Disaster in the Naugatuck River Valley

By Francis Tarasiewicz

On the afternoon of August 18th, a roughly 31-mile swath of the Naugatuck River Valley in southwestern Connecticut experienced a generational rainfall event. Over a 6-hour period, a series of training thunderstorms brought 1 in 1000 year rainfall totals to this highly sensitive and populated watershed.

The human impacts, while highly localized were no less devastating. As of the writing of this blog three fatalities have been confirmed along with dozens of swift water rescues and nearly 100 evacuations taking place during the storm. Some of the hardest hit were folks living near rivers and streams, and motorists trapped on rapidly flooding roadways. The recent approval of a federal disaster declaration will help unlock resources and aid to support the affected areas, and the collective effort of responders and community members will be vital in the rebuilding process. The road to the recovery will be long but if my stint in emergency management taught me anything, whenever disaster strikes, our disaster response institutions guarantee that an army of help is not too far behind.

In this blog I hope to shed some light on what exactly led to such a disaster. I will warn that this blog is going to contain a lot of technical information, and that, frankly, as someone who spent 18 years in Connecticut, this one is more personal than usual. Growing up, my strategy for getting over my storm-related anxiety was to learn about how they work. I’d like to think of this blog as my way of fighting back and, hopefully as a way to arm folks with the knowledge they need to be better prepared when a similar disaster strikes in the future.

Before the Event

The absorption of rainwater by the ground is lessened when the soil is either, too wet, and, paradoxically –too dry. Think of pouring water on an overly dry sponge. Rain is best absorbed in a sort of “Goldilocks” zone where the soil has a normal amount of water in it. The first alarm bells for this event rang several weeks before the first drops of rain fell from those fateful cumulonimbus clouds. Heavy rains throughout the spring and early summer had created a recipe for oversaturated soil. Between July 31st, and mid-August, soil moisture anomalies – differences from normal, increased to 2.7 inches above normal.

The Setup

The choreography of this event was led by air flowing around 40,000 feet (300 millibars) above the surface. As air is a continuous fluid, divergence in the upper levels means that other air parcels are more than happy to fill the void. The parcels that take up this role are usually much closer to the surface. The parcels converge and rise to fill the void. This rising motion results in clouds and, more critically, precipitation. On the afternoon of the 18th, the jet stream above the region was diverging. Air over western New York was flowing from south to north while air across southern New England moved from SW to NE. A simple 50 degree directional difference transformed this day from a typical Sunday to a disaster.

Troubling signs were also brewing lower in the atmosphere. At around 18,000 feet (500 millibars) an area of lower pressure was located over southern Ontario and was beginning to push east. Troughs like this are sources of lift in the atmosphere, and can further enhance precipitation chances. To simply explain the dynamics of troughs, lift from them results from the separation of areas of north-moving warm air ahead of the trough and south-moving cold air behind. Troughs are like ripples in a pond, and counter-clockwise moving air around them are responsible for shuffling airmasses around. On this day, the trough was well to the west of the area meaning that a southerly flow ahead of it was pushing abnormally warm, moist air into the region. This warmer air was a source of buoyancy and moisture – critical ingredients for the development of heavy rain.

A look at air pressure near the surface reveals an elongated area of low pressure spanning across southern Ontario and Quebec. To the west, higher pressure was centered over Nova Scotia. As a rule of thumb, wind flows from flow from high to low pressure. The configuration of the low and high pressure on this particular day caused the flow between the two clashing features to come from the south and southeast. This configuration of winds served a dual purpose. Convergence and moisture transport. The southerly component of the wind allowed moisture to flow off the ocean and into the area. Convergence squeezed air near the surface resulting in, you guessed it, more vertical motion!

Moisture was not in short supply as precipitable water (PWAT) (we love our acronyms in meteorology) values approached 1.8 inches. PWATs are a measure of the amount of water that would instantaneously fall from the sky if all the moisture were “rung out”. This means that at any given moment in time on the 18th the sky over southwest Connecticut could quickly dump nearly 2 inches of water. Of course, PWATS are not a 1:1 forecast for precipitation, a PWAT value of 1.8 inches does not mean 1.8 inches of rain will fall. Other factors like storm motion, convergence, and precipitation coverage determine the precipitation amount. Think of PWATs as a reservoir of water that storms can tap into.

The necessary convergence to ring out atmospheric moisture was laser-focused on the Naugatuck River valley. Surface observations near New Haven showed a temperature of 75°F and an incredibly high dew point temperature of 73°F. More importantly, the wind direction at this station was from the southeast. Just 60 or so miles to the north, at the surface weather station near Springfield Massachusetts, sensors indicated significantly lower dew points and, more critically, a northeasterly wind. These two stations acted a sort of canary in the coalmine, surface winds were broadly converging over west-central Connecticut. It’s no surprise then that the axis of greatest moisture convergence extended from New Jersey to southwestern Connecticut. Ominously, the absolute maximum values of moisture convergence were found along the weak boundary indicated by surface observations, and over the areas most impacted by flooding.

