The challenges faced by these elfin forecasters are immense. Anyone who has been on a long trans-Atlantic or trans-Pacific flight knows the value of a strong tailwind and the delays posed by strong headwinds. Santa needs to take advantage of the former and avoid the latter as much as possible. Turbulence? If you think it is bad on an airplane, imagine traveling at high speed on an oddly shaped sleigh loaded with gifts, many of which are fragile. Visibility? The addition of Rudolf has reduced, but not eliminated, weather delays due to clouds, fog, and snow.
For short-range forecasting in the mountain west of the United States, the elves naturally favor the NCAR ensemble and many of the specialized products we produce on weather.utah.edu. However, for medium-range forecasting, which is critical today as Christmas Eve still a few days off, the European Center for Medium-Range Weather Forecasting (ECMWF) is now providing forecasts from the deterministic and ensemble forecast systems at no charge to Santa.
Unfortunately, the Grinch has intercepted those forecasts, preventing their broad distribution from Santa to the free world. Thus, we are stuck with the GFS and NAEFS. While certainly not a lump of coal in the stocking, it is a bit like receiving a Hershey bar instead of Swiss chocolate. Nevertheless, we shall plow forward.
Christmas Eve and Day look to be very exciting for northern Utah not just for kiddies, but also meteorologists. This morning's GFS run shows a deep trough sweeping through the western U.S at 1800 UTC (11 AM MST) Christmas Eve with a cold front moving through northern Utah.
By 0600 UTC (11 PM MST) Christmas Eve, the front is through and colder air is ushering into northern Utah. If this forecast were to verify (key word: if), it would be a White Christmas along the Wasatch Front.
Of course, the elves know there are two major problems with relying on the GFS at such long lead times. First, it is a single model run, and if we look at other modeling systems and ensembles, there are some important variations in the strength and track of the upper-level trough and cold front. Second, the GFS has a horrible overforecasting problem when it comes to precipitation in our area, so it will always raise the hopes of skiers and White Christmas lovers, sometimes followed by great disappointment.
If we look at downscaled NAEFS forecast plumes for Salt Lake City Airport, we see a range of projected snowfall totals between about an inch (remember, this is on the valley floor) and nearly 8 inches.
Ok, that's good news, but we're still a few days out. The elves allowed me a very quick peak at the ECMWF forecasts and they have a deeper, slower upper-level trough. That would delay the precipitation and cold air some, but we'd probably still see some snow on the valley floor for Christmas.
Don't jinx it. Be sure to be a good girl or boy the next few days.