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Posted at 11:00 AM ET, 09/06/2011

Rainy week: what’s the cause and how much?

Doppler estimated rainfall totals from Monday night. (National Weather Service)
The rain poured down fast and furiously last night producing 1-2” of rain across the immediate metro region, and up to 2-4” to the west and southwest. The rain, which prompted several flash flood warnings, was just the first of several waves of rainfall likely over the next few days.

By the end of the work week, cumulative rainfall totals may reach 3-6” in the immediate metro region (slightly less east of I-95), with 4-8” towards the mountains. Within both of these areas, locally higher amounts cannot be ruled out. A flood watch is posted through Wednesday afternoon and may need to be extended.

What’s causing this super soaker scenario?

NOAA’s Hydrometeorological Prediction Center (HPC) describes the overall setup nicely:

Rainfall projected by NOAA’s HPC through Friday. (NOAA)
The attendant frontal zone associated with [the remnants of tropical storm Lee] will be stationary across the southern mid-Atlantic and southern Appalachians through Tuesday before then lifting back north as a warm front on Wednesday. abundance of tropical moisture will continue to stream northward up across the mid-Atlantic and southeast coastal plain while then wrapping back westward into central Appalachians...Ohio Valley and also portions of interior New England. The result is expected to very heavy rainfall over the higher terrain...and more specifically portions of western Virginia...West Virginia...Western Maryland...and central and western Pennsylvania through early Thursday. These rains may cause life-threatening flash floods and mudslides.

Lee and its remnants have already produced record rainfall in parts of the deep South and Tennessee Valley. Meteorologist Jeff Masters at Wunderground reports Jackson, Mississippi received 11.68” in a 24 hour period yesterday, which is that city’s heaviest 24-hour rainfall on record, and Chattanooga, Tennessee also set its record for the wettest 24-hour period in its history, with 9.85” falling yesterday.

As the above HPC discussion notes, the axis of the heaviest rainfall is likely to be located west of the immediate metro region. Whereas the rain towards the mountains and into central Pennsylvania may seldom ease, our rain will tend to come in waves, with breaks in between where showers will be more isolated and the sun may even peek out at times.

Weather map showing the near stationary position of the remnants of tropical storm Lee through Thursday, when hurricane Katia, shown off the NC coast, is blocking it from moving away.

Why is the rain sticking around so long? Interestingly, it’s related to the interplay between Lee’s remnants and Hurricane Katia. Our local National Weather Service Office in Sterling, Va. explained last night:

Lee remnants won’t be leaving the area after midweek in part due to the movement of Katia off the NC coast...which may actually cause remnant low to retrograde and keep mid atlantic rainy (!) through the end of the work week. My memory bank can`t recall a situation like this.

Katia, while forecast to remain offshore, is acting like a block - essentially backing up the flow in our atmospheric river.

In case you’re not tired of rain by the end of this week, AccuWeather is advertising additional opportunities for heavy rainfall into the middle part of September:

Current indications are that at least two more systems have a shot at wandering close enough to the east coast of North America to bring heavy rainfall to troubled areas dealing with flooding.

Hurricane and Tropical Weather Coordinator Dan Kottlowski states that a new area in the southwestern Gulf of Mexico and the latest tropical wave to roll off the coast of Africa are the concerns.

“One or both of the systems has a window of opportunity to reach the Eastern U.S.,” Kottlowski said.

It’s quite premature to say anything about specifics, but I agree with AccuWeather in the sense that the pattern overall supports wetter than average weather beyond this week. Stay with the Capital Weather Gang as we will keep you informed on any new tropical systems and their potential to impact the region.

By  |  11:00 AM ET, 09/06/2011

Categories:  Latest, Floods, Local Climate, Tropical Weather

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