Saturday, January 8, 2011

Tuesday Storm Update - Saturday 7 PM

Here are the maps of the ECMWF and GFS ensembles from earlier this morning. Both computers have a very similar track with rapid cyclogenesis (storm intensification) from 7 AM to 7 PM Tuesday. It's during this time that the heaviest snow bands will form.

As with previous runs, the GFS is hinting at a mid-level warm layer moving into southeast Virginia. I've posted the forecast sounding for Tuesday afternoon. The right pink line is the temperature forecast. The bottom is the surface, the top is around 50,000 feet. I've drawn in red the freezing line. Everything to the left of this line is below freezing, everything to the right is above freezing. Notice the red arrow at around 5,000 feet, where temps are very close to freezing. If it gets a little warmer in this location, then snow would melt, then refreeze in the colder air below. In other words, we would get sleet (ice pellets).

Although I think we could get some sleet in Richmond, most of the mixing will occur in south central and southeast Virginia. That's why I'm giving a higher probability to the ECMWF solution below. Notice how the heaviest band starts in central Virginia, right when the low intensifies off the coast. This heavy band then extends south of D.C. and into Philly.

If we see more sleet, then our snow accumulations will be less. Right now I think this is less likely, but still a plausible outcome.