Here is the 850mb temperature plot from Thursday morning. It shows the temperatures (colors) and wind speeds around 1,500 meters or 5,000 feet up in the atmosphere. This chart is usually good for seeing areas of cold or warm air advection as well as areas where gusty winds may be present. The giant swirl in the atmosphere at this time over the Great Lakes is what has helped bring the cooler air and gusty winds to the D.C. area, but it is not often southwesterly winds will bring a colder air mass to the Mid Atlantic.
- 850 Temps Thursday AM
This low had a path which took it just to the west of the mountains, which placed the center to the northwest of D.C.
If the D.C. area is east of the low, there will be southwesterly winds over the region in response to the pressure gradient forces. Well the way this system set up, the cooler air finally wrapped around the low late last night as the surface frontal system pushed through the region. With nothing to push the main low to the east, it continued to move north over the Great Lakes, ensuring a southwesterly wind over the area and a cooler airmass as well.
Oddly enough, the opposite happened a few weeks ago when there was an area of high pressure to our west and northwesterly winds brought warmer air. In that situation the warmer air stretched all the way around the high from the Plains, to the Midwest and eventually to our area. Yes, I'm a weather nerd, and this is what interests me. It's really not something you'll see every day or even again this season. At least the clouds will depart by the weekend!