On Monday, extremely heavy rainfall caused major flooding in one of the most populated metropolitan areas in the United States. The governor of New Jersey declared a state of emergency. Service at major airports in the New York City region were disrupted, and residents were warned of potential flooding to roadways and basement apartments. Video of flooded subways were reminiscent of what New York and New Jersey experienced during the remnants of Hurricane Ida (2021). Yet, this was not a hurricane. What caused the major flooding in the U.S. Northeast.
The National Weather Service had a good handle on the flood threat in the days leading up to the rain. The NWS discussion at 3:53 am on July 14 noted, “Scattered to numerous instances of flash flooding possible across NE NJ and LoHud. Isolated to scattered instances of flash flooding possible for NYC and SW CT. Flood Watch in
Effect from 2pm to midnight.” The primary weather factors were:
- Daytime heating which primes the atmosphere for rising motion.
- A shortwave feature that helped trigger the tropical, moist environment.
- Interactions of cool air (outflow boundaries) from morning rainstorms and a sea breeze front that was sprawled across the region.
- An area of lower pressure ahead of a front approaching the region by Monday evening.
The NWS discussion went on to say, “Primary threat will be for torrential downpours from tstm activity with 2+”/hr rainfall rates likely in a tall and narrow CAPE profile environ, PWAT approaching 2.25″ (+2 std), deep warm cloud layer of 12-13kft, and weak SW steering flow (15kt) aligned with trough/sea breeze/outflow boundary.” I know all of that jargon and shorthand sounds like gibberish so let me translate. CAPE is an indicator of how unstable the atmosphere is and how strong the rising motion in storms might be. PWAT is an indicator of the precipitable water. In other words, how much moisture (in inches) is in the atmospheric column if you condensed or precipitated it out the air? Typical of extreme flooding scenarios, it was near the upper end of expected values. The trough, sea breeze, and outflow boundaries are mechanisms that lift air to convert that moisture and convective available potential energy to intense rainstorms.
With such conditions, it is not surprising that the NWS Weather Prediction Center placed the region under increased alerts for excessive rainfall. Extreme rainfall also fell in other parts of the Northeast including Washington D.C., Maryland and Pennsylvania. Flash flooding and car rescues were reported throughout the D.C. region. Meteorologist Matthew Cappucci wrote on his Facebook page, “D.C. gang – this is rather crazy; Tyson’s Corner recorded 0.55 inches of rain in 5 minutes between 5:35 and 5:40 p.m. That’s an equivalent rainfall rate of 6.6 inches per hour – albeit briefly. Cappucci, a meteorologist with the Washington Post, went on to say, ”With that sort of air mass being pumped into storms, it’s no surprise we saw such exceptional rain rates!”
Ok, that’s the meteorology. It was a somewhat typical scenario with an optimal alignment of ingredients. However, there is a climate backdrop that most be discussed. Several reports, including the National Climate Assessment, have reported that rainfall intensity has changed dramatically in every region of the U.S. Climate Central has also looked at trends for U.S. cities. They found that Some 88% of 144 cities in the U.S. have experienced greater hourly rainfall intensity since 1970. Of those, hourly rainfall rates are roughly 15% higher.
As you see jaw-dropping flooding around the U.S., it should resonate that, on average, it rains harder now. A warming atmosphere means more moisture availability to storms. It’s basic physics captured in the Clausius Clapeyron relationship. Additionally, water temperatures are warmer in the Gulf and other water bodies, which also boosts available moisture these summer storms. Heavy rain has increased in every part of the U.S. over the past few decades.
While people contort themselves with alternative theories, the answer has been in front of us all along. This is not a surprise to climate scientists. In fact, we have warned of an accelerated water cycle for decades. Yes, rain storms happen naturally, but they are more “juiced” now. And by the way, storms with this type of moisture and dynamic profile are not impacted by cloud seeding, which has mostly been proven to be inconclusive anyhow. As Professor Tom Gill reminded me, if cloud seeding was a match to a flood like what we have seen recently, it would be like striking a match to a raging inferno.
