What is the climate of New Jersey?

New Jersey is located nearly halfway between the equator and poles, at the east end of North America and the shores of the Atlantic Ocean. This makes it a battleground of sorts - a battleground for air masses throughout the year, especially in the cold season. Clashing air masses make for interesting and quite variable weather throughout the year, particularly as the seasons change. Additionally, while a relatively small state, differences in latitude, elevation, and proximity to the ocean as well as even the urban heat island of central NJ allow weather conditions to often vary within the state.

Summer

Summer in NJ can be very warm and humid, featuring afternoon thunderstorms in unstable air masses and frequent sea breezes on the coast. The average high and low temperatures in July range from the low 80's/ low 60's in the highlands and near the water, to the upper 80's/ upper 60's in inland central and southern NJ. The urban heat island effect is especially prominent on summer nights, where the temperature in the city remains several degrees higher than in surrounding areas. This is especially uncomfortable during heat waves, when the low can struggle to dip below the mid to upper 70's. Heat waves occur every summer to varying extents, often at least at some point raising temperatures into the mid 90's and heat indices over 100 degrees, especially in cities. Temperatures above 100 are rare but not unheard of, and will occur every 2 - 3 years (statewide) on average. Here are some record highs (in Fahrenheit) during the summer for selected NJ stations:

  • Runyon 110 (1936) State Record
  • New Brunswick: 106 (1918)
  • Newark: 108 (2011)
  • Sussex:: 101 (1954, 1955)
  • Atlantic City: 106 (1966)
  • Hammonton: 105 (1966, 1966)

Most precipitation in the summer comes from afternoon thunderstorms resulting from an unstable airmass; these thunderstorms rarely turn severe. Thunderstorms also occur as cold fronts traverse the state on average about once a week, and these have more potential to be severe. The primary threat with thunderstorms in New Jersey is hail and damaging wind, though neither typically reaches the severity of those experienced in much of the Midwest. The state averages 25 to 30 thunderstorms per year, with fewer near the coast where local circulation interferes with the storms. Tornadoes are relatively rare, averaging about 5 per year, though usually quite weak.

Sea breezes develop during hot afternoons as the air over land is heated much more rapidly than that over water, resulting in a circulation of air towards the coast. This brings cool marine air onshore, sometimes only near the water, but other times extending dozens of miles inland. The opposite occurs during the night as the air blows offshore, mixing the air and keeping the temperature warmer than it may otherwise be. This occurs most frequently early in the season when the ocean water is cooler.

Fall

Signs of autumn begin in September, when Arctic air masses are finally able to exert a stronger influence at these latitudes after a couple months of lengthened nights. The first frost often occurs during October throughout much of the state, obviously occurring earlier to the northwest and last along the marine-influenced coast.

Fall months are climatologically the driest months, though not by a wide margin, averaging 8 days with measurable precipitation (as opposed to 9 - 12 in other months). However, fall (particularly late fall) is Nor'Easter season, whereupon potentially powerful low pressure systems can develop offshore and move up the coast. These often bring heavy and at times flooding rains in addition to their characteristic northeasterly winds as they pull away. These winds are sometimes powerful enough to cause extensive power outages and considerable beach erosion on the coast (easily comparable to a hurricane). Later in the season, they can bring heavy wet snow further inland and up in elevation.

The average high temperature is still in the 80's to begin September, but quickly begins dropping as the seasons transition and falls into the low-mid 60's by the end of October with low temperatures averaging in the 30's in all locations except the coast.

Precipitation occurs most often from low pressure systems, particularly later in the season, as opposed to the type of thunderstorms observed during the summer and into September. Brief but intense rain and thunderstorms can also occur with the passage of cold fronts, which become more frequent as the season progresses, and tend to be strongest during this season. Along with brief heavy rain, they can bring a very sharp contrast in sensible weather conditions as colder and dryer Arctic air is ushered in behind the front.

Snow is very rare in October, and November is more often than not snow-free in most parts of the state. Heavy snows have been recorded in November, especially at the end of the month and in the northwest part of the state, but these are exceptions to the rule and the air mass accompanying them is rarely cold enough to keep the snow on the ground for long.

