New Brunswick, NJ

The ninth month of 2015 served as a bookend of sorts to an extended warm season. The average statewide temperature of 70.7° was 4.5° above the 1981–2010 normal and ranks as the 3rd warmest on record (since 1895). This ranking is the same as that achieved in May, with the intervening months all above average, though not nearly as much so as these shoulder months. Five of the ten warmest Septembers have occurred since 2002. A look back at the past five months finds that this most recent May through September interval was the third warmest on record. Even more notable than the recent run of warm Septembers, eight of the top ten warm seasons occurred within the last 18 years and four within the last six years.

What had been a rather dry month with drought concerns ended on a damp note on the 30th with a wet episode that extended into early October. Through the 29th, the month ranked as the 14th driest. However, an approximate statewide-average rainfall of 1.50” on the 30th brought the monthly average to 3.44” (-0.63”) and with it the rank of 63rd driest.

This month had quite a variety of weather conditions around the state. This resulted in some areas experiencing flash flooding and others encroaching drought conditions. Damage resulted from the flooding and one storm even produced a nocturnal “heat burst.” Some minor brush fires and declining river, ground water, and reservoir levels accompanied subnormal rain totals. The dry conditions helped produce some wide daily swings from cool nighttime temperatures to consistently warm daytime maximums. Preliminary values show August 2015 to have a statewide average rainfall of 2.18”. This is 2.03” below normal and ranks as the 13th driest since statewide records commenced in 1895 (Table 1). The 74.5° average temperature was 1.1° above the 1981–2010 mean and ranks 21st warmest.

Scientists are carefully monitoring sea-surface temperatures in the tropical Pacific Ocean for a potential El Niño event. An El Niño occurs when warmer-than-average waters start to form in the eastern Pacific Ocean, specifically near the equatorial latitudes. Easterly winds (blowing from the east) typically move warmer water to the western Pacific (near Indonesia), permitting cooler water to upwell to the surface in the east (near South America). When these winds are weaker, or if they reverse direction, the warm water stays in the eastern Pacific. This difference in sea surface temperatures and winds creates a new dynamic between the ocean and atmosphere, distinctly affecting weather patterns across the world. No two El Niño events are alike; they vary in magnitude and location of the largest temperature anomalies. El Niño events can be classified as Strong, Moderate or Weak.

May 2015 was a warm and dry month across New Jersey. As the month ended, drought concerns were looming large, though rainfall in the north during the afternoon and evening of the 31st and continuing into the first days of June resulted in at least a temporary braking of the downward slide. Based on data gathered at long-term National Weather Service Cooperative Observing (COOP) stations, May rainfall averaged 1.08" across NJ. This is 2.92" below the 1981-2010 mean and ranks as the 3rd driest May since records commenced in 1895.

As those who have been reading these monthly narratives for some time now know, precipitation that falls at COOP stations after observation time on the last day of the month gets recorded as falling on the first day of the next month. Most COOP stations observe in the morning, thus the heavy showers that fell across north Jersey on the 31st were not factored into this May average, except for a few stations such as Newark Airport (which registered 3.83" on the 31st alone) that observe at midnight. The last time this situation had a notable impact on monthly rainfall was a daytime heavy rain event on April 30, 2014.