Posted: Jan 10, 2013 9:43 AM by Meteorologist Matt Elwell
Updated: Jan 10, 2013 10:00 AM
Winter storms can be tough to predict in Montana. We've been tracking this storm since last week in the STORMTracker Weather Center, but exact amounts on snowfall are difficult for several reasons. One of the biggest factors that we run into is depth of the cold air in the atmosphere and timing will impact the moisture to snow ratio dramatically.
Typical winter storms in most of the country will run at an 11:1 ratio, which means for every inch of liquid that falls we would see 11 inches of snow. Montana often runs closer to 20:1 ratios, or 20 inches of snow for every inch of rain. In Bozeman, the models are showing an average of 0.60 inches of moisture (if we were to melt the snow down to liquid form) for the storm. At the 11:1 ratio the range on the computer models is setting at 4-6 inches total snow accumulation. At 20:1 we are more like 4-8 inches of snow. Butte has a little different spread. At 11:1 it is more 3-8 inches and 20:1 it is 3 to 10 inches.
That is a lot of numbers, I know. The point is that the spread on this storm is pretty high. Some forecasters have had the area in the 8-12 inch range which looks pretty high for Southwest Montana. A couple of other impacts are evident from the water vapor imagery. I see some dry air sliding into southwest Montana around a developing low along the Wyoming, Idaho border. This is what is known as ‘dry-slotting' meaning that we get some dry air sneaking in and breaking up some of the snow. I believe that this will move out and we will still see a good snowfall, but not on the 8-12 inch side unless we are talking about the mountains.
At this point, my forecast is calling for 3-6 inches for most of the area for Thursday and Thursday evening, with another 1-3 possible for Friday. With the colder air moving across the area, we will see some overrunning of snow over the cold front (which typically means great conditions for snow) but the cold air behind this front is pretty deep which will limit our moisture to snow ratio back to around 15:1 for the vast majority of the storm.
We will get you more information when the newest data is available.