The New RPM guidance has just come in and shows a surprise snow event for parts of NE PA. The latest trend in short term guidance has me a little concern we might be dealing with a more significant winter storm for parts of NE PA. The NAM does not show Allentown, PA getting just rain at all with this system:
Verbatim this is a sleet sounding. The warm layer at 750mb is enough to melt the snowflake and it refreezes in the lowest levels. However, the RPM says dynamic cooling will be a factor here and as we outlined above a good deal of elevated instability will exist from around 600mb down to 675mb. The only problem here is the strongest forcing and instability are below the snow growth zone, so snowfall rates will not be that impressive. However, if you head north a little bit to Scranton,
The SREF members are all agreeing on strong frontogenetic forcing coinciding with this unstable layer:
Towards the end of the event the ptype changes over to freezing rain for Allentown and it is defined by a very narrow area of freezing rain then plain rain to the south. This line does not move all that much producing a mesoscale circulation between the snow to the north and rain to the south...this allows another heavy band of snow to develop and hit NE PA.