Last night’s models have changed little from previous runs.   (Please see my previous posts.)

Light precipitation starts about 8AM Thursday in PHL (earlier to the south).  Low level temperatures remain at or below freezing for an extended period of time, while the temperature profile in the upper atmosphere is too warm to support snow.

While the precipitation may start as some light snow, it will quickly change over to all sleet in PHL and the immediate surrounding counties. Some freezing rain is also possible early.  

QPF values in the morning are about 0.20 inches water and during the afternoon, 0.70 inches water.   Much of this will fall as rain. 

The sleet will mix with and eventually change to rain. Cold air damming scenarios have sometimes lasted longer than predicted by the models, so it’s difficult to tell when this changeover will occur.   An extended period of sleet can occur in far northern and western suburbs. (Allentown)   East of the city, in NJ, any sleet will change to rain even earlier.

Surface temperatures at or below freezing about 7 AM Thursday but rise slowly into the mid 30s.

The expected secondary storm formed by the merger of the upper low Thursday night now appears to be too warm for any snow.   Rain is expected late Thursday night into Friday morning.

I’ll update tonight about 9:30 PM.

Just a quick update,based on tonight’s NAM model data that just became available.

Much of what I wrote about earlier today still holds.  While the lower levels of the atmosphere remain below freezing for an extended period on Thursday due to “cold air damming”, the upper levels of the atmosphere are forecast to be too warm to support snow Thursday.

I believe this will be a sleet event, eventually changing to sleet and rain; rain possibly heavy in the afternoon. While some early snowflakes are possible, it should quickly change to sleet.

I’m sure there are forecasts out there for snow, based on preset precipitation type algorithms. But thermal profiles have worked well for my forecasts in the past and I’m going to stick with it this time. This forecast is for Philadelphia and immediate suburbs. Areas far north and west may have more freezing rain and sleet accumulating.

The scenario with the upper low enhancing a secondary surface low Thursday night into Friday morning is also supported by tonight’s NAM.  However, it’s looking less likely that enough cold air will allow a changeover to snow .

Things will clarify by tomorrow evening.