Spring Vermont Flood Outlook: New Snow Brings (Slightly) Higher Risk

Water rushes across Route 15 in Cambridge, Vermont during
spring flooding in April, 2014. The risk of similar
flooding this spring is definitely lower, but is 
still a possibility

Last Thursday, the National Weather Service office in South Burlington, Vermont updated their late winter and  spring flood outlook, as they do every other week.  

Our belated semi-winter in late February and early March has actually forced some changes in their predictions. Earlier this winter, with a lack of snow cover and incredibly warm temperatures for January, meteorologist regarded the chances of late winter and spring flooding really low. 

As this update indicates, Vermont isn't exactly primed for March or April flooding, but the chances of it happening have increased a little. 

The report does focus on the two week period ending March 16. Forecasters can practically guarantee no flooding between now and then. It will be too cold for a lot of snow melt. In general, the chances of big storms seems low through mid-March and if any substantial storms do hit, we'll probably have too much snow or mixed precipitation involved to give us any kind of high water. 

The risk of flooding later this spring is still below normal in most of Vermont, but not as far below normal as it had been. Northwestern Vermont is actually right around normal as far as chances for spring flooding. 

The change is due to the fact that snow has finally started to accumulate, at least in the mountains. The second half of February brought a series of storms that managed to bring snow cover a lot closer to normal for this time of year. 

When the Burlington National Weather Service office completed their update on Thursday, the snowstorm on Saturday hadn't happened yet. But the meteorologists there were anticipating it. The snowstorm added a half inch or more of water to the the snow pack in most of the state. That water is now stored as potential runoff. 

Vermont snow depths are now not far from average for this time of year 

For instance, the snow depth atop Mount Mansfield was well below normal for most of this winter. But in late February, it started to catch up.  As of Saturday, the snow depth there was 64 inches, which is just three inches shy of normal for the date. 

At sort of high elevations, snow cover has gotten healthy, too. In Corinth, snow depth Sunday morning was 32 inches. Sutton was close behind at 31 inches. A number of places at 1,000 feet elevation or more in Vermont have around two feet of snow on the ground. 

That's actually fairly typical for early March, but of course it's no where near any record. 

Snowmelt water in the spring is often a big source of spring flooding, of course. 

The National Weather Service flood potential update showed that as of Thursday, there was about 2.5 to 6 inches of "rain" locked up in the snow pack at elevations of between 1,000 and 2,500 feet.  

It's hard to predict precipitation trends, but at this point, the amount of additional snow or rain we can expect in rest of March looks somewhat close to average. Even possibly a little below average, we shall see. 

In the shorter term, the weather this week should add to the mountain snow pack, mostly in the central and northern Green Mountains. The deeper valleys should only see between one and five inches of new snow between Tuesday and Friday. 

But the northern Greens will probably see more than six inches of snow during the upcoming long period of snow showers. It's not out of the questions that some summits, like Mount Mansfield and Jay Peak, could see an additional foot of snow gradually pile up over the next five days.

 Unlike most springs, ice jam flooding isn't a real big threat in Vermont this year. It's been too warm for much river ice to form. Some far northern rivers, like the Missisquoi, have enough ice to potentially cause some trouble, but the risk isn't all that great.

Since we spent most of the winter until the second half of February melting snow immediate after it falls, Lake Champlain is about a foot higher than normal this time of year. 

I suppose that increased the flood threat on the lake a little bit as we head into spring. But unless we have tremendous amounts of rain and snow between now and early May, it doesn't look like Lake Champlain will create a major problem.  

Minor Lake Champlain flooding is actually good for the ecosystem. At this point, it looks as if major flooding on the lake is unlikely. 

Predicting spring flooding can be difficult. The snow pack that piles up in the mountains by March is one important factor. So is the general weather in late March and April. 

If we have great maple sugaring weather, the flood potential goes way downhill. Afternoon thaws melt some snow and overnight freezes pause the melt. Under that circumstance, water from melting snow in the mountains drains gradually into rivers, so no high water.

 If it stays cold through March, and snow piles up a lot, then we abruptly get very warm weather a lot of rain in April, then we're screwed.

We have no promises either way. In late March and the first few days of April of 2001 and 2011, we had near record snow depth and water equivalent in the snow in Vermont. Both years brought high fears of spring flooding. 

In 2001, April turned out to be quite dry, with a number of subfreezing nights to slow runoff. We did have quite a bit of minor flooding that year, but nothing especially destructive. 

In 2011, on the other hand, we had record rainfall in April and May, which resulted in a series of highly damaging floods.

So, we might have some sort of surprise with flooding later this spring. But for now, things are not looking dire at all. Just potentially kind of wet underfoot.