The latest NAM data is becoming available.  Precipitation, very light, expected to begin about 7 am Wednesday.  As I’ve been saying since last night, temperatures in the upper atmosphere will be too warm to support snow in Philadelphia and the immediate adjacent PA counties and south Jersey.

In the lower levels of the atmosphere and at the surface,  temperatures remain below freezing until 9-10am in and around PHL,  so precipitation will fall as a brief period of light sleet and then light freezing rain, changing to all rain by late morning.

The issue for tomorrow morning will be icy conditions on untreated surfaces, not accumulations.  Even a thin coating can be slippery.

One more thing…QPF values are very low in the morning, less than 0.09 inches water.  The heavy rain will be during the afternoon.


No Snow, maybe brief freezing rain changing to rain

The morning model runs continue to show that upper air temperatures are too warm for snow formation tomorrow morning in PHL and the immediate suburbs, despite the TV forecasts.

Ground temperatures, however, may be below freezing at the start of the precipitation (GFS starts somewhat earlier than the NAM ) between 6-8 AM, which may result in a brief period of sleet or freezing rain before temperatures warm up rapidly in all levels of the atmosphere. So icy conditions are a concern early.

Heavy rain is expected for much of Wednesday, although the latest NAM has backed away from the 1 inch totals.  I’ll nail down the forecast this evening, a little after 9 PM.