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Heavy Rain, Thunderstorms on Sunday Create Flooding Concerns for New England

Updated: Jul 16, 2023

A cold front will approach New England from the west Sunday morning. The front will interact with a tropical-like air mass over the area that has been brought to New England by a very persistent southerly flow. These two main components have led to a risk of a flooding event.


THE SET-UP


The moisture-rich atmosphere can be seen in expected precipitable water (PWAT) values. Unbelievably, PWAT values have trended even higher, at least for southern New England. It is now possible for these values to be around 2.4-2.8 inches. PWAT is defined as the amount of water vapor contained in a column of air if it were to be condensed and collected. Basically, the higher the PWAT value, the more rain could fall. The PWAT value does NOT correspond to how much rain is expected to fall, meaning that a PWAT of 2.6 to 2.8 inches does not immediately mean 2.6 to 2.8 inches of rain is expected.


Expected PWAT values Sunday afternoon, notice New England has around the same values as the deep south:



Once you get to the 1.5 to 1.75 inch range of PWAT, that is considered a moisture-rich atmosphere, where storms that develop can drop heavy rain. So, when it comes down to it, PWAT values at 2 inches or more can lead to a lot of rain being dropped in a short amount of time, which is just what's needed to produce flash flooding. The PWAT values are as high as they can get in New England. For reference, the highest PWAT value ever recorded in the area is 2.73 inches in Chatham, MA, according to the National Weather Service. The PWAT values forecast are approaching the maximum value for model runs in New England!



TIMING


It currently looks like there will be two waves of rain and thunderstorms. The first wave of showers and storms will likely develop and move into western areas early Sunday morning. This wave will slowly push eastward during the day. As it does, daytime heating will likely aid in the formation of more numerous thunderstorms. This first line will be rather slow moving, and storm training (when multiple storms move over the same area) will be possible.


HRRR guidance on expected weather at 9am Sunday. The first wave has fired up and moving eastward:


This first line, at present, is the bigger concern. Slow moving storms will be capable of producing rather extreme rainfall rates. With PWAT values where they are, torrential downpours may be fully tropical in nature, leading to 1-2+ inch an hour rates. The storms will be scattered in nature, so rainfall rates will not be the same throughout the day. Sometimes it will be a torrential downpour and other times it may just be drizzling.


The second line of storms will likely move into western New England Sunday afternoon. This line should, overall, move much quicker than the first line, however, due to the short "break" between lines, this line could exacerbate the flooding issues in western areas. I put break in quotes since it won't necessarily stop raining between lines, it's more likely that downpours will become more isolated to scattered rather than widespread between the waves.


Expected weather at 5pm Sunday. Storms will continue to be scattered around. You can also see the second wave in upstate New York, where the darker green is:



What's stated above is one of the reasons why the area of greatest concern has expanded westward to include much of western New England. This second line should begin to dissipate as it pushes eastward, especially as the sun sets and daytime heating is lost.


Overall, most of New England (except eastern Maine) can expect scattered to widespread downpours throughout the day Sunday. Again, it needs to be emphasized that the rainfall rates will not be consistent all day. The event will wind down for most of New England by midnight. Eastern Maine will be in it a bit longer as the rain is last to reach them (naturally). Rain will likely persist through Monday morning for eastern Maine, however, the flood threat is lower there as more of the event will take place after daytime heating is lost, which will likely limit storm growth.


Expected weather 8am Monday. Much of New England is dried out, eastern Maine is still in the rain:



RAIN TOTALS AND FLOODING


This event is extremely unlikely to be a repeat of last week's flooding event in Vermont. Catastrophic flooding and damage is not expected. Despite this, residents should be ready for potential road closures and flash flooding. At this time, flooding looks like it will be more localized than widespread, thanks to the scattered nature of downpours.


The area where numerous flash floods are possible has expanded to once again include areas closer to the coast and in some cases, all the way to the coast. This is thanks to the PWAT values trending higher as it now looks like thunderstorms will be able to stay organized all the way to the coast. This is especially true across Rockingham County, New Hampshire and York County, Maine. Coastlines in these counties are the most likely to see flash floods across New England's coast.




It is looking likely that most of New England will see 1-3 inches of rainfall total. Since the storms and downpours will be scattered in nature, rainfall amounts could vary quite a lot from location to location. There will likely be thunderstorms that produce extreme rainfall rates, so some areas could see upwards of 3-5+ inches. Where these thunderstorms end up setting up is uncertain. Right now, it looks like that could happen in the southern 2-3 inch area on the map below, but this won't be locked in until storms actually start forming tomorrow.



I will say that the areas with the greatest concern for flooding will be in areas with more terrain, such as the White and Maine mountains, as well as areas that have seen flooding rains recently. The entire region is over-saturated, which lowers the amount of rain needed to cause flooding problems. Local flash flood threshold guidance is as low as 1.5 inches in an hour and 2.5 inches in three hours. Rainfall rates that meet this threshold are likely at this time.


Infrastructure (roads, dams, embankments, etc) has already been stressed from weeks of flash flood issues, especially across northern New England. Each event is starting to compound on each other, and less and less rain is becoming needed to cause problems.


SEVERE STORMS


The possibility of severe thunderstorms is rather low, given a thick cloud cover will be around all day limiting instability. Despite this, there is a slight chance of strong to severe storms developing across the southern two thirds of the region. Damaging wind gusts would be the primary threat.


There is also a very small threat of an isolated, weak tornado or two. Like this past Thursday, there is a 2% chance of tornadoes occurring across southern New England (minus Cape Cod), southern New Hampshire and southwest Maine. This threat will likely be around all day, not just limited to the afternoon. Again, this is a small chance, but whenever the threat of tornadoes is above 0% in New England, it's worth bringing up.


Widespread severe thunderstorms will not happen, and the main threat will be flash flooding.


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