As we get closer to the weekend storm, determining the type and duration of the precipitation type is turning into quite a challenge. The latest GFS Ensembles (tweaks and reruns of the GFS) show a great deal of uncertainty regarding where the freezing line aloft (850mb/~5,000 ft) will set up. Any temperature above freezing aloft would be too warm for snow (sleet or ice), while subfreezing temperatures aloft would increase chances for snow. Basically, if you’re above 0°C/32°F aloft, you can forget snow. So knowing where this demarcation will be is critical to determining pcp type.
The rather messy “Spaghetti” plot below shows the individual runs of the ensemble’s temperature placement as a solid line, with the critical “0” line (0°C/32°F) above the ground in the cluster of lines right over MD. This very important rain/snow indicator could end up anywhere from northern Virginia to the PA line based just on the GFS and its Ensembles alone! This does not even begin to account for what other models are saying… and highlights how difficult the p-type will be with this storm.