Tuesday, January 15, 2013

Frozen Slop North of City Tonight

Winter weather advisories are out for the Lehigh Valley, Poconos, and Berks County for tonight and tomorrow morning with the second wave of precipitation that's projected to move in later today and tonight. It's going to be a mixed bag event of sorts to the northwest, with a general rainfall likely in Philadelphia, the immediate suburbs, and farther south where between a half inch and three-quarters of an inch of rain is likely with this system for the city and south.  North and west will see a bit less precipitation but temperatures will be colder...and thus, the potential for some frozen or freezing precipitation across the outer northern and western burbs, generally along and north of a Reading-Pottstown-Doylestown line.

The NAM from last night shows the general idea of this but using I-78 as the benchmark of frozen/non-frozen precipitation.  Each computer model varies in placement of the rain/non-rain line a bit -- but it's generally across the Lehigh Valley or northern burbs.  Above I-78, it's pretty likely that this is a mostly snow event although some sleet or freezing rain could fall initially as some milder air aloft remains.  Along I-78, you're in line for a little bit of everything -- sleet, freezing rain, some snow, or even liquid rain depending on temperatures.  You're in the slop zone with this event in the Lehigh Valley and across most of Berks County.

We've indicated that some of the frozen precipitation could extend down into the outer burbs like Quakertown, Pennsburg, or perhaps Pottstown and Doylestown depending on temperature.  However, since you're on the fringe of the rain/non-rain line the likelihood of the northern parts of Bucks and Montgomery Counties getting more than just a mix is low.  Temperatures could be close enough to freezing that side roads and sidewalks could be a bit slick later tonight across the northern burbs, with decreasing quality of travel as one travels north.

The best chances of a couple of inches of snow reside across the Poconos and northern parts of Lehigh and Northampton Counties -- some spots could see more than three inches of snow tonight with this system.  The pink shaded regions across the Lehigh Valley could see a coating to an inch...if temperatures are colder and more frozen precipitation falls, two inches may accumulate along I-78.

For everyone else, rain.

Timing-wise, this system pushes in this evening from the south, with frozen precipitation reaching the Lehigh Valley around 10 PM and continuing into the morning rush hour. Rain and frozen precipitation will taper off from the southwest between 6 and 9 AM (latest northeast), with drizzle lingering through the balance of the morning around the region.