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Weekend Downpours Continue Flash Flood Concerns for New England

New England's unsettled pattern has evolved into a bit of a hyperactive pattern. After flash flooding occurred last Friday (7/7), Sunday-Monday (7/9-7/10) and yesterday (7/13), heavy downpours are in the forecast throughout this weekend for parts of New England. The threat of flash flooding is beginning to shift further east.


FRIDAY


A cold front that worked into New England yesterday and touched off a potent line of severe thunderstorms has stalled across the region. This will allow plumes of moisture to ride along the nearly-stationary front. With an extremely moisture rich, tropical-like atmosphere remaining in place, heavy downpours will be possible, with localized rainfall amounts of 1-3 inches possible.




As has been the case for a while now, coverage will be scattered in nature, so some areas will be spared much of the rain while other areas get hammered. For this afternoon and evening, it is currently looking like western Massachusetts, southern Vermont and northwest Connecticut will see the most widespread activity. There is much more uncertainty than there should be for the day of the event on just how widespread the coverage of storms and downpours will be. Despite this, it is highly unlikely that storms will be as widespread (or intense) as they were yesterday


What we know is that downpours will break out this afternoon, mainly in western areas early in the afternoon, and begin to spread eastward as the afternoon wears on. Scattered storms and downpours will continue on and off for most of the region throughout the evening into at least the start of the overnight. Storms should slowly begin to dwindle as the night wears on.


Scattered showers and storms breaking out around mid-afternoon:


The areas with the least likelihood of seeing storms and downpours are downeast and the midcoast of Maine.



SATURDAY


Saturday will give New England a one day "break" from the flooding concerns. I put break in quotes since the day won't be totally dry. Much of the region will remain dry for most of the day, but isolated to scattered showers will likely pop up in places during the afternoon. The Weather Prediction Center does have New England (minus Maine) in the marginal category for excessive rainfall, but this overall threat is very low for the day. This broad designation is to cover a very isolated chance of a flooding issue arising. The day will be warm with tropical-like humidity remaining in place.


GFS showing very isolated showers Saturday afternoon across New England:



SUNDAY


There is an increasing concern for potential flash flooding on Sunday. The atmosphere remains extremely moisture rich through the weekend. Precipitable water (PWAT) values may approach 2 to 2.5 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 2.5 inches does not immediately mean 2 to 2.5 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. In fact, the National Weather Service states that the all-time record highest PWAT value in Chatham, MA is 2.73 inches. Combine this with the fact that the region is already over-saturated and you have a decent concern for flooding at this time.


Whether or not all the pieces of Sunday's puzzle line up to create a widespread flash flooding event remain to be seen (PWAT values are just one piece of the puzzle, remember). At this time, the Weather Prediction Center has portions of New England in the moderate range for excessive rainfall on Sunday (which is the second highest level). Where exactly the heaviest rain sets up is still a bit of a question mark as is just how widespread rainfall will be, so stay tuned for future forecasts.


BEYOND


This ridiculously stubborn (for summer) blocked up pattern is not breaking down after this weekend with unsettled weather looking to continue into next week.


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