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  • Writer's pictureBryan Norcross

Looking at the forecast for Ida in Louisiana versus New York; Hurricane Larry grows in the Atlantic

While Louisiana is still trying to come to grips with the calamity left behind by Hurricane Ida, the New York City region is trying to figure out how to reconcile the catastrophe that occurred Wednesday evening from the remnants of the same storm. More people died in the horrendous floods in New York and New Jersey than in Louisiana and Mississippi. It was horrific.

To compare the two events, we can start with the official forecasts. While the Louisiana forecasts were outstanding, the New York City forecasts were not. They were too little too late.

No experienced emergency planner should have been surprised by the events that unfolded in New Orleans and across southeastern Louisiana. People were well warned while the skies were still sunny with clear descriptions of the kind of damage that could occur.

But the New York situation highlighted the serious limitations in our messaging and alerting capabilities in weather situations that have the potential to produce life-threatening floods. Strong alerts weren't issued until after the event was underway.

On Monday afternoon, the National Weather Service issued a Flash Flood Watch for the New York City region. The bulletins talked about the potential for rivers and streams to flood – not much of an urban-area concern. The forecast was for 3 to 5 inches of rain with higher amounts possible. That’s a lot of rain in the north, but the tone of the messaging wasn’t especially urgent for city dwellers.

On Tuesday morning, the forecast was reduced to 3 to 4 inches with possibly higher amounts. That was a tactical mistake, even if the data showed less rain. The implication was, the threat was reduced by 20%, even though the same alert, a Flash Flood Watch, was still in effect. In fact, the threat of flooding was still high, though again concern was only expressed for rivers and streams.

On Wednesday, the day of the event, the Flash Flood Watch continued. The rain forecast was bumped up to 3 to 6 inches and possibly more. That’s a lot of rain in New York and should have triggered concern in the city administration that a significant flood event was possible, though the National Weather Service bulletin continued to focus on rivers.

At 6:51 PM on the evening of the flood, a Flash Flood Warning was issued for New York City and the surrounding area until 10:00 PM. Oddly, the total rainfall forecast in the bulletin was only 2 to 3 inches, although flooding was underway in surrounding area.

At 8:59 PM, the Warning was extended until midnight, but the total rainfall projection was only increased to a max of 4.5 inches. This forecast will require close examination because that was the hour that 3.15 inches fell in Central Park.

At 9:28 PM, a Flash Flood Emergency was declared. Clearly, forecasters had caught up with the catastrophe that was already unfolding. Sadly, it was too late for people caught in the maelstrom of water surging down stairwells, streets, and into the subway.

How could this happen in the modern world of radar and instant communications? That’s a very good question that will have to be answered.

It’s true that forecasting where extreme rain is going to fall is very difficult – in fact, it’s impossible. We can see that the ingredients will likely come together to create dangerously heavy rain, but not exactly where that’s going to happen. Thus there’s reluctance to commit to a strong alert until it becomes clearer where the ingredients are coming together.

It is fair to ask, however, when did forecasters at the local National Weather Service office realize there was a significant risk of a deluge of 6 or more inches in New York City? The Flash Flood Watch bulletins would seem to imply that they thought there was some chance of that fairly far in advance.

And if they did forecast that risk, who in the city administration knew about it? Why didn’t the mayor go on TV like he would for a snowstorm and raise awareness?

This comes down to threat analysis and responsibility. What is the threshold for the National Weather Service to start waving the flag that something extreme could happen? And who in the city is then responsible for taking action?

This situation can occur in any city in the country. When a flood event could occur on a scale beyond what’s normal, what is the alerting protocol? A run-of-the-mill Flash Flood Watch is not enough. They are issued all the time for relatively small events.

After the disaster was underway, the Flash Flood Emergency was issued. What is the equivalent of a Possible Flash Flood Emergency before the fact, and what is the probability threshold for that sort of special alerting for a potential extreme event?

These are not easy question to answer, but it will be a bigger tragedy if we don’t learn from this situation.

The answer might be that general flooding was foreseeable, but not a catastrophic flood. If the science doesn’t allow better alerting, then let’s know that as well. That knowledge will help focus future research.

The circulation that was Ida is bothering Atlantic Canada and is no longer a menace to the U.S. The tropical moisture tail stretches all the way to South Florida, however. The excess moisture will continue to cause heavy thunderstorms across the southern Florida peninsula.

Out in the Atlantic, Hurricane Larry is slowly organizing and growing larger. It is expected to be a large powerful hurricane when it heads in the general direction of Bermuda next week. It’s too early to know if it will directly affect the island. Atlantic Canada will also have to watch its progress.

It should stay away from the U.S. except for producing large, dangerous swells along the East Coast beaches.

In the Caribbean, Disturbance #1 is tracking over Central America and the Yucatán Peninsula of Mexico. If it survives its trek over land, we’ll have to keep an eye on it in the Gulf of Mexico next week.

Over near Africa, Disturbance #2 has faded out, and conditions ahead look increasingly hostile.

Nothing else appears to be in the works.


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