Sunday, December 21, 2014

Christmas Potential High-Impact Winter Storm

Model guidance is still wreaking havoc on forecaster confidence, meaning this post will be laced with numerous caveats.

Instant Weather Maps
The image above shows the 12z GFS model forecast for snowfall totals from this Christmas storm. That thin swath of snow in northwest Indiana into western Michigan is the accumulation from this storm. That's a far cry from the projections of 6"+ earlier this week, even twenty-four hours ago.

This big change came about late last night, when the energy associated with this system finally crashed into North America, and the weather balloon network could take measurements of the system. This process, called 'sampling', as the weather balloons sample what the system is doing and how strong it is, gives weather models more confidence in a solution, as there are no weather balloon launches out in the Pacific.
At least, that's usually how things go. Late last night, when this first sampling took place, model guidance took a significant jump eastward and greatly weakened the overall storm, bringing about minimal snowfall accumulations. The theory among forecasters was that this would be the final solution.

But the drama appears far from over.

Instant Weather Maps
The above image shows the 18z GFS forecast for snow from this storm, the most recent forecast we have to work with. In this projection, the storm jumped west again, like model guidance was favoring about twenty-four hours ago. In this forecast, we see amounts generally of 3-6" draped across east Missouri into west and northern Illinois, with a sliver of southeast Wisconsin getting into the action.

Now's the time when I would give my personal analysis of what will happen here. But to be frank, I have no clue. This is one of the worst performances of model consistency I've seen in my five years of forecasting, if not the worst performance. I'm preparing to go about this storm on a now-casting basis, where the forecast is made as the storm actually happens.

For now, most of the population living in St. Louis on eastward will likely have to contend with travel difficulties, as this storm system still looks to be a strong one. Snow forecasts are useless at this juncture, as it's possible people could get a foot, or could get nothing. On top of that, location of this snow is nearly-impossible to predict at this point in time.