The last night’s 06Z  data has become available- both the NAM and GFS models have increased the QPF values to an average of about 0.50 inches water.

This morning’s NAM data also becoming available- the trend is for heavier QPF, and warmer atmosphere!

The NAM has increased the average temperature of the mid-level of the atmosphere to a level that is too warm for snow formation for much of the storm in Philadelphia and immediate northwest areas, as depicted by this map.  (Lower levels of the atmosphere (except at ground level) are at or below freezing, allowing sleet at times.)

Here’s the current best forecast-

Precipitation starts between 4-6 PM today, possibly as light snow or a mix of rain, sleet and snow.

A changeover to rain and rain-mixed with sleet will occur early.  Much of what falls will be rain and possibly some sleet;  at times, some snow may mix in at peak intensity if dynamic cooling occurs.   This event will be mostly wet, not white, in the area depicted south of the red line  in the inset map.

Precipitation may changeover to a brief period of light snow before ending about 2 PM.

Accumulations- zero to a coating, mostly on grassy surfaces.

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.