The freeze warnings for the DC Metro area are highlighted in light blue on this map provided by the National Weather Service.
A Freeze Warning is in effect for the entire DC Metro area east of the Beltway tonight into Saturday morning. This is because it would be the first time, for many locations, for the temperature to dip below freezing if it happens tonight. As you can see in the picture above, even the District and the counties surrounding the Chesapeake are included in the Warning.
If the temperature drops low enough for frost to form in DC tonight, it will be earlier than usual, but not the earliest by a stretch. The earliest first frost, as our Chief Meteorologist Topper Shutt mentioned in his blog post, was on October 10th. On average, the first frost happens on November 7th. So if it happens tonight, it will be about 2 weeks earlier than "normal".
The weather tonight is creating almost ideal conditions for the temperature to take a big drop.
The first element: a clear sky. Clouds will hold onto the heat we got during the day, and they even radiate some of that heat back down toward the ground during the overnight hours. This is why people will sometimes say that the "clouds act like a blanket" to keep in the day's warmth.
The second element: calm winds. Breezy conditions prevent the day's heat from exiting straight up into the higher atmosphere. The wind stirs the air, preventing any part of it from getting much warmer or much colder than another part. So, a lack of wind allows the day's heat to shoot straight upward and away from the ground.
The third element: low humidity. Water holds onto heat much more efficiently than air. That's why the towns that are closest to the Chesapeake tend to have later frost and freeze dates than inland locations. The humidity in the air has this same insulating effect. So, on a night like tonight, with a low amount of water vapor (low humidity) in the air, the temperature can drop more quickly than if there is more moisture present.
A Twitter follower asked me if we might see flurries tonight. Based on what you've just read, can you figure out why the answer is "no"? It's because we're going to have clear skies and low amounts of moisture all night. Since it's only October, I would venture to guess that if the humidity had been higher today, our temperatures wouldn't get nearly cold enough for snow. The time is not yet right for the elements of snowmaking to come together. We'll probably have to wait at least a couple more months for that!