What caused such a sudden rush of torrential rain in Vermont? – The Boston Globe

Sometimes heavy rain events are widespread and sometimes they’re more localized. Either way, they can cause tremendous amounts of damage, ranging from major to even catastrophic. Last night and early Tuesday, one of these localized heavy rain events went through a small area of the Northeast Kingdom of Vermont.

The torrential rain was caused by an unusually well developed low-pressure area that had a lot of moisture and was moving slowly. This combination of high precipitable water — the amount of moisture available in the atmosphere — the slow movement of the storm system and probably even some topographical enhancement all contributed to dumping anywhere from 5 to 8 inches of rain in a short amount of time.

Notice how fast the Passumpsic River rose and then started to fall due to the heavy rain. This is why flash flooding is so dangerous.

When you receive this much rain in such a limited amount of time, it overwhelms streams, rivers and catch basins and the water has no place to go.

This is why flash flood warnings end up being issued and why damage can occur really fast.

Some scientists are calling this devastating flooding a 1-in-1,000-year event, but it’s not that simple.

Last night’s rain in VT had a 1 in 1,000 statistical probability of occurring. There is a 0.1% chance of that event occurring in any given year, but you don’t have to wait 1,000 years between them. You could even have these events in back-to-back years or even two in one year.

In the graphic below, the curved line marks a hypothetical list of all possible extreme rainfall events, with the caveat that the total area under the curved line must equal 1.0 or 100%. The percent chance of any single rain event being more than a specific amount is the percent of the total area to the right of that rainfall amount. The percent chance of a rain event less than or equal to that threshold can be found by subtracting the area to the right of the threshold from 100.

This recent event is being compared to some other big Vermont flooding events as far back as the 1920s and as recently as 2011, when the remnants of Hurricane Irene brought devastation to many of Vermont’s covered bridges. Those events were the type that impacted many more people and were not so localized.

There are more chances of rain in the forecast for much of New England, including the Northeast Kingdom of Vermont. On Wednesday, a more widespread area of showers is likely to develop and I suspect at least some appreciable precipitation will occur.

There is the chance for more downpours, but it’s highly unlikely we will see a repeat of Tuesday morning’s event. Nevertheless, any additional rainfall will be unwelcome.

Lastly, while individual events like what we saw this morning are just simply weather in a warming climate, these events are forecast to become more frequent.

Indeed there has been a notable uptick in heavy precipitation events worldwide and the Northeast is likely to experience both localized micro events and more widespread flooding events in the coming decades.