More rain tomorrow – NBC Boston
Monday was a particularly bad day for Leominster, Gardner and Pepperell, as central Massachusetts experienced catastrophic flooding.
The American Meteorological Society’s meteorological glossary defines flash floods as moderate to severe rainfall on highly saturated or impervious ground surfaces, generally occurring within minutes to several hours of rainfall.
Heavy rains lead to a rare flash flood emergency
These floods were sufficient to close main roads, sweep homes from their foundations, and collapse roads. This resulted in a local state of emergency being declared for the City of Leominster. Six to nine inches of rain fell in northeastern Worcester County on Monday, triggering a rare flash flood emergency from the National Weather Service.
You’re probably familiar with flash flood watches and warnings. Watching for flash floods means you should be prepared. Conditions are favorable for flooding on that day. A flash flood warning means that a threat is imminent or occurring. At this point, if you are in a flood-prone area, you should get to higher ground immediately.
When this threat is of greatest concern, a flash flood emergency is issued. It is issued in extremely rare cases when extremely heavy rain results in a severe threat to human life, and Catastrophic damage From a sudden flood.
Before Monday, the last time such a warning was issued for the NWS Boston forecast area was 1,168 days ago, in late June of 2020.
These events are catastrophic, in part because of the speed with which they can occur. Flash floods are floods that begin within 6 hours, often 3 hours, of the onset of heavy rainfall. The intensity of rainfall, location and distribution of rainfall, interaction with the ground, vegetation, and soil density all play a role in how quickly flash floods occur.
Why does Leominster get so much rain?
A combination of a stalled boundary, a tropical air mass (high humidity) and bits of energy swirling over southern New England caused locally heavy rain over just one area. There was so much moisture in the air that it was all compressed into a specific place: think of it like a big, plump sponge sitting on top of us and being squeezed.
More rain is likely on Wednesday
Although the heavy rains have ended for now, and no new floods have occurred, washed-out roads and flood areas still remain. Although Tuesday is a relatively calmer weather day with some scattered showers here and there, some renewed flooding is likely on Wednesday, as another disturbance approaches the area.
The peak flood threat will be Wednesday afternoon and evening. Most communities will see rainfall totals of less than a half inch, but localized amounts of around an inch will be possible. An isolated severe thunderstorm is also possible, so it’s definitely a weather awareness day. Not everyone will see flooding, but with this record wet stretch and the ground already saturated, it won’t take much for additional flooding to occur.
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