Flash Flooding Likely to Be Significant for Some WesternCentral New England Communities Wednesday Evening | NECN
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Meteorologists' Observations on the Weather

Flash Flooding Likely to Be Significant for Some WesternCentral New England Communities Wednesday Evening

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    The map above was valid as of late Tuesday night for Wednesday. I expect new flash flood watches to be hoisted in the light green area during Wednesday.

    Although I don't expect Wednesday to start out with more than scattered showers - and I also expect those showers to contract west, allowing Eastern New England to dry out, and even see some sun, Wednesday afternoon, that western consolidation of rain will be problematic for others.

    For as bad as flash flooding was in some communities Tuesday, I expect worse in some of Western New England Wednesday. Although the exact same communities may not be impacted, Central/Western CT, Western MA, Vermont & Western NH will see localized flash flooding Wednesday.  The amount of water vapor in the atmosphere Wednesday afternoon will be nearly 3 standard deviations above normal - in other words, Western New England's sky will be totally saturated, so precipitation production will be extremely efficient, falling on already waterlogged ground.

    TECHNICAL: Meteorologically, we have a weak vorticity maximum that moves through Central and Eastern New England Wednesday morning, and that will be responsible for scattered showers in these areas Wednesday morning to midday.  By Wednesday afternoon, slight subsidence (sinking air) sets in behind that disturbance, so Eastern New England can break some sun out.  Meanwhile, a stronger disturbance will be crossing Upstate New York Wednesday afternoon, entering Western New England by late day and early evening.  This disturbance will encouter a combination of diffluent air aloft with anticyclonic shear, mid-level cyclonic vorticity advection, low level positive theta-e advection owing largely to a surge in 850mb dewpoints, and a baroclinic zone in the boundary layer associated with the retreated backdoor front. Together with air approximately 3 standard deviations above normal for precipitable water, very efficient precipitation production will result with downpours and thunder drifting east from New York into New England.  Near the New York State border and in far Western New England, some damaging downbursts are possible as the 850mb low level jet increases - just a bit farther east, specifically either side of the Connecticut River (though also in orographically favored areas in mountainous terrain, outside of that corridor), the low-level jet weakens as speed convergence is present in the low-levels, meaning flash flooding is by far the greatest concern.  This is especially bad news for the communities either side of the Connecticut River from Connecticut through Massachusetts, Vermont and New Hampshire that have been hit so hard.