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September 29, 2009

Big low over E. Canada sinks barometers here

Canadian low 

Weather watchers in Maryland noticed something odd on Monday. Their barometers were skidding to unusually low numbers, while the skies outside seemed strangely clear. Normally, when the barometer is very low, we're in the middle of a pretty impressive storm.

Fred Weiss noticed and shot me an email message from Baltimore: "What is causing this low pressure?" he asked.

I checked, and he was right. The official National Weather Service barometer out at BWI-Marshall Airport sank to 29.44 inches. Out on the WeatherDeck in Cockeysville, the console said 29.37 inches. And here at The Baltimore Sun, Calvert and Centre streets, the weather station read 29.45 inches.

Steve Zubrick, science and operations officer for the NWS at Sterling says the last time BWI saw a reading that low was back on April 7, 2009.

Anyone else make a note of their low reading for Monday?

For comparison, I checked the low barometer reading back on Sept. 19, 2003, when Tropical Storm Isabel blew through the region. The low reading at BWI that morning was 29.58 inches.  So, I checked back ever farther, to Hurricane Hazel, on Oct. 15, 1954. The low barometer that day was 28.93 inches.

So Monday's low reading was a bit lower than during Isabel, but not nearly as low as during Hazel's passage. 

So what was going on here Monday? And more specifically, why was the sun shining?

Our low barometer was caused by an intense low-pressure system that was - and still is - drifting across the Great Lakes and southeastern Canada. A trough of low pressure extends south and west from that storm center, which is what is keeping our barometric readings so low even today - Tuesday. The Sun's barometer has only rebounded to 29.69 inches at 11 a.m.

Here's a nifty satellite loop of the spinning low.

The low is rotatring counterclockwise. And on Monday morning, it was setting off torrential rains along the Great Lakes, where I was visiting my in-laws in Erie, Pa. It also touched off a band of intense showers along the I-95 corridor in the late afternoon at one point (where it whacked us again as we drove back into Baltimore.) We recorded a quick tenth of an inch here at The Sun.

The big low was also dragging a wedge of dry air in from the southwest, which gave Baltimore the sunny skies we saw for a time Monday. Meteorologists call that a "dry slot," and it's not unusual with these strong lows. You can see it in the satellite image above, now just off the East Coast.

Some clouds have moved in behind it as cooler, drier air is pulled down from the northwest around the backside of the low. 

The forecast calls for windy conditions today as the atmosphere continues to flow around the low to our north and east. But we should remain sunny, with a high near 70 degrees.

As cooler air moves in with high pressure builds into the region behind the low, we'll start hanging up in the mid- to upper-60s for the next few days.  We'll stay sunny until the next cold front approaches late Friday into Saturday. 

Posted by Frank Roylance at 10:48 AM | | Comments (2)
Categories: By the numbers


No self respecting weather buff would be caught dead without at least two of the following tools: anemometers, thermometers (indoor and outdoor), rain gauges, barometers, atomic clocks, weather maps of all kinds, including NASA and NOAA weather maps and even travel weather maps, hygrometers, psychrometers and any other weather instrument that they can get their hands on.

Sometimes the scale is adjacent to the glass on a brass plate attached to the mounting board. When the air pressure rises, the mercury in the glass tube of the barometer will also rise. Conversely, as the pressure goes down, so will the mercury.

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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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