A look at why Wednesday may be a "mix"
Posted February 10, 2014
You've probably noticed how we've had to treat parts of the developing winter storm as a bit of a moving target, one that may produce varying amounts of precipitation at varying times over the next 2-3 days as multiple upper level disturbances and surface waves of low pressure affect the region. Right now, it appears the best chance of wintry weather on Tuesday, including accumulating snow, will be over roughly the southern third to half of our area.
On Wednesday, a deeper upper level trough to our west and what appears to be a stronger surface low should sweep deeper moisture into more of the area, for a better chance of substantial precipitation, but that process may also pull some warmer air into the eastern and central parts of our area, not as much at the surface as a few thousand feet up, and how extensive that air becomes will play a huge role in how the weather turns out in any given location.
One of the tools we use in making our forecasts is a program called BUFKIT, which allows us to take an hour-by-hour look at how temperature, moisture, winds and more are projected to evolve from the surface up through around 30,000 feet above the ground. I've included a couple of images here showing two different model results for the same time period, Wednesday afternoon at 4 PM.
The first is from a model called the Global Forecast System. What you see are a red line that shows the temperature at many altitudes, a green line showing the dew point, and a line in between showing the wet bulb temperature. There is also a white line that shows us what altitudes the air is rising or sinking and how fast, along with arrows/flags along the right side of the image that indicate wind speed and direction, which can vary greatly with height. Finally, notice the slanted blue dashed lines. These indicate temperature, with the one labeled "0" (in Celsius) at the bottom showing the freezing line.
On the GFS model, the actual temperature is below freezing all the way through the atmosphere. When this is the case, snow is the most likely precipitation type, and this model indicates a good chance of that starting later Wed morning and continuing at least into the evening.
However, as a comparison, the second image here is from a model called the North American Mesoscale, or NAM. At the same time of day, notice the substantially warmer bulge of air in the region of strong winds blowing at least partially from the east. The horizontal dashed line marked "5" indicates 5000 feet off the ground, and the above freezing layer here extends from about 2,000 to 6,000 feet up. If this is how the atmosphere looks at that time, then snow aloft will melt in that layer and likely reach the ground as a liquid that then freezes as a glaze, or possibly refreeze just in time to reach the ground as ice pellets (sleet).
So far, a few other models support each of these outcomes depending on how strong low pressure is and how far it tracks east or west relative to our location, with small differences in track making a big difference in precipitation type. For now, we have to be ready to deal with either on Wednesday, and we'll be watching to see if one or the other comes to dominate in forecast projections over the next 36 hours.