Flood

The second half of the 2016 NJ weather and climate year began with plenty to talk about. Maximum temperatures were 90° or higher somewhere in the state on 20 afternoons. This, along with many warm nights, helped boost the mean monthly temperature into the top 10, based on statewide records back to 1895. Yet, remarkably, this was only NJ’s 5th warmest July in the past 11 years. The 77.2° mean was 2.2° above the 1981–2010 average.

On 15 July days an inch or more of rain fell somewhere in NJ. This included eight days with greater than 2” in spots and four days where a few locations exceeded 4”. Statewide, the average rainfall was 6.85”. This was 2.33” above average and ranks as the 14th wettest July since 1895. It was the wettest July since 2004, and the 4th wettest in the past 41 years. The northern half of the state (Mercer/Somerset/Union northward) averaged 7.27” (+2.49”, 14th wettest), while the southern region averaged 6.77” (+2.38”, 12th wettest). This all came in a month where the northern half of the state remained classified as being in moderate drought (D1) on the US Drought Monitor, with about half of south Jersey considered abnormally dry (D0). Also, on July 26th the NJ Department of Environmental Protection placed the northern half of the state in a drought watch. These actions were the result of notable rainfall deficits dating back to the early spring, with the warmth of the month exasperating low soil moisture, stream flow, ground water, and reservoir conditions. Clearly, the late-month heavy rainfall in a good portion of the state warrants a re-evaluation of drought status as NJ heads into August.

Toward the end of May, the threat of a significant drought loomed over the Garden State, as May proved to be the 3rd driest on record. Crops were in bad shape or not growing at all, and reservoir levels were declining at a faster than seasonal rate. However, New Jersey had yet to reach the point where average and timely rainfall could not remedy the situation. Not only was this prescription filled, it was done in abundance. June rainfall averaged 8.21" across NJ. This was 4.19" above the 1981-2010 normal and ranked 4th wettest since 1895 (Table 1). It joined four other Junes in the past 13 years to rank in the top eight over this 121-year period. As explained in last month's report, the rains that fell during the daytime and evening hours of May 31st factored into the June total, much as the localized afternoon and evening rains on June 30th (discussed below) will count toward the July total.

Extremely heavy rain drenched portions of the Mid-Atlantic during the daytime hours on August 12 until after sunrise on the 13th. Starting off in the Washington-Baltimore area and moving up into central Long Island, a narrow ribbon of rainfall exceeding 5”, and over 10” in a few locales, resulted in flash flooding that resulted in water rescues and many damaged roads and vehicles. Excessively heavy rains, occasionally accompanied by lightning, traveled up a frontal boundary that was associated with an unusually strong August low-pressure system situated over the Great Lakes. The atmospheric impulses riding up this front joined forces with abundant atmospheric moisture (in the top 1% for the region) to bring multiple inches per hour rainfall rates…for multiple hours.

The heaviest rain was situated within less than a 10-mile wide path. Within 20 miles on either side, totals fell off to a mere inch or two, or even less. Such is the nature of these events, where despite the abundant atmospheric moisture, there is a finite amount of water available. The dynamics concentrated the atmospheric lifting, thus the condensation of the majority of the moisture and resultant rainfall, while adjacent areas balanced out the lifted air with subsiding air that greatly limited rainfall totals.

Statewide, the first six months of 2014 averaged 45.8°. This is 2.2° below normal and ranks as the 31st coolest since 1895. It was the coolest start since 2003, which was 45.0° and ranked 15th. Before then you have to go back to 1982 (45.3°), which ranked 20th. The chilliest was 1907 at 43.2°.

Precipitation (rain and melted snow) averaged 24.76" across NJ from January through June. This total is 2.11" above normal and is 26th wettest. Since 2003, four January-June intervals have been wetter, including last year. 2011 ranked one notch drier at 27th. The wettest first half of the year was 1983 at 32.51".

May 2014 had a difficult time establishing an identity. What began with a storm that carried over from April 30th and resulted in the 7th largest flood of the past century in the Raritan basin on the 1st (see the April narrative for discussion of this event), later included some warm days, late freezes in a few locations, severe thunderstorms with hail in others, and a spectacular Memorial Day. Overall, May averaged 62.1°, which is 1.3° above average (compared to the 1981-2010 average). This ranks as the 35th warmest (tied with 1962) in the 120 years back to 1895. Precipitation averaged 5.18", which is 1.18" above average and ranks as 19th wettest. This value includes the considerable rain that fell on April 30th at National Weather Service Cooperative Stations that report during the morning hours (see April narrative for a full explanation).