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Freeze tonight, cool week ahead

Cool, dry Canadian air rushing into the region with building high pressure will keep us 5 to 10 degrees below the seasonal norms early this week. As the high moves off at mid-week, a coastal storm could bring us some rain.

And when the northwest winds resume behind that storm, Maryland's far-western counties could see some SNOW showers at higher elevations. (There, I've mentioned the S word for the first time

this season.)

The overnight lows at BWI-Marshall didn't quite make it to the freezing mark this weekend. The lows Saturday and Sunday were 35 and 36 degrees. The airport thermometer slipped to 33 degrees this morning. We were right around the freezing mark on the WeatherDeck, and I had to pull the ice scraper out of the trunk and clear the windshield before I could head out this morning - another first for this season. 

But it should get colder tonight. Forecasters at the National Weather Service office in Sterling, Va. have posted Freeze Watches again (blue on the map) for all of Central Maryland, from Washington County east to Cecil, south to Charles, Prince George's and Anne Arundel counties, as well as the northern portion of Maryland's Eastern Shore. 

UPDATE, 3:30 p.m.: Much of the area north and west of I-95, (and on the northern portion of the Eastern Shore) which had been under a Freeze Watch tonight, has been upgraded to a Freeze Warning (light blue on the map). That means freezing temperatures are now expected. The forecast calls for lows in the upper 20s to near 30 in that area. Earlier post resumes below.

The overnight lows are forecast to reach the lower 30s in Baltimore and Annapolis, southern Baltimore County away from the bay shore, Prince George's and Arundel. But they could touch 30

degrees in parts of Northern Baltimore, Howard, Harford and Montgomery Counties, and the upper 20s farther west, in Carroll, Frederick and Washington counties. The lows on the upper Eastern Shore will approach the freezing mark, forecasters said.

A Freeze Watch means sub-freezing temperatures are possible and could kill crops and other sensitive plants.

By Wednesday, as the high moves to the southeast, we'll pick up the return flow. That will mean slightly milder temperatures, in the upper 50s at BWI-Marshall, and increasing moisture. A coastal low is forecast to develop late Wednesday into Thursday, with the possibility of rain spreading across the region. Subtropical moisture from the Gulf of Mexico could drive up rain totals, but most of the precipitation is expected to stay offshore.

By late in the week, we should be looking at more cool, Canadian air arriving on northwest winds "with the potential for upslope snow showers possible Thursday night through Friday night along and west of the Allegheny Front," forecasters said.

(SUN PHOTO: Frank Roylance 11/1/10)

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