Instead of a typical early February snowstorm, we are about to witness a catastrophic ice storm (well maybe not for Philly itself or the immediate suburbs). Locations in the vicinity of 40 – 50 miles North and West of Philadelphia will get the worst of this storm as a very severe cold air damming situation is setting up. The effect known as “the wedge” is the most widely known example of cold air damming. In this scenario, the southern storm system will bring warmer air with it above the surface (at around 1500 meters). This warmer air will ride over the cooler air at the surface (also known as “overrunning” precipitation), which is being held in place by the high pressure system to the north. This temperature profile will lead to the development of freezing rain or sleet, depending on the exact depths of the warm and cold layers. A thinner cold layer will lead to conditions favoring freezing rain, whereas a thicker or stronger cold layer will lead to sleet. Ice storms are extremely difficult to forecast, as the difference in less than 10 miles can be the difference between a much more manageable heavy sleet accumulation, as opposed to a very dangerous freezing rain accumulation. One of the known weaknesses and biases of the computer models is to underestimate the amount of low level cold air “locked-in” at the surface. For this reason, the computer models sometimes depict a warmer solution as compared to what actually occurs. Currently, the NAM and other short range models have shifted slightly colder in the past few runs, while the GFS is consistently warmer. I would think that the cold air hangs around slightly longer than the models indicate and allows for around a 0.25 inch freezing rain (ice) accumulation in and around the Philly metro area before a changeover to rain Wednesday morning. The Lehigh Valley will probably get the worst of this storm with localized 0.50+ inch ice accumulations. This will cause power lines to come down and make for virtually impossible driving conditions. As we all know from this winter, tomorrow morning may bring a completely different view of what will happen, but that is the fun of understanding and studying the weather!