Winter

Winter weather in New Jersey varies depending on location as well as the dominant weather pattern for the moment. Cold spells bring temperatures below freezing for days or even a week or two, whereupon rivers freeze and eventually become ice-jammed. In particularly cold and stormy patterns, as occurred in much of the winter of 2009-10, snow can be frequent and at times heavy. Conversely, sharply different patterns can produce prolonged spring-like weather, or cold and dry conditions where the temperature only warms up just enough to rain. Regardless of the pattern, the coast will be the warmest part of the state as the relatively warm water moderates winter's cold. The coldest part of the state is nearly always to the northwest, where elevation especially keeps it colder.

The January average high temperature ranges from the mid 30's in northern and northwestern NJ to the upper 30's in much of central NJ to the lower 40's on the coast and in the southern part of the state. Low's average in the mid 20's on the coast to the upper teens in northwestern NJ to the lower 20's in between. Some record low temperatures (in Fahrenheit) are listed below:

  • River Vale -34 (1904) State Record
  • New Brunswick: -16 (1934)
  • Newark: -8 (1935)
  • Sussex: -29 (1994)
  • Atlantic City: -11 (1979)
  • Hammonton: -9 (1982)

Snow can come from a variety of sources, including moisture-starved weak systems originating in southern Canada, modestly-moist systems coming from the central United States (and at times tapping into Gulf of Mexico moisture), and very moist systems originating in the Gulf of Mexico and/or gathering moisture and strength from the Atlantic Ocean ("Nor'Easters"). Snowfall over the course of the season averages around a foot on the coast to around two feet through the remainder of the lowlands, and 40-50 inches in northwestern NJ. Despite these modest averages, especially away from the highlands, snow can be very heavy when it does fall. The abundance of moisture sources means, when the conditions are right, individual storm systems can drop well over a foot of snow. However, this does not happen on average every year in any part of the state. Furthermore, snow that does fall does not tend to last more than a week, especially late in the season when the sun is relatively high. Again, the winter of 2009-10 was an exception to this.

Thaws in mid-winter occur nearly every year to some extent, and temperatures can soar well into the 60's and even 70's in places for a few days, even in mid-winter. Rain occurs most frequently from "Lakes Cutters", where NJ finds itself in the warm sector of a low pressure system that is moving through the Great Lakes, or similarly when a coastal storm moves too far inland to keep the state in the colder part of the storm. In the latter case, there is often a mixed bag of precipitation, spanning the spectrum from rain to freezing rain, sleet, and snow. Ice storms do not tend to be as severe in NJ as they are just to the west primarily because there is little in the way of topography for cold air to dam against with an east wind.

Spring

The onset and nature of spring can vary widely from year to year. Spring arrives early in March in some years as the temperature can begin rising into the 80's, while other March's and even April's are mired in a cool, "extended winter". Heavy snow can occur well into March, but snow begins to get quite rare by the end of March. It has snowed at most locations during April, but this is very uncommon. By April, spring is often well underway and the average high temperature has risen into the 60's with low temperatures well above freezing in the 40's. The last frost often occurs during April throughout much of the state.

Precipitation occurs primarily in association with low pressure systems, especially early in the season. These can bring both cold, grey and drizzly days as well as days with flooding rains in stronger storm systems. Later in the season as it warms up, more precipitation is of the convective nature as it is in the summer. Additionally, cold front passages are common during this transition season, which bring potentially heavy (but brief) rain and thunderstorms and the potential of severe weather.

Severe weather is more common in May and early June than any other month, though as mentioned, it pales in comparison to that of such regions as the American Midwest. It is most typically associated with the warm sector (ahead of the cold front but behind the warm front, generally south of the center of low pressure) of a passing intense low pressure system. In this region, high temperatures and humidity combine with the potent atmospheric dynamics of the storm system to generate severe thunderstorms. While damaging winds, relatively large hail, and isolated tornadoes do occur, the deadliest effects of thunderstorms in this region are flooding (from slow moving or stalled storms) and lightning.

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