It's something we talk about often on TV, but sometimes don't have the time to thoroughly explain what's happening. It's called a "wedge" or "cold air damming". This setup can shoot down forecasted temperatures by 20° (as it nearly did), and block out the sun for days.
It happens when an area of high pressure is to our north/northeast, and cold air moves in from the northeast - typically a chilly day. As a warm front is on its way from the south, most Meteorologists anywhere else in the country would say a nice warm up is a day or two away.
Our trouble maker is the mountains to our west. The cold air is trapped, or "wedged, between the mountains and rising warm air ahead of the warm front.
The result is a cloudy, and MUCH colder day - especially over central and northeastern Virginia.
Then why did we still forecast 70°? Because the wedge CAN be broken, especially with a strong enough warm front. Forecast models have a difficult time picking up on C.A.D. (Cold Air Damming), but it's tough to ignore when they still have expected temps in the mid 70s. The clouds DID break apart yesterday, as we said, just not enough. Look at the temperatures around 3pm (Sat Jan 12th). The sun came out southwest of Richmond, but the skies just didn't clear enough in central VA.
We have a better chance of 'breaking the wedge' today, but there's still the possibility of a cloudy and much cooler day. Let's hope the sun comes out before cold air finds its way back into VA tomorrow night.
-Kevin
