Although yesterday saw pretty good agreement among most computer model guidance regarding a major snowstorm on Monday, we were still four days away from the storm, which is a pretty long time in the realm of computer modeling. In more plain terms, this means that despite this agreement, we were still susceptible to changes in modeling that would ultimately affect the outcome.
The current weather pattern is dealing with a strong storm system crashing into southwest California, as well as a strong Polar Vortex in Southern Canada, supplying cold air into much of the country. This storm system in southwest California is forecast to move eastward, and as it does, will be increasing the warmth and moisture out ahead of it as it interacts with the Gulf of Mexico. Additionally, the Polar Vortex is forecast to exert its muscles and intensity the cold around it and just to its south. This leads to a strong source of anomalously cold air in the north, and anomalously warm air to the south — creating a strong temperature gradient which should lead to widespread, heavy precipitation just north of this gradient. This part seems easy.
But it’s much more complicated than that. The Polar Vortex in a way is a double-edged sword. While it can supply the cold air to strengthen the gradient, it can also be so strong that it shears out the initial storm system. If the initial storm system gets sheared out, then its ability to generate warm air advection out ahead of it and spread northward is decreased, which means the thermal gradient weakens and gets pushed to the south.
Let’s take a look at some computer models. Today’s 18z NAM showed a major snowstorm of 10-15″, whereas today’s 18z GFS only shows a few inches. Below, we will see the 18z NAM valid for 10:00pm, Saturday night.