Monday, January 25, 2010

Measuring Snow in FEET Again???

OK. I'm hyping this one even though it is 4 days out, mainly because I missed the last big snow here back in December, and I want to see another bomb. Definitely going out on a limb following forecast models that are still 4 days out. For those who don't know much about weather forecast models, 4 days away is a long time and leaves a lot of room for error. That's why everybody always blames the weather-guy for a bad forecast- he's looking at model data to make it, and if it is wrong, usually the meteorologist is wrong. Basically, that's just my CYA statement in case I'm wrong. But it is important to understand that the system that will affect us in Charlottesville on Friday is just coming into the West Coast, and better data is obtained once a system is on land, therefore the closer we get, the more accurate the models become because they have more accurate surface and upper-air observations fed into their initializations which future-cast the weather.

With all that said, at this point I would say 75% chance of 6" of snow between Friday and Saturday. 50% chance of 1 foot. 25% we see 18". But percentages aside, right now I think we'll see approximately 1 foot of snow here in Cville. Lots of factors will affect accumulations, the important ones are how cold is it at the surface and where snow crystals form. It is probable that towards the heaviest part of the snow we'll start seeing ratios as high as 15:1. That means for every tenth of an inch of precip (.10) 1 1/2" of snow would fall. Typically it's around 10:1, so .10 = 1". The picture I posted is the latest operational run (18Z) of the GFS model which shows 1.25"-1.75" of liquid around our area. Doing the math using 1.5" of liquid means 15" of snow at 10:1 ratios and averaging to 13:1 for beginning to end equals 19.5" of snow with the same 1.5" of liquid. Again, this is subject to change, but not to the point of looking at green grass all weekend. We just might not be in this sweet spot. I expect to see 1" of liquid (QPF) generally, which is why I'm forecasting 1 foot. I'll update as we get closer. Stay tuned.