The latest NAM model data shows a consistent scenario  of low pressure developing along a frontal boundary to our south and east Saturday evening.  QPF values still in the 0.60 inch range.

The usual rain-snow line appears to run through Philadelphia.  Here’s the difficulties with the snow forecast — temperatures at the surface may initially be above freezing, limiting accumulations, especially on paved surfaces.

With the usual “critical thicknesses”  for snow where they are, I would ordinarily be forecasting a mix of rain, sleet and  some snow.  BUT, with the precipitation rate as high as forecast (0.60 inches/6 hours), I am thinking that dynamic cooling may become a big factor with this event. As a result, I am leaning towards more wet, large flake snow than I would ordinarily forecast.  From Philadelphia north and west, 4-5 inches appears possible on grassy surfaces, less on paved surfaces.  Areas far north and west may have 6 -7 inches.

Again, I’m counting on the NAM high precipitation rate for dynamic cooling for this to be snow in Phladelphia; otherwise it will fall mostly as a wet  mix with significantly lower accumulations.

Precipitation starts about 5-6 pm Saturday and ends about 3 am in the morning Sunday.  Whatever falls starts melting Sunday.

Still 48 hours before this starts, so expect changes in the forecast.



As mentioned in my post yesterday, there’s much uncertainty with all aspects of the storm that is expected to develop Saturday evening.    Much of the details won’t be resolved until the immediate 24 hours preceding the event. Here’s what we know now-

  • It’s a fast mover.  Over in about 7 hours.
  • Precipitation starts during the late afternoon or early evening Saturday and ends well before daybreak on Sunday.
  • Temperatures appear marginal for snow in the immediate PHL area and immediate western suburbs.  The NAM which showed snow likely for PHL has flipped to a warmer scenario and is now favoring rain or rain maybe with sleet.
  • Temperatures on Sunday, with sunshine, will melt much of whatever falls, if it falls as snow.  There’s no post-storm freeze.

So, anyone thinking this will be “February-style big snowstorm” is likely to be disappointed.