Fall

September 2020 was a month of widely-varying temperatures, several episodes of heavy precipitation, and an extended dry spell. Put it all together and conditions averaged quite close to normal. The statewide monthly average temperature of 66.8° was 1.0° above the 1981–2010 mean. While it was the seventh month of 2020 to average above normal, it was the first of these not to rank in the top 10 for warmth. Rather, it ranked 33rd warmest of the past 126 Septembers. Divisional departures from normal ranged from +0.7° in the north to +1.5° along the coast. The statewide average maximum temperature was 76.6°, which is 0.3° above normal and ranks 47th warmest. The average minimum of 56.9° was 1.6° on the warm side and ranked 24th mildest. High temperatures were likely reduced by several degrees during a mid-month period when a veil of smoke from western US forest fires blanketed the sky at an altitude of about 15,000 feet when skies otherwise would have been blue, especially on the 15th–16th. It is uncertain if this kept minimum temperatures somewhat warmer than they would otherwise have been. A cold front moved through from the northwest on the 18th, clearing the skies and sending temperatures plummeting. What followed was a four-day spell of temperatures that were well below average, including four nights with below-freezing temperatures at three to four Rutgers NJ Weather Network sites. While freezes resulting from cold air draining into valleys in some northern locations are not unprecedented before the end of astronomical summer, it is highly unusual for the cold to persist over four consecutive nights.

As the 10th month of 2019 commenced, attention was on the continuing flash drought conditions across the Garden State. Lawns had gone brown and shallow rooted trees were losing their dull-colored leaves early. By the second week of the month, the US Drought Monitor showed all of NJ to be in either abnormally dry (D0) or moderate drought (D1) status (full disclosure: my recommendations are taken under consideration by the national author of each week’s map). Several rain episodes in the first half of the month, particularly in the north, began to stall any worsening of conditions. However, it wasn’t until the last half that five events deposited an inch or more, three of them with two inches or more, at a number of locations around the state. Thus by month’s end, only portions of southwest, southeast, and central NJ were rated D0, and the remainder, as we like to say, in “D nada.” This turn to storminess didn’t come without consequences. This included localized flash and small stream flooding on occasion, several episodes of strong winds, and some minor to moderate coastal flooding. More on all of this below, but first to further summarize the month. In the precipitation department, the statewide average was 5.79”. This is 1.90” above the 1981–2010 normal and ranks as the 15th wettest dating back to 1895. North Jersey came in with an average of 6.72”, some 2.41” above normal and ranking 10th wettest. In the south, the 5.28” average was 1.65” above normal and ranks as the 20th wettest.

As if the wet month was not newsworthy enough, the statewide monthly temperature of 58.0° tied with two other years as the 9th warmest back to 1895. This is 3.5° above normal, with northern and southern portions coming in at 3.2° and 3.8° above, respectively. The 2nd was arguably the hottest October day on record, thus also the warmest for so late in the season. Meanwhile, several locations experienced the first freeze of the season on the 4th or 5th, with a more widespread frost and freeze on the 19th. Still, the month ended with only 18 of the 63 NJWxNet stations having fallen to the freezing mark.

October was a tale of two half-months. Summer-like weather prevailed through the 11th, quickly transitioning to fall-like conditions on the 12th. Passage of the first strong cold front of the season was responsible for the change. Moderate to heavy rain accompanied the front in northwest NJ during the daytime hours of the 11th. That evening, and into the morning of the 12th, moisture from the remnants of once major Hurricane Michael ran up against the front and brought heavy rain to south Jersey. This one-two punch was followed on the 17th by the first freezing temperatures of the season at a few locations and a more widespread freeze on the 19th. Generally cool conditions prevailed the remainder of the month. The remnants of yet another major hurricane, Willa, provided energy and moisture to a quick-hitting nor’easter on the morning of the 27th that brought some of the worst coastal flooding since Sandy in 2012. Michael was an Atlantic storm that made landfall in the Florida panhandle, while Willa was an eastern Pacific storm that came ashore on the west coast of Mexico.

Even in a rather tranquil month, there are always interesting aspects of New Jersey’s weather and climate that can make for memorable situations. Such was the case during the 11th month of 2017. On the heels of a mild October and first week of November came one of the coldest early-season blasts on record on the 11th. Then the 19th saw some of the strongest winds of the year across the state. Overall, dry conditions by month’s end put the northern Highlands in the D0 (abnormally dry) category on the US Drought Monitor map, threatening to expand further into the state should dry conditions persist.

Rainfall averaged 1.68” across the state. This is 1.93” below the 1981–2010 mean and ranks as the 19th driest November since 1895. It was the driest since 1.30” fell in 2012 and well above the record low 0.54” in 1917. The statewide average temperature of 44.3° tied with 1938 as the 75th coolest (or 49th mildest) on record. The average was 0.9° below the 1981–2010 mean, but 0.8° above the 1895–2017 mean. This demonstrates how New Jersey has warmed in recent decades compared to earlier in the 20th century.

While remarkable deadly and destructive hurricanes pummeled portions of the Caribbean and southern states, September weather conditions in the Garden State were generally quiet. The most notable exceptions were the dangerous rip currents at the Jersey shore associated with the latter stages of Hurricanes Irma and Jose, which resulted in several drowning deaths and numerous water rescues. Minor to some moderate back-bay flooding and beach erosion accompanied the tumultuous surf, particularly between the 18th–21st. Otherwise, little more than some clouds and light rain reached NJ from these storms, while earlier some remnant energy and moisture from Hurricane Harvey brought one of the month’s few notable rain episodes. Dry conditions prevailed through much of the September, especially the second half. As a result, statewide precipitation averaged just 2.23”. This is 1.82” below the 1981–2010 average and was the 28th driest September since 1895.

While the first half of the month had an average temperature of about a degree below normal, September finished on a warm note. Some locations experienced three consecutive days with maximums of 90° or higher from the 23rd–25th. This backwards turn to warmth while the calendar suggested there should be a decline in temperature resulted in the month averaging 2.6° above the long-term mark. The 68.4° average made this the 10th warmest September on record. The last three Septembers and six since 2005 all fall into the top 10.