Snow for the Appalachians and Rain for the East Coast
It's weekends like this that make me glad I'm not in Happy Valley (State College, PA). It's only October, but the NWS has issued a Winter Winter Advisory covering most of central PA from Happy Valley to the Allegheny Plateau from Thurs. night into Saturday morning. What's going on? Well, it's a bit complicated.
A low forming off the Carolina coastline is expected to move NE while deepening and producing a wide shield of rain. As the low moves past the Chesapeake Bay region, the rain will turn into snow in the higher elevations of the Appalachians of VA, MD, and PA. Also, the circulation around the low will pull cold air down from Canada, making snow possible in the lower elevations of central PA east of the Allegheny Ridge. State College will be right at the edge of the rain-snow line.
From personal experience, I can tell you what it's like. First comes the rain, not rain showers but a sustained, heavy drizzle with temperatures in the low 40's/high 30's Then as the cold air moves in, snow starts mixing in with the drizzle with minimal accumulations. As the temperature drops, the precipitation changes over to wet, heavy snow that can fall in big heavy clumps. No matter how it falls, the snow will start sticking to trees, while it might melt on the roads. Since the leaves haven't fallen yet, there's more for the snow to stick to and tree branches start snapping as the wet snow accumulates. Power outages are frequent with this type of event. For you Nittany Lion fans (and misguided Gopher fans) who are going to be at Beaver Stadium Saturday afternoon, the current game-time forecast is for rain and 38 deg F, but I highly recommend using our Game Day Weather page for the current forecast.
This weekend's weather event isn't limited to snow. The rain associated with the coastal low is expected to produce over 2 inches of rain over the Baltimore-Philadelphia region according to the Hydrometeorological Prediction Center precipitation forecast for Oct. 15-17 in Figure 1 with reduced precipitation along the NY/New England coast.
Fig. 1 72 hour Quantitative Precipitation Forecast (QPF) for October 15-17. Image courtesy of HPC
After the low moves out, another low will affect the NE coast. The timing is still a bit uncertain, but an upper-level trough in the Midwest/OH river valley will trigger surface low formation east of the Delmarva peninsula on Saturday. The current HPC forecast holds that rain from this event on Oct 18-19, Figure 2, will not be as heavy as what occurred before. Still, it ought to be enough to keep everybody indoors on a Sunday.
Fig. 2 48 hour QPF for October 18-20. Image courtesy of HPC
Stay warm and dry out there.