The snow-accumulation forecasts include a lot of wiggle room, but don't be surprised if the actual results vary even more wildely than the forecasts suggest.
We wouldn't be shocked if the totals called in by National Weather Service spotters in Philadelphia and the surrounding counties tomorrow ranged from 2 to 10 inches. And the differences won't necessarily be along the traditional lines, southeast to northwest.
At this writing, it appears that no one near Philadelphia will see significant accumulating snow during the day, and the region should skate through the afternoon commute without much trauma.
Why? With the coastal storm migrating off the Carolina coast, the region is getting an easterly component to the winds (which are swirling counterclockwise), thus warmer air off the Atlantic, where surface temperatures are in the upper 30s.
Temperatures around Philly are expected to stay above freezing during the day, and the colder air won't build in until the coastal storm -- which is going to ingest a weaker cousin from the Great Lakes -- moves north and intensifies.
The latest computer guidance suggests a changeover around 6 p.m. in Philadelphia, with snow occurring earlier north and west, and the line moving southeast, said Dave Dombek, meteorologist at Accu-Weather.
That changeover will have some impact on totals, but the bigger factor with a storm of this intensity could be where the narrow bands of the heaviest snow set up.
Without getting geeky, certain areas, sometimes no more than 10 miles wide, end up with perfect conditions for snowflake building.
Another complicating factor will be figuring out where the shut-off lines occur. Air in a storm rises madly, which causes the condensation and convection, but it sinks near the borders of the storm, and that dries out the air.
Obviously, the outcome depends on the path and ultimate intensity of the storm. In the early going, it looks like a potent one.
Dombek said thunderstorms have popped up off the Carolina coast, a sure sign of friskiness.