This may help to explain the frequently higher snowfall totals along the north shore as compared to locations across the sound on the Connecticut coast.

True “sound effect” snow, when it does happen, occurs mostly on the east end with a cold west wind and the proper conditions present both at the surface and overhead. (see sidebar). When this does occur, it usually consists of just some light snow showers. Sound and Ocean effect snow showers can also occur along the north shore and even into parts of New York City and northern New Jersey with an east or east-northeast wind. These too, tend to be light.

Enhancement of snow fall rates caused by Sound Effect can occasionally cause noticeably heavier snowfalls in very localized areas. One such example occurred on January 14, 1999. Very cold high pressure was drifting east of New England while a warm front was located well to the south across the mid-Atlantic. Very light snow and flurries fell across the area through the day with little more than a dusting as temperatures remained in the teens. However a very narrow band of moderate to heavy snow developed in the northeast wind over the sound. This band was primarily over the sound but stretched southwestward into parts of the Port Washington area and remained nearly stationary for a few hours. The result was as much as 5 - 6 inches of snow in a small area stretching through parts of Sands Point, Manorhaven and north Port Washington. Some businesses in the area, perhaps assuming that this was part of a more wide spread snowfall dismissed workers early, but as they left it took just minutes to drive out of the mini-snowstorm. During that same day, a much more significant ocean-effect snowfall occurred in eastern New England. An extensive case study of the event in the Boston area can be found at the UCAR web site.

More recently, a significant local enhancement of snowfall near the Long Island Sound occurred during the evening of January 7, 2002. Click here to go to the case study on a Long Island Sound enhanced snowfall.  Use the your browser 'Back' button to return to this page.That evening, an upper level disturbance caused some very light snows over Long Island, with most accumulations being a half inch or less. Along the immediate north shore, a stationary band of snow formed and persisted for a few hours dropping up to 3 inches of snow in spots between Huntington and Stony Brook. It was not a major storm, and while not strictly speaking a pure sound-effect snowfall, it was clearly a sound enhanced event. In the narrow area effected, the vast majority of the snow fall received was due to the local enhancement provided by the Long Island Sound. This image of radar estimated precipitation for that evening clearly shows northeast to southwest oriented bands of heavier precipitation There were several factors involved in causing the enhanced area of snowfall over the north shore, including colder air temperatures above the surface with a relatively long trajectory over the warmer waters of the sound, and convergence of these winds with surface winds from a different direction right at the immediate shoreline. Click here to view a brief case study of this event.

Proximity to the New York City urban heat island

Another factor that periodically influences snowfall accumulations across Long Island is the proximity to the urban heat island of NYC, and to a lesser extent localized heat islands in some of the more densely developed areas, particularly in Nassau County.

Buildings and paved surfaces tend to store and then release heat. In certain situations, the effect can be significant enough to cause snow to melt faster as it settles on the ground, or even to cause areas of rain embedded in a more general area of snow.

Snowfall totals for March 19, 2004.  Click here for complete map This seems to occur most often during springtime snowfalls when the temperature is very close to or just above freezing. The heat added to the lowest levels of the atmosphere in urban areas may be just enough to cause most of the snow to melt as it falls. At times the impact can be quite dramatic, especially in areas close to New York City. This may have been the case on March 19, 2004 when 2 to 6 inches of snow fell in areas surrounding New York City, while only trace amounts were reported in Manhattan and urban New Jersey. The map to the right shows some reported snowfall totals for this storm.

A similar situation occurred during a strong nor'easter on April 9-10, 1996. The storm caused heavy precipitation through much of New Jersey, New York City, Long Island and southern New England. The precipitation was in the form of heavy snow throughout this area, except for in New York City and urban northeast New Jersey where slightly warmer temperatures caused the snow to mix with and at times change to rain. This kept accumulations there under one inch, even as areas just to the south received up to 8 inches of snow and parts of Long Island received over a foot of heavy wet snow. This snowfall map and chart shows snowfall and liquid equivalent totals for this storm.


Elevation

The terrain over much of southern half of Long Island is a low lying gently sloping glacial outwash plain. The Harbor Hill moraine and the Ronkonkoma moraine, both hilly deposits of debris left behind by retreating glaciers run nearly the length of Long Island across the north shore and over the center of the island, respectively. While the elevations in these hilly areas are not great, they do range up to between 300 and 400 feet above sea level at a number of locations. Generally speaking, these elevations are not high enough to result in significant orographic enhancement of precipitation, although it is likely that there is some minor enhancement from added lift near Long Island's morainal hills.

Although the contribution of orographic lift to snowfall on Long Island is relatively minor, differences in elevation can sometimes produce noticeable effects during individual snowfalls. Small differences in temperature resulting from a combination of higher elevation and relative isolation from the warmer layer of marine air over the sound can make a difference in snowfall amounts. The temperature differences are not great; in saturated air (as might be observed during a precipitation event), the temperature usually falls about 1 degree for every 300 feet increase in elevation. This can be important when temperatures are borderline for snow. Instances where elevation has played a role in precipitation type and snow accumulations occur predominantly during very wet and typically light snowfalls.

Causal observers will notice relatively dramatic evidence of the effects of elevation on precipitation type from time to time along the hilly north shore where the elevation changes from sea level are relatively abrupt. An interesting effect of elevation has been observed in the past when crossing the Port Washington peninsula from the Hempstead Harbor side to the Manhasset Bay side on Beacon Hill Road, a distance of only a little over a mile. During a light wet snowfall, rain falling at the bottom of the hill on the Hempstead Harbor side changed to snow going up the hill with nearly two inches on the ground at the top. Back down the hill on the Manhasset Bay side of the Peninsula rain was also falling. The maximum elevation on the road is only about 190 feet, so it is possible that given the marginal temperatures, the water may have contributed enough heat right near the shores to result in rain immediately adjacent to the bays. Most likely this has happened on other occasions.

Similar effects have been noted along roadways such as the Northern State Parkway in the South Huntington / Dix Hills area where light accumulations of snow have been observed in areas where the roadway reaches elevations near 300 feet, while snow did not accumulate at lower elevations. It should be noted that it is not always a given that such differences are due to elevation. When a mix of rain and snow is present, areas that receive heavier precipitation are more likely than surrounding areas to receive snow, rather than rain. That is sometimes the real reason for differences in snow cover over small areas.

Snowfall zones on Long Island

Long Island can be divided into several different zones which average differing amounts of snow during the winter season. The snowiest parts of the north shore may average as much as 50% more annual snowfall than the least snowy parts of the island. Limited data available suggest that average annual snowfall at the snowiest location on Long Island may approach 35". Anecdotal evidence suggests that parts of the West Hills area of Huntington town may be in the running for the snowiest place on Long Island. The barrier islands off of the south shore are almost certainly the least snowiest parts of the island.

Thus far, quantifying the amount of snow that does fall in different locations on Long Island is proving quite difficult, but in a follow up to this article we will publish a discussion of the existing climate record and the detailed snowfall zones on Long Island. Only a handful of long-term snowfall records exist for locations around the island and most of these tend to be spotty and inaccurate. The record at the Brookhaven National Lab, dating back to 1948, is the most thorough and complete. It is maintained by meteorologists employed by the lab. The remainder of the available snowfall data for Long Island is from records kept for varying periods of time at several airport locations and by cooperative weather observers. Most of this data cannot be taken at face value, and will require additional analysis to identify data errors and fill in the gaps. When the analysis is complete, the resulting snowfall data will be used to produce a more refined picture of Long Island's average snowfall.