Localized downpour swamps Lutherville area
Reports are now trickling in from the torrential but extremely localized rain that fell yesterday afternoon in the Towson, Lutherville, Timonium area. One official NWS spotter in Towson reported 3.6 inches of rain in an hour.
The amateur CoCoRaHS network reported very little rain across the state yesterday. But what fell was significant and very narrowly focused. The high readings:
Cockeysville: .85 inch
Jacksonville: 0.67 inch
Long Green: 0.54 inch
Towson: 0.47 inch
We clocked 0.12 inch on the WeatherDeck in Cockeysville. Here at Calvert & Centre streets, we saw nothing. BWI saw nothing. The National Weather Service in Sterling has posted its Cooperative Observer measurements for yesterday. They have no observers in the Baltimore area, but do show some very heavy, very localized accumulations in Virginia and West Virginia.
But much more impressive than the numbers have been the eyewitness reports I've been receiving, like this:
"Dear Mr. Roylance,
On Wednesday evening (August 13th) 99% of the Baltimore area was rain-free. The exception was the Towson-Carney-Parkville area where a compact - but persistent - thundershower parked (or "trained") over the area for 2-3 hours. There was wave after wave of torrential rain and gusty winds. Busy intersections were awash and there was considerable tree damage - especially in the area of Goucher Boulevard and Joppa Road. Several times the skies brightened and it appeared the storm was moving out of the area but it drifted back as strong as ever. My question - what atmospheric conditions can cause an isolated storm to behave that way? Thanks! - Ted Lingelbach, Parkville"
You can read some other rain reports in the reader comments I posted here late yesterday. Here's a private station in Towson with more than an inch. And here's one in Timonium with nearly 4 inches in the can.
I called Brandon Peloquin, a NWS meteorologist at Sterling. He said the problem yesterday afternoon was the lack of wind to push the thunderstorms along. So, with plenty of moisture in the atmosphere, and strong sunshine to heat it up, convection began to carry the warm, moist air high into the atmosphere, where it cooled, condensed, and began to rain out.
"It was very persistent and localized - one storm that just sat there and did not move," he said. "The flow in the atmosphere was very weak, and storms that didn't have any wind to push them along." After one storm peters out, "something redevelops right over the top of what was there before."
One NWS spotter in Towson reported 3.6 inches of rain, Peloquin said. The weather service did issue a flash flood warning for an area east of Towson because of the heavy rain. Some basements were reportedly flooded, but there were no official reports of street flooding or damage. Only a few miles in any direction, it was "an entirely different ballgame." Streets were dry, or nearly so.
"It will be different today," he promised. Although showers and thunderstorms are in the forecast, but there will be "more push" to the atmosphere. Storms may be locally heavy, but they will be moving toward the east and should not persist as long in one place.
