I know the more urbanized parts of the metro (DC/Arlington/Alexandria) are not representative of most folks here, but the warning signs are there. I mean it basically doesn't get below 20F without some severe shot of Arctic air (like what is coming this weekend). Frankly without some reinforcing shot of cold air, it struggles to get into the upper 20s. For the most part, most of the winter (at least over the past few years), DC stays above freezing, even at night.