Tuesday, February 25, 2014

Vermont Spring Flood Threat? Well..... Maybe

In Vermont, the Burlington Free Press put out an Associated Press article last week discussing what might be a rising chances of spring floods in the Green Mountain State this spring.
Severe flooding on the Lamoille River near Cambridge, Vt.
after torrential thunderstorms and rapid snowmelt in April, 2011.  

The headline was "Vermont Snows Prompt Worries Of Spring Flooding"

That's true, but at this point, it seems that chances are spring flooding won't be extreme, unless some incredibly extreme weather comes into the picture in the next few weeks

It's true the snowpack has increased in recent weeks and rivers are very, very frozen, two factors that could contribute to floods.

However, I don't expect a big spring flood disaster like the state got in 2011 unless something unprecedented happens.

The Burlington Free Press/AP article did not say we would get a flood like 2011, but understandably the piece brought the idea up. People are still spooked by that disaster, and the even-worse Tropical Storm Irene floods three or four months later in 2011.

The spring of 2011 was a special case.  It really was a worst case scenario.

The late winter snow cover that year was among the deepest ever seen in Vermont. Many feet of snow covered the ground in the beginning of March, especially in the mountains.

Worse, that 2011 snow cover had an unusually high water content. That meant there was the potential for massive amount of water to be released when the spring thaws came.
Strong winds over a badly flooded Lake Champlain
send waves crashing onto a North Hero, Vermont
highway in April, 2011.  

And that's what happened. Still, the flooding wouldn't have been too bad if the spring of 2011 was relatively dry.  Instead, it was easily the wettest spring on record in Vermont.

Early on, in April, torrential rainstorms caused river, street and basement flooding.  A storm that caused the worst tornado outbreak in United States history in late April of that year sent record warmth and very uncommon, torrential thunderstorms  into Vermont, causing severe flooding in some rivers, especially in the Lamoille River Valley.

The huge volumes of water poured into Lake Champlain, and meanwhile, the torrential rains continued to pour down from the sky during May. The result was by far the worst flood on record in Lake Champlain.

The flood also lasted a long time, well into June, due to the repeateded heavy rain. Millions of dollars in damage hit homes, camps, roads and other properties near the lake in Vermont, New York and Quebec.

Needless to say, all those bad weather spells in spring, 2011 coming all at once is pretty much impossible to replicate.

So I don't expect anything nearly as bad in 2014 in Vermont or surrounding states.

Still, there are some things to watch for, which could cause some real flooding trouble as we head into the spring.

This Year: 

The biggest flood threat this year, at least in my mind, is ice jams. Some thaws in midwinter have left jumbles of broken ice piled up in some rivers, and those piles threaten to jam up again if higher water moves them.
Camps severely damaged by record flooding
along Lake Champlain in May, 2011.  

All the rivers have also re-frozen solidly in the face of February cold spells.

Far colder than normal weather this week will extend into March, so the river ice will continue thickening, spelling more and more potential for trouble once we get the spring thaw.

Thick ice chunks make more bigger, more solid ice jams than thin, breakable ice.

If we get ice jams, chances are they might form in fairly remote areas, so they won't be that much of a problem. So what if Farmer Bill's field goes under water for a couple days in late March?

But if the ice jams form near communities, we could have a real disaster, like the ice jam flood in Montpelier in 1991. Needless to say, the ice breakups in northern New England rivers are going to have to be watched carefully this spring.

In January, the snow pack in the mountains of northern New York, and in Vermont and New Hampshire were way, way below normal, which, had the trend continued would have greatly minimized the threat of spring flooding.

But the snow cover caught up in February and it's now near normal. The water content is pretty high in that snow, too, since last week's rainstorm largely soaked into the snow rather than melting most of it.

Still the amount of snow, and the amount of water in it, is much, much less than it was in 2011.

The longer we go into March without a good thaw, the greater the chance it will turn really warm later in the spring and melt the snow suddenly, which could cause some river flooding if we get a lot of rain.

The good news is this week, only light amounts of snow will add to the snowpack, so the potential water for floods won't increase markedly (Although there are some highly uncertain hints northern New England could get a pretty good sized snowstorm next week, which would definitely add to the snowpack)

Finally, there's frozen Lake Champlain. Water levels are a little above normal now in the lake, because of some winter rainstorms. The lake is mostly iced over, so water can't evaporate from the lake that much. Of course, evaporation only would have lowered water levels only a tiny bit.

Flood stage in the lake is 100 feet, and I think there's a pretty good chance the water in Lake Champlain will reach that height as the snow melts this spring. But that's still way different than 2011, when the water got up to 103 feet above sea level.

I can't imagine the lake remotely close to that height again this spring, unless the impossible happens and we get a lot more rain this spring than the record amounts in 2011.

The bottom line around Lake Champlain and surrounding areas:  Yeah, watch for spring flooding because there's about a normal chance that could happen. But don't panic over it. There's no reason to see signs of a watery apocalypse.

Elsewhere in the nation, places like the Upper Midwest, the Great Lakes and the rest of northern New England could be in for some flooding this spring, given deep snow cover in those areas.

As in Vermont, it depends on how much rain falls in the spring, and how fast it finally warms up.

Here's a video I took of the severe flooding on Lake Champlain, in Colchester, in May, 2011.


  1. Think you meant 2014?

    "So I don't expect anything nearly as bad in 2013 in Vermont or surrounding states."

    Doesn't the rather extremely thick amount of ice still around affect all of this? Even with multiple 40+ degree days still unbreakable layer of it.

    1. Yep. Meant 2014, thanks for the correction. That extremely thick amount of ice could worsen the ice jam flooding on rivers. The big blocks of ice could pile up more readily in narrow spots when the ice breaks up. Thick ice on the lake probably won't make much difference in the flooding. Thanks for the question!