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September 23, 2011

In for another soaking

Looks like Central Maryland will be in the chute again as stalled weather systems to our east and west conspire to channel a few more days of tropical moisture, showers, rain and thunderstorms this way.

With nearly 20 inches of rain in the bucket since the beginning of August, forecasters are saying we should prepare for as much as another 3 to 6 inches in the next few days. And the I-95 corridor could see the worst of it, if you can believe some of the forecast models.

"The model consensus for the axis of heavy rain fall is near the I-95 corridor, with west to east variability of this axis ranging from just east of the Blue Ridge to the Chesapeake Bay. Rainfall totals of 2 to 3 inches with isolated amounts of 4 to 5 inches are possible within this band, which is AccuWeather.comenough to produce flash flooding," said National Weather Service forecasters in Sterling, Va., in this morning's forecast discussion.

Flash Flood Watches are in effect for the entire Western Shore, from Frederick County east to the bay and south to the Potomac. "The best chance for the heaviest rain will be near and east of the I-95 corridor," the watch said.

UPDATE, 11:30 a.m.: A Flash Flood Warning was issued for portions fo Frederick and western Carroll counties as heavy rains crossed the area with rain rates of up to an inch an hour.

Eric the Red, a professional meteorologist in Baltimore and frequent contributor here, said this rain event won't compare with that from remnants of Tropical Storm Lee earlier this month, but he had this analysis to offer:

"Stalled upper-air low to our west and a stationary high to our east puts us between two spinning pinwheels, pulling all sorts of tropical moisture into our area. The problem with this forecast is there is no easy-to-identify trigger mechanism (no stalled front or tropical storm) ... so we're left with a ton of moisture, high rainfall potential, but nothing to grasp in terms of where this rain will fall. This is readily apparent in the models, which all have heavy rain falling, but they are all over the place as to where."

Wherever is falls, he said, "This rain will come down in torrents ... so be prepared to deal with street flooding and all the other fun that comes with this. Throw in the 10-15" of rain that has fallen over the past 2-3 weeks, and you can see why we're under a Flash Flood Watch."

Here's's take on what's ahead.

For the record, August ended with 10.38 inches of rain at BWI-Marshall Airport, the fifth-wettest on record for the city. Through midnight last night, the airport had received 9.57 inches. Three more inches this weekend would make this the wettest September on record for Baltimore, beating the 12.41 inches in 1934. 

We've had more than a foot of surplus (above average) rain since Aug. 1.

Posted by Frank Roylance at 10:32 AM | | Comments (5)
Categories: Flooding, Forecasts


What's the average monthly rainfalls for a comparison to the actual totals in August and September?

FR REPLIES: The 30-year average for August at BWI is 3.29 inches. The average for September is 4.03 inches. That's a total of 7.32 inches. As of midnight last night, we've had 19.98 inches, or 12.66 inches above the average. With more on the way.

auto-insert stock man-made global warming blame

FR REPLIES: That's you raising the issue, Alex, not me.

How thick or what is the minimum height a could has to be before it produces rain?

FR REPLIES: It's not about height or thickness. It's about moisture content and temperature. When invisible water vapor in the air is cooled to its dew point, it condenses into tiny liquid droplets, which we see as clouds, or fog. If there is enough moisture, the droplets will combine until they grow large enough to fall as rain. If the air is dry, they may evporate again on the way down without reaching the surface. That's called "virga."

For the record, I was being facetious and satirizing the propensity of some folks to reflexively attribute every weather anomaly to global warming.

Some of us are old enough to have heard about (or even remember) an era in the 1950s when "those atomic bomb tests" in the Nevada desert and the Soviet Union was "blamed" (seriously or facetiously) for every anomaly, from a hot day to a nasty sunburn to a blizzard to the car not starting to radio static to the dog having puppies........

And those 1950s atomic bomb tests have what to do with the wet weather we're having?

Oh, and (if you're going to be bringing up the atomic bomb blasts) don't forget the sonic booms that were also common in the 1950s. They provided a lot of sonic commotion, too, and many people blamed them for causing sudden downbursts of precipitation, and/or droughts. Not that they did, but many people thought they did.

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About Frank Roylance
This site is the Maryland Weather archive. The current Maryland Weather blog can be found here.
Frank Roylance is a reporter for The Baltimore Sun. He came to Baltimore from New Bedford, Mass. in 1980 to join the old Evening Sun. He moved to the morning Sun when the papers merged in 1992, and has spent most of his time since covering science, including astronomy and the weather. One of The Baltimore Sun's first online Web logs, the Weather Blog debuted in October 2004. In June 2006 Frank also began writing comments on local weather and stargazing for The Baltimore Sun's print Weather Page. Frank also answers readers’ weather queries for the newspaper and the blog. Frank Roylance retired in October 2011. Maryland Weather is now being updated by members of The Baltimore Sun staff

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