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'1-in-1,000 Year Event': How Vermont's Improbable Storm Came Together

The long road to recovery is now well underway more than a week after catastrophic flooding struck the state. Just 12 years later, this storm challenged 2011's Tropical Storm Irene flooding, which set the benchmark for modern flooding in Vermont. What's especially impressive about this event is that this was not a big tropical cyclone, nor was it a powerhouse nor'easter. This storm was made up of a few rather mundane pieces that just happened to align perfectly in just the right atmosphere to unleash a devastating amount of rain.

This event has widely been labeled a "1-in-1,000 year" rain event over the last week. This label does not indicate that this type of storm occurs once every 1,000 years on average. What this label really means is there is a one in 1,000 chance of this kind of event happening in any given year, or a 0.1 percent chance of everything lining up the way it did.

The stage was set for this storm a few days before the event. A cold front began to approach New England the previous Friday (July 7). This cold front touched off a line of potent thunderstorms that afternoon. By mid-afternoon, there were a multitude of flash flood and severe thunderstorm warnings issued across northern New England. The region had been in a very unsettled pattern leading up to this event and less and less rain was becoming needed to cause flooding problems.

This cold front slowed to a crawl on Friday and eventually stalled out due to a slow moving area of low pressure off the coast of Cape Cod. In a forecast post the morning of July 7, New England Storm Center wrote:

"Into this weekend, the front will stall across New England, allowing a wave of low pressure to ride along it. This will set up another round of potentially heavy downpours for Sunday through Tuesday."

The front would end up stalling just to New England's west, across upstate New York. This placement would be crucial to setting up Vermont's disaster. The stalled frontal boundary would hang in New York for a couple days before slowly pushing eastward once again as the area of low pressure offshore of Cape Cod started to pull away. On Saturday (July 8), the cold front remained in place, but started to weaken. This led to less numerous storms erupting across New England that afternoon, with growing concerns over Sunday night.

In a forecast post on the evening of July 8, New England Storm Center sounded the alarm over a possible significant flooding event. We wrote:

"The frontal system is so slow moving, it will take a while for the rain shield to get further east. Significant flash flooding is possible, mainly in Vermont, where widespread flash flooding occurred on Friday. Those who live in flood prone areas of Vermont have been asked to have plans in place in case evacuations are needed Sunday night. There will likely be hours of steady rain with embedded heavy downpours, potentially moving over the same areas all night."

For much of this summer, New England has been entrenched in a trough thanks to an unseasonably strong Greenland Block. These blocks are much more typical of the spring time and cause storm systems to move very slowly over the northeast. This slow movement is a big reason why hours and hours of rain fell over the same areas all night Sunday and throughout Monday.

At this same time, an area of low pressure began to form to the south of New England along the front in the mid-Atlantic. This area of low pressure would ride up the front into southern New England Sunday into Monday (July 9-10). This low pressure system did not strengthen all that much, the central pressure was not all that low, staying above the 1,000mb mark.

This setup on its own would create a rainy day under any circumstances, but not historic flooding. This setup would need just the right environment to produce massive rainfall amounts and extreme rainfall rates. This right environment is extremely moist air. While the northeast does get plenty humid in the summer, the moisture in the air during this event was exceptional for the region.

Prior to this event, the northeast (and New England in particular) had been stuck in a very persistent southerly flow. This flow drew moisture from the tropics and funneled it all the way up the east coast.

This moisture-rich air rode up the entire east coast. Before the area of low pressure arrived in New England, it dumped large amounts of rain over areas just to the south and west, hours before the first flash flood warning was issued in New England Sunday evening. Dozens of these alerts were seen across Pennsylvania, New Jersey and New York. Significant flooding occurred in these areas. This would be ominous foreshadowing for Vermont.

By this time, a significant flooding event was locked in for Vermont. The Weather Prediction Center put much of Vermont into the "high" category for excessive rainfall. This is the highest category and is rarely used. The description for this level is as follows: "Severe, widespread flash flooding. Areas that don't normally experience flooding, could. Lives and property in greater danger." The National Weather Service of Burlington wrote on Sunday:

"Expect significant to potentially catastrophic widespread flash flooding through Monday evening, with possible impacts not seen since [Tropical Storm] Irene."

As the initial shield of rain worked into southwest New England on Sunday afternoon, the storm system was able to tap into the extremely moist atmosphere and it did not take long for the rain to pour down. With more unstable weather just ahead of the cold front, and with the cold front situated in New York slowly moving east, this placed Vermont in the (unfortunately) perfect place for the worst of the rainfall.

Vermont, being a rather mountainous state, has trouble dealing with large rainfall amounts. Excessive rainfall runs off the slopes of the mountains and right into the valleys, where rivers fill up fast and towns and cities become inundated. The state has a long history of intense floods, dating back to the great 1927 flood. This flood killed over 80 in Vermont, destroyed over 1,200 bridges and thousands were left homeless. This flood is still considered to be the worst natural disaster in the state's history.

Back to 2023, by Monday morning (July 10), a concerning development began to evolve. An area of low pressure over western New York began to close off. This would allow rainfall to fill in over the populated area from Burlington to Montpelier. The National Weather Service of Burlington stated:

"Earlier this morning, we noticed a potent 572 Dm low trying to close off over Rochester, NY with the upper level trough increasingly taking on a negative tilt...The [7am] HRRR and NAM 3km picked up on this development and backs/blossoms the precipitation over the populated Winooski River Valley from Montpelier to Burlington through the afternoon hours...The existing flash flood warnings across northern VT might need to be updated/upgraded in severity."

This low pressure over western New York would prove to be the final piece of the puzzle to bring Vermont a catastrophic rain event. By Monday evening, the consequences of this blossoming of rain over the Winooski Valley became apparent. Montpelier was under a flash flood and river flood warning at the same time as rain continued to pour on the city while the Winooski river rose to major flood stage from runoff. The Winooski River surpassed flooding seen during Irene.

The above graphic shows the full puzzle of this flood put together. As noted in the beginning, there isn't one big, powerful low pressure system or strong cold front. It's a few pieces of energy, none of which stand out on their own, that lined up in just the right environment to produce 5-9 inches of rain right over the most vulnerable areas, which were already over saturated by previous rain.



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