Meteorology doesn’t only happen on a flat surface, it occurs in three-dimensions, well, four if you count time. To get a sense of the third dimension – height, the National Weather Service launches 92 weather balloons with specialized sensors attached daily, sometimes more if there are significant weather events. These give meteorologists a sense of how temperature, wind and moisture are changing through the troposphere. One of these balloon launch sites, located at the National Weather Service office in Upton New York had its regular balloon launch at 8 am, just like any other day. The data retrieved from the day’s launch showed a full cast of ingredients that came together for the day’s deluge.

The first, and perhaps main character was a deep plume of nearly-saturated air. This area of saturation extended from the surface to nearly 34,000 feet. Air this close to saturation needed very little in the way of lift to produce tall, prolific rain-making clouds. Lift came from the tall and narrow column of buoyancy that was evident on the sounding. Convective available potential energy, CAPE, is a measure of the vertical acceleration of air that results from the difference in the observed temperature from the balloon, and that of a pocket of air that would be forced upward from the surface. The greater (more positive) this difference is, the greater the upward buoyant force. For the more conceptual thinkers out there CAPE is positive when the air of the environment is cooler than an idealized pocket of air being lifted from the surface is. In short, warm air rises, and the warmer the air is relative to its surrounding environment, the faster and more violently it rises. On the 18th, CAPE values were near 1,500 joules per kilogram of air. Add that up over 40,000 feet, and you can imagine the magnitude of energy at play that fateful day. The final character in this horrific play spent its time between 5 and 35 thousand feet. Here I am talking about the direction of winds in this layer. Winds in the day’s observed profile were unidirectional (moving in the same direction). Winds at these heights steer thunderstorms, and when they’re blowing in the same direction storms get a chance to line or “train” up. When storms train, a specific, localized area may get hours and hours of extreme rainfall rates. As the rain piles up, flash flooding begins. Now that I’ve talked a bit about the meteorology that characterized this event, let’s take a look at the results and dive into this disaster.

The Event

Over the course of just 6-8 hours, a series of thunderstorms developed and dumped between 6 and 12 inches of rain from Wilton to Waterbury. Radar estimates at the time were nothing short of incredible with the highest estimates near the town of Oxford. Another way to look at these numbers is to look at their “return interval” or, how rare they are. In many areas, the 12-hour rain totals were a 1 in 200 to 1 in 1000 year event! This means that in any given 12-hour period, an event like this had between a tenth and a half of a percent chance of occurring! The hardworking folks at the National Weather Service Upton compiled radar totals as well as dozens, if not hundreds, of rainfall reports to give the most accurate possible summary of the event. Its not just the eye-watering official report of 12.17 inches of rain near Oxford that jumps out at me, I am also a bit awestruck at the area covered by the 2-inch isohyet. The atmosphere was absolutely loaded with moisture. Southwest CT wasn’t the only place that suffered with heavy rain and flooding. I would be remiss if I didn’t mention the 8-10” totals that fell across north-central Long Island as they had to contend with their own train of locally extreme rainfall.

So, what did 6-12 inches bring to those below the towering cumulonimbus clouds?

River gauges tell the story.

Two gauges in particular, the Housatonic River at Stevenson, CT, and the Naugatuck River at Beacon Falls showed us what happens when 6-12 inches of water runs into rivers. Both rivers went from a late-summer trickle to experiencing major flooding. At Beacon falls, the water height rose from just 2 feet to well over 16 feet in a little over 3 hours. These nearly-vertical hydrographs are a tell-tale sign of major flash flooding At these heights, flooding impacts numerous roadways, and begins to inundate the towns of Waterbury and Naugatuck. Similarly devastating water rises came to the Stevenson gauge as its reading spiked from 3 to 20 and a half feet in a similar timeframe. At these levels, widespread inundation occurred from Stevenson all the way to the busy city of Bridgeport.

The epicenter of the event, the 862 square mile New Haven County, is home to 863,700 people. The population density per square mile ranges from 500 to 5,000 people. As meteorologists it is imperative to understand that there is no such thing as a “natural disaster” disasters are the result of natural events impacting humans. Every time there is a rare event, be it, a tornado, hurricane, or flood, I am confronted with complex emotions. On one hand, each event is unique and helps to give scientists another piece of the puzzle. On the other hand, real people are impacted by these “case studies”. When this happens it becomes personal. Meteorologists should always be thinking of weather science in terms of human impacts, and that’s exactly what happened with this event.

At around 3pm that afternoon, the heroes at the National Weather Service issued a rare Flash Flood Emergency. These rare statements are a step above Flash Flood Warnings, and indicate that flash flooding is causing real, human emergencies. In these instances, rivers may running out their banks, normally small streams may already be turning into lakes, and there be high-water rescues occurring. Sadly, all three of these situations were occurring with this storm. A silver lining with this event, if we can find any, is that there were multiple river gauges impacted by it.

Emergency managers who distribute hazard mitigation and recovery funds to help affected communities recover and be better prepared for future instances of flooding can use these numbers to make a case for recovery money. While the road to recovery may be long, the solidarity and determination of those involved will undoubtedly play a significant role in helping the community bounce back.