Hey folks, long time no chat. How's it going?
Since ZR is in the mix for most I just thought I'd pop in and share this paper with you all: https://journals.ametsoc.org/doi/full/10.1175/WAF-D-15-0118.1.
This is the research behind the FRAM (Freezing Rain Accumulation Model) that the NWS uses. (Or at least, in Central Region we use. I can't speak to those living the Bohemia lifestyle 😉.)
The gist is that ice-to-liquid-ratio, ILR, is often actually even less than 1:1.
The FRAM is a function of 1) rain rate, 2) wind speed, and 3) surface Tw. Rain rate shows the most robust statistical relationship. And it might not be what you think. Higher rain rates decrease the ILR - see Figs. 10-12. Remember that there is latent heat released when liquid freezes on contact. More liquid freezing at a time means more latent heat release and thus more warming of the surface. Secondarily, a higher rain rate also means more splash-off and thus less of the liquid remaining as ice. As for the other factors... more wind helps to transport that heat away from the surface and thus increases the ILR. For Tw, the relationship is non-linear, but as you can see, the peak ILR occurs with a surface Tw of around -1 to -3 C. That means a surface temp of 32.0 F usually doesn't cut it. Even 30-31 F can be iffy for significant ice accrual.
So if we were to believe the NAM (famous last words), in Philly you're getting most of your ZR in the optimal temp range, so that's a plus for significant ice accrual. You should also have a 10-15 knot wind, which helps. Buuut a big chunk of the precip is coming while rain rates are still pretty high. Looking at the NAM in BUFKIT, I'm seeing a rain rate around 0.1"/hr during the hours where the Bourgouin gives ZR. That may end up really limiting the ice accrual - the median ILR for that rain rate is 0.5, and the innerquartile range is ~0.3 to 0.65.
So all in all I'm not ready to bite on a major ice storm just yet.
Now that said, I was also really bearish on the November snow event, so... 🙃
P.S. We'll cross 100" on the season up here by the end of the weekend. Sitting at 99.1" right now, with 19" on the ground. Continuous snow cover since November 9th. And this is entirely normal!