Cross-Polar Flow to Bring Historically Cold Valentine's Weekend!Whenever weather models start throwing out extreme data, such as record challenging cold temperatures, the first question that a meteorologist should ask is, is this a realistic solution? During the process of determining if the model’s “credibility” a good meteorologist should be able to discover the process or processes that could (or could not) lead to such an extreme outcome. In the case of the cold that is currently forecasted for this weekend, it all comes down to the path the jet stream winds eventually carve out. Let’s take a look at the projected jet stream map (300 mb winds) of the Northern Hemisphere for Saturday (Valentine’s Day) according to the latest run of the Euro.  To most folks, a map like the one above probably looks more like a bowl of spaghetti than an upper level wind chart. But those colored “ribbons” here show where the upper level winds (jet stream) are projected to be this weekend. One key to understanding what you are looking is to visualize the jet stream as a “river of air” that travels around the globe separating air masses of different densities and, most importantly, of different temperatures. In the Northern Hemisphere, the jet stream winds flow from west to east around the globe with warmer temperatures (lower densities) on the right side and colder temperatures on the left (higher densities). Embedded in the core of these large scale swirling winds is the heart of winter’s arctic cold. As we’ve heard about so much in the media the last couple of winters, the “Polar Vortex” lives above the heart of that cold. In this case, and as is typical for this time of the year, the polar vortex has split into two distinct lobes with one located near Siberia and the other over Northern Canada. Now look closely at the flow of air (jet stream winds) around these two lobes of low pressure (Polar Vortices). More