Significant Flash Flooding and Mild Temperatures in September

October 10, 2023 // Article by: Steven Weinstein

It was another very active month for most across the Northeast in September, which featured continued bouts of flash flooding (some of which were significant), severe weather, and a few tropical systems. Additionally, although the start of the month featured temperatures running well above average with many daily records set, the second half of September completely flipped the script and saw temperatures run near to below average. The cooler weather and lower humidity was especially felt during the nighttime, allowing many to get their first taste of a more fall-like airmass. Overall, warm readings beat out the cool conditions and monthly temperatures ran a bit above average for the month. 

As the calendar flipped to meteorological fall, a generally dry, tranquil, and hot first several days of the month resulted under the influence of high pressure. It sure wasn't feeling anything like fall yet with high temperatures running 10 - 15 degrees above normal during the first week for most of the Northeast. This led to daily record highs being set across many climate sites from Richmond, VA to Boston, MA in the 90s to around 100 degrees. A pattern change then arrived on the 7th as an upper level disturbance and associated frontal boundary stalled out across the east coast. This brought increased humidity as the heat persisted and multiple days of unsettled weather with rounds of afternoon showers and storms a frequent occurrence. In addition to localized heavy rainfall, many of these storms were strong to severe which led to widespread downed trees and powerlines all along the I-95 corridor during the span of the 7th - 9th. Scattered large hail reports were also recorded, most of which were around quarter sized, though a potent storm on the 9th led to hail up to the size of golf balls across Central New Jersey! The severe weather aspect then relaxed in the following days, though an active pattern and repeated / heavy downpours continued, especially across New Jersey and into New England, leading to flooding problems. However, some of the most significant issues occurred in and around Leominster, MA on the 11th where 8-9"+ of rain fell.

The end of the second week of September continued to be active with a cluster of strong storms the night of the 12th in Maryland leading to sporadic wind damage and producing torrential rainfall, especially in and around Baltimore, where 3-4" fell in a short while. This prompted a rare flash flood emergency and led to many water rescues. Activity then shifted into New England the next day leading to another swath of very heavy rainfall across parts of Connecticut and Rhode Island, where a few stronger storms also occurred. One of these storms produced 4 tornadoes from Western Connecticut through Rhode Island, three of which were given an EF-1 rating with estimated winds up to 100 mph. The next prominent weather event was Hurricane Lee, which thankfully missed well offshore for most into the 16th, though increased surf / waves and rip currents were felt all along the Eastern Seaboard. It was Cape Cod and coastal Massachusetts that got more of a glancing blow from this system, leading to coastal flooding issues, and downed trees and powerlines from the 50 - 60+ mph wind gusts that resulted. This was followed up by a coastal system on the 17th - 18th which again brought heavy rainfall to parts of New England with 2-3"+ falling across Eastern Massachusetts. 

Tweet from NWS Boston discussing the tornadoes that occurred in New England on September 13th, 2023. 

Temperatures really began to trend more seasonable to even slightly below average into the third and fourth weeks of the month with highs and especially nighttime lows feeling far more cool, crisp, and fall-like across many locations. A few drier stretches also occurred at times before Tropical Storm Ophelia made landfall in the Mid Atlantic on the 23rd bringing heavy rainfall and strong winds, which downed some trees and powerlines (particularly across Maryland and Virginia). Coastal flooding also occurred, with gauges pushing into moderate to even major flood stage along the East Coast. Indeed, high tides even continued to run above normal into the last week of September owing to a persistent onshore flow regime. The remnants of Ophelia then remained offshore to begin the last week of the month on the 25th and 26th keeping rainy and dreary conditions locked in.

Otherwise, a rather significant and impactful flash flooding event took shape to end the month during the overnight of the 28th and into the 29th in association with a rather potent disturbance. Persistent and repeated downpours / storms led to a deluge of rainfall with as much as 5-8"+ falling across the 5 boroughs of New York, Western Long Island, and parts of coastal New Jersey with slightly lesser amounts found into the Lower Hudson Valley and Connecticut. Rather significant issues resulted in these areas with numerous road closures, water rescues, inundated homes / businesses, and flooded subway stations. Unfortunately, these issues were exacerbated with the heaviest rainfall coinciding with the morning and evening rush hours, which caused cars to stall and become submerged, people to become stranded, and had traffic at a standstill. This life-threatening flash flooding prompted the issuance of a state of emergency for New Jersey and New York. JFK airport, meanwhile, recorded a total of 8.05" of rainfall on the 29th, making it the wettest calendar day on record since record keeping began in 1948! This event ultimately pushed many climate sites in the area to rank within their top 5 wettest September's of all time.

Tweet from NWS New York showing an all time rainfall record that was set on September 29th, 2023 at JFK airport.

NWS New York tweet highlighting rainfall records at various climate sites that were set during the month of September 2023.

Overall, high and low temperatures ran a degree or two above normal across the entire Northeast. Precipitation wise, much like July, it was another anomalously wet and active month for most and while many ran 1-2" above normal (with some stations a tad closer to average), the precipitation surplus was much more notable in areas across New Jersey, Long Island, and into New England that ran 4-8"+ above normal! These extreme anomalies were certainly a byproduct of the historic rainmaker to end the month. However, parts of coastal Massachusetts were an exception to the extreme rainfall and saw anomalies run closer to average to around 1.5" above normal. Given the rainy month, drought conditions are now essentially nonexistent outside of a stray area here and there down into the Mid Atlantic running abnormally dry.