Due to New Jersey's middle latitude coastal location, precipitation is relatively abundant and consistent, at least in a 30-year climatological sense, throughout the year. February is the driest month throughout most of the state, with totals at most locations within a half-inch of 3.00". July and August vie for the wettest month of the year (do not be fooled by the lack of standing water on land during these months, this is a result of evaporation exceeding precipitation during the summer). Summer monthly totals generally range from 4.00" to 5.00", except approximately an inch lower in southern coastal counties.

Most every month of the year exhibits a similar pattern of precipitation distribution across New Jersey. A maximum is found in the north central Highlands, with a minimum along the southern coast. The cool season months show a secondary maximum along the northern coast, and in late winter and early spring over inland southern areas. Summer precipitation is most abundant in inland portions of the state, particularly in the west central counties.

The primary precipitation pattern is a result of a broad maritime influence in the southern coastal areas that inhibits warm season rainfall associated with thunderstorms. In the cool season, this area of the state is at times farther from a storm track through the Ohio Valley and Great Lakes, and south of nor'easters that may not attain their greatest strength until off the New England coast. Northern areas of the state are closer to winter storms. Highlands maxima are also a function of elevation-induced enhancement. As moist air comes in from the west or off the ocean, it is lifted, condensation increases and more precipitation falls. Conversely, a flow from the southeast crosses over the Highlands and then sinks once past them to the northwest. This results in a minor rain shadow or diminution of precipitation in the northwest valleys. Winter precipitation patterns are evident in January precipitation and standard deviation maps.

July maps best depict the warm season pattern, where inland lower elevation precipitation is enhanced by a dominance of convective, showery type precipitation over these warmest areas of the state. Cooler conditions in the Highlands and northwest valleys result in less convection than farther south in the more atmospherically unstable Piedmont and Inner Coastal Plain. The nearby cooler, more stable ocean and Delaware Bay inhibit convection in coastal areas, particularly the southern coast, thus the primary producer of warm season precipitation is diminished.

The amount of precipitation falling within a month or season and the characteristics of its regional distribution are critical in many decision making process, ranging from transportation to civil engineering to agriculture to water resources. So too is the frequency of precipitation events. This includes not only how often it precipitates, but also how often events of different magnitudes occur. Therefore four map pairs were generated in this collection based on station observations of just how many days each year precipitation may be expected that exceeds 0.01", 0.10", 1.00" and 2.00" (number of days and standard deviations). Roughly one third of the annual daily measurable (at least 0.01") events are less than 0.10". Unless the rainfall is of a short, intense nature, significant flooding will not occur until more than two inches falls in a day. With these larger values, we are mainly speaking of rainfall, as snowfall events having liquid water contents of such large magnitudes are relatively scarce in New Jersey, and obviously pose problems other than flooding.

Measurable precipitation falls on all areas of the Garden State approximately 100 to 130 days each year. As the daily precipitation amounts increase, the number of days with such events decreases, as do ranges between lowest and highest counts across the state. Generally, a tenth of an inch or more falls on 70 to 90 days, at least an inch on approximately 10-15 days, and two or more inches on 1 to 4 days. As best seen on the standard deviation maps, there is generally a two-region pattern of precipitation days for each of the four thresholds, that being a north maximum and south minimum. However within each of these regions there are some interesting inter-threshold differences. In the north, days with measurable precipitation are greatest in the northwest, while the Highlands have the most one inch or larger events. Inland southern counties have fewer precipitation days than in the coastal south, however have more days of inch or greater precipitation than their coastal neighbors. The northern pattern may be explained by the proximity of the northwestern corner to the often-lingering cloudiness and showery precipitation of the inland higher elevations of Pennsylvania and New York. The Highlands enhancement is a result of the wettest airflows being lifted by the hills as moisture streams in from the ocean. In the south, coastal areas experience lingering clouds and showers more often than inland, but are spared from as many soaking convective summer rains than inland reaches.

More information can be found here

Climatological maps were developed under a grant from the New Jersey Department of Transportation.