From the ABC 7 Weather team

First Frost D.C. : When Will the Growing Season End?

September 20, 2012 - 05:00 AM
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Autumn is almost here . . .is the first freeze coming?

Say it isn’t so… just at the very beginning of the month temperatures were in the middle 90s and now much more refreshing lower 70s are coming down the pike for the latter half of the weekend. The days are getting shorter by the day and the jet stream is dipping farther south as the sun gets lower in the sky. This all equals one thing: Fall!

The big question now before we race into winter is……how much longer will the flowers be in bloom and the grass stay green before the big killing frost hits?

To answer that question, let’s look at what past weather records show for the first frost.

localfrost

Note that the climatological first frost is now a month away in the District but coming up in the next two weeks for the Shenandoah Valley. In western Maryland, temperatures are just about cool enough for a light frost this morning! So, we’re not too far off from Oakland, MD’s first frost date. Oakland, though, is situated at about 2,000 feet where the air is thinner and loses heat more readily at night than urban corridor centers.

Now take a look at typical first frost dates around the Mid-Atlantic and Northeast. Pittsburgh averages its first frost in late September but New York City and Philadelphia have to wait until the end of October, usually.

regionalfrost

Philadelphia’s urban heat island affect tends to moderate temperatures here while the slowly cooling Atlantic waters influence the frost date in New York City and especially Boston. Even though Boston is so far north, it’s location near the Atlantic Ocean influences temperatures inland.

Now let’s come back home to the nation’s capital. A big factor in the first frost date inside the Capital Beltway is the Potomac River. Temperatures tend to be warmer around water bodies in the fall because rivers, lakes, etc., are slower to cool off due to the heat trapped inside the tightly packed water molecules that influences the lower atmosphere around them. The opposite happens in the spring; areas near rivers and lakes are usually cooler because the water warms up slower after coming out of the colder, darker winter days.

Take the temperature at Reagan National for instance. This is the official weather observing site for Washington. Note the water temperature at the nearest gauge at Little Falls; 73 degrees! This has an impact on the minimum temperatures at the airport, which is right beside the river.

watertemp

Take for instance the average low temperature at the end of September at Reagan National versus that of Dulles International, which is just 23 miles to the west. Reagan National’s average low right now is 61 degrees while Dulles’s is 55 degrees; that’s a 6 degree spread for an AVERAGE low temperature in 23 miles (not even the distance of a full marathon!).

It’s a safe bet to say you won’t have to be taking potted plants inside anytime within the next four weeks inside the Capital Beltway but the growing season isn’t too far off for the Blue Ridge and Shenandoah Valley regions! Stay with ABC7 and WTOP Radio for the latest forecasts!

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