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Leominster's Perfect Storm: Why the City got so Much Rain so Quickly

It's now been more than a week since a deluge dumped nearly 10 inches of rain on parts of Massachusetts and Rhode Island. Recovery efforts are well underway in the city of Leominster, a city that saw 9.5 inches of rain fall in less than six hours. About 200 homes and buildings were damaged, along with heavy damage to roads, culverts and bridges. Recovery from this flood will be a weeks long (and in some cases, likely a months long) process.


The rain total from Leominster mirrors that of the highest rainfall total Vermont experienced during the Great Flood of 2023. The highest total in that flood was 9.2 inches in Calais, Vermont. Leominster's September flood and Vermont's July flood can hardly be compared, though, as the two events have many more differences than similarities.


One of the biggest differences is that Vermont's flood was forecast to be a potentially historic 36-48 hours in advance while Leominster's flood seemed to come completely out of the blue. There was very little warning of a historic deluge, but the warning signs were there as every facet of the storm began to line up, one piece at a time. Here's how it happened.



Leominster's flood was set up days before the September 11th event. The week prior to this flood featured full on summer heat with temperatures shooting well into the 90s and dew points in the 70s. Relief from this heat would approach New England as a cold front from the west.


The problem with this is that there was a strong area of high pressure just off New England's coast. This high pressure slowed the cold front to a crawl, and eventually caused the front to stall out right across New England. Normally, cold fronts pass through New England within hours, bringing thunderstorms for one afternoon before clearing out. This front took nearly five days to pass through New England.




This led to days on end of unsettled weather and storms popping up across the region. When the front first arrived on Friday, September 8th, it brought a round of severe thunderstorms that injured four and killed one, as well as knocked out power to over 100,000 customers. Typically this would have been the end of the front as it would have passed through the region, however, the front hung around, leading to continued storms on Saturday and Sunday, September 9th and 10th.


Stalled frontal boundaries lead to unsettled weather as waves of energy and low pressure systems can "ride" along them, bringing an almost constant threat of rain and clouds as long as they are around. In this case, an area of low pressure formed along the front in southern New England by the evening of September 10th. In a forecast post on September 8th, New England Storm Center wrote:


"The frontal boundary will remain stalled over New England through Monday. This will allow a system to ride along it, bringing the chance for a more widespread rain to occur Sunday night through Monday afternoon."



The morning of Monday, September 11th, the pieces were lining up for a potential flash flooding event. In a forecast post that morning, New England Storm Center wrote:


"With a lot of rain possible in a short amount of time, especially on top of what has fallen over the past couple days, flash flooding could become an issue today...scattered flash floods could crop up as they did Sunday. A flood watch is in effect for eastern Massachusetts (minus Cape Cod and the islands) from 11am through tonight. Storm training (when multiple storms move over the same area) is also a possibility."

So that was the basic setup; a stalled frontal boundary that brought days of rain prior to the event, saturating the ground ahead of time with an area of low pressure passing through the area bringing the most widespread rain yet.


However, for nine inches of rain to fall on one city over the course of several hours in New England, there needs to be more in the picture. The beginning of this post mentioned the full summer heat in place over New England to start September. Since the front couldn't clear through New England, humidity remained elevated. All this moisture in the atmosphere allowed for basically an endless supply of water vapor for storms to create very intense rainfall rates. The storm basically wrung out the atmosphere right over Leominster.



On top of this, thunderstorm training was a major threat that day. Thunderstorm training occurs when multiple storms move over the same area again and again. With a stalled front right overhead, constant waves of energy could move through the area, giving the thunderstorm over Leominster renewed energy instead of the storm just fading away or moving out of the area.


The basic setup gave warning that a flash flood event was likely going to happen somewhere in New England, but it was those two "extra" ingredients (endless water vapor and energy rippling through the area) that propelled this from a more standard flash flood event to a catastrophic deluge.



What all this led to was an extreme rainfall event that lasted for hours on end in one particular spot. The issue with this kind of event is that it's very difficult to pinpoint exactly where the worst of the flash flooding will occur. We knew flash flooding would occur somewhere in the yellow area below, but exactly where, and how bad it would be, couldn't be predicted.


New England Storm Center forecast map from the morning of September 11, 2023


It just so happened that all of this set up over Leominster. Even communities next to Leominster escaped the flood with far less rain. Next door neighbor Lunenburg picked up a modest 7.26 inches. Fitchburg, next to both Leominster and Lunenburg, saw 5.1 inches, nearly 4.5 inches less than Leominster.


Naturally, 9.5 inches of rain over the course of one afternoon led to very serious problems. A flash flood emergency was issued for Leominster and all surrounding communities. These alerts are issued for "exceedingly rare situations when extremely heavy rain is leading to a severe threat to human life and catastrophic damage from a flash flood is happening or will happen soon. Typically, emergency officials are reporting life-threatening water rises resulting in water rescues and evacuations."


All of that was occurring in Leominster as flood waters rose in a flash (hence the name, flash flood). Numerous vehicles were stranded, with their drivers needing rescue. Raft after raft of residents were pulled through the streets by firefighters. Homes and businesses were flooded and heavily damaged. Streets were destroyed and bridges washed out.


Route 2 in Leominster the afternoon of September 11, 2023


Significant flash flooding also occurred along the Massachusetts-Rhode Island border that afternoon as well, with Attleborough, Massachusetts seeing nearly seven inches of rain. Officials reported up to 42 emergency calls within an hour in that community.


By the time the high pressure moved along, allowing the front to clear, another system was backed up behind the front, needing to move through New England. This system brought the prospect for more flash flooding. Thankfully, that system was not a repeat of September 11th and flash flooding was limited. That system did spawn four separate tornadoes, however. Hurricane Lee brushed the area the following weekend, however, it stayed too far away to bring any notable impacts to central Massachusetts.


So, Leominster's disaster was a case of everything lining up just perfectly in just the right atmosphere. It just happens that the city was the unfortunate place where all of this came together.



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