Why So Much Rain in the East?

October 1, 2010; 3:00 PM ET
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Tropical systems often bring heavy rain, but why was the rain so intense over such a big area for so long?

It literally poured for five days in parts of North Carolina and southeastern Virginia, leaving some places with rainfall of 18 to 24 inches, or half of their annual average rainfall.

What first set the atmosphere on a path to the deluge was a routine dip in the jet stream over the lower Mississippi Valley last weekend.

While this dip brought an end to the relentless summer heat over the southern Plains and Deep South on its western side, it also began to funnel moisture northward on its eastern side from the Gulf of Mexico, the Atlantic Ocean, and the Caribbean.

A non-tropical storm formed just to the east of this dip in the jet stream and brought much of the Atlantic Seaboard its first dose of rain early in the week.

However, the jet stream failed to drift eastward, like it typically does in the wake of a storm. This caused a surface cool front to stall along the Atlantic Coast, prolonging rain in some areas.

To top things off, a second non-tropical low pressure area formed along the front at midweek.

Next, along came Nicole in the Caribbean, which had plenty of moisture left over to work with in the wake of Matthew.

Since the door remained opened for all of that moisture to come northward, along with Nicole, the stage was set for flooding in the East.

As Nicole moved northward it interacted with the stalled front, so you not only had Nicole's moisture, but all of the locked-up energy from the front and the slew of tropical moisture already in place in the southwestern Atlantic basin.

The lingering front helped to focus the rain from Nicole in a narrow north-south band. You were either in the band or you were not. Forecasting exactly where that band would set up and if it would shift to the east or west was the forecasting challenge.

Winds at the surface and aloft kept funneling moisture northward from deep in the tropics.

Since this band did not shift until the very end of Thursday night, rainfall of close to a half a foot was produced as far north as the southern tier of New York state.

Since the jet stream finally swung eastward, it allowed the rain to shift eastward through New England Friday.

A word of caution, the tropics in the Caribbean region will continue to cook up systems in coming weeks.

The jet stream could take additional dips over the middle of the nation, allowing more opportunities for that moisture and new systems to come northward during "Troptober."

The onset of chilly weather in the Northeast and less humid conditions in the South will not necessarily mean an end to that threat.

We no longer have drought conditions to negate excessive rainfall like we just had. Therefore, even an event with half of the rain that just fell could potentially cause even bigger problems.

Related to the Story:

2010 Hurricane Center

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