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Major Flash Flood Event Possible Sunday in New England


There is an increasing concern for a potential major flash flooding event on Sunday. The atmosphere remains extremely moisture rich through the weekend. Precipitable water (PWAT) values may approach 2 to 2.3 inches over a widespread area of New England. 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 to 2.3 inches does not immediately mean 2 to 2.3 inches of rain is expected.


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 just about as high as they can get in New England. In fact, the National Weather Service of Gray, Maine has stated that PWAT values could near record levels.




This, combined with a low pressure system moving across the region and the fact that the ground is already over-saturated is leading to growing concern over a potentially significant weather event. This post is not meant to sound drastic or dramatic and a full forecast will be coming tomorrow morning, so stay tuned as this threat could increase or decrease by then. This post is just to let everyone know about a potential weather event.


The Weather Prediction Center has a large swath of New England in the "moderate" category for excessive rainfall for Sunday. Moderate is the second highest level and means numerous flash floods are possible. New England is rarely placed in this category on day one, so the fact that the region was placed in this category on the morning of day three shows confidence in a "major slug" of rain.


Weather Prediction Center excessive rain outlook for Sunday:



Right now, it looks like 1-3 inch rain totals are in the cards with localized higher amounts IF storm training develops. Storm training occurs when multiple storms move over the same area over and over again. Whether or not storm training occurs will likely determine how significant flooding will be. Flood watches will very likely be posted sometime in the next 12-18 hours should everything detailed here hold up (and it looks like it will).

At this time, this event does NOT appear to be a repeat of the catastrophic flooding that struck Vermont last week. Significant, possibly. Catastrophic, no. Nine inches of rain is very unlikely anywhere. The main area of concern, as seen above, is further east than last week.



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