This afternoon’s models continue with the fact that this is a difficult forecast—too many boundary conditions which affect the precipitation type and amount.   The GFS has a QPF of only 0.30 inches water, the NAM has a consistent QPF of 0.60 inches water.

Here are the issues making this forecast difficult-

• Temperatures during the daytime Saturday are expected to be well above freezing, with the GFS much warmer than the NAM.
• Ground temperatures, especially on paved surfaces are slow to drop, reducing accumulations.
• Critical temperatures in the lower atmosphere support snow, BUT the models agree that critical thickness levels (a measure of the upper atmosphere temperature and density) are a bit warm to support snow at several points during the 7 hour precipitation window.

As a result, it looks like a mix of snow, sleet or rain is likely in PHL and the immediate suburbs.    So what falls from the ground may be snow, but it may switch over to  sleet and rain and then end as snow.   Add the warm ground temperatures to the mix and it’s truly not possible to predict accumulations.

More specifically, it looks like it starts as snow about 4 PM, but I think it will mix with sleet and freezing rain for a portion of the duration, then switching back to snow before ending about 2 AM Sunday.   Most accumulations will be on non-paved surfaces.

Areas in northern Montgomery and Bucks county will likely have more snow.

Temperatures are key- the next model run with the data I need will be available about 9:45 PM.