From the ABC 7 Weather team

D.C. fog, crazy regional temperature spreads: what’s the deal?

January 13, 2013 - 08:03 AM
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If you look at the temperature plots on ABC7 Live Super Doppler, you’ll see a rather uncommon trend for mid-January. Why is it warmer in the mountains and cooler in D.C.? What explains the weekend fog?

We’ve had an unusual stretch of weather this weekend with warmer temperatures in the Blue Ridge and Allegheny Mountains and cooler weather in the valleys. So, what’s going on with these wild winter temperature swings and why all this fog?

The temperature and moisture profile in our atmosphere is where we need to start. Below is what the atmosphere above us (Dulles International) looks like. There are a few key features that explain the bazaar temperatures and the fog. Notice the area circled on the sounding below. Notice how the lines bend to the right as you go up from the bottom of the sounding. This indicates warming temperatures aloft (or higher in elevation).

 

sounding

This is further justified by the raw data (this image below). The left column is the height in meters above the surface followed by the temperature in Celsius (next column over). The surface temperature at Dulles International at 7 a.m. this morning (when this data was observed) was 44 degrees but at 628 meters (2,067 feet) above Dulles, the temperature warmed to 14.20 Celsius or 58 degrees Fahrenheit.

sounding

Therefore, this explains the temperature map from 7 a.m. Sunday. Temperatures were in the middle 40s in the D.C. area but in the middle 50s in Elkins, W.Va. (circled) at an elevation of 1,979 feet.

sounding

This also explains the warmer temperatures seen at Shenandoah National Park (closer to the nation’s capital but at a much higher elevation as well).

sounding

Why the fog? In the sectioned off sounding below, notice how the lines are together near the surface. A warm front is slowly pushing north, bringing a steady increase in moisture and temperatures on the heels of southwest winds aloft. However, since dew points (temperature at which dew or water droplets form) are “catching up” with temperatures because of the typical nocturnal cooling trend, the air has become saturated with water. Where the lines on the sounding meet there is saturation or near 100% humidity; where they are separated, drier air exists.

sounding

Fog needs to be mixed out; there has to be a mechanism to move the air around so it can dry. Since warmer air rises and cooler air sinks and the warmer layer is above the cooler layer, winds are calm and can’t mix through the lower atmosphere. It requires a higher angle sun to “burn off” the low-level moisture. Since the sun angle is lower in January than August, the fog will linger longer than you would see during a foggy late-August or September morning.
Eventually enough moisture will evaporate to allow “breaks” to develop in the fog and overcast skies so that temperatures will warm into the 50s today inside the Capital Beltway.

sounding

As the radar and satellite image above shows, a cold front is knocking on our door. The front will help “mix” the atmosphere after a foggy early morning Monday and drive in cooler temperatures on the heels of northwest winds. Of course, the north to south band of rain seen across the Midwest and Tennessee Valley will also come at the expense of the front, so windshield wipers will be working Monday.

Be sure to stay with ABC7 and WTOP for the latest forecast.

 

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