SATURDAY FORECAST UPDATE

As mentioned earlier this morning, things have changed considerably with the model forecast for Saturday. The low pressure that was expected to slowly move along a sinking frontal boundary, with heavy precip over-running the front now appears to move more quickly- the precipitation moves through quickly and the low doesn’t intensify until it’s further to our south.

As a result, there’s not enough cold air in the mid levels to support snow around PHL until after the precip moves through.   Also, there won’t be enough heavy precip to cause enough dynamic cooling.

The timing has changed.  Most of the precip occurs before sunrise and ends before noon.  Most of the precip will be rain.  There may be a brief period of rain mixed with snow around daybreak, but will likely change back to all rain maybe mixed with sleet early morning before ending by noon.  There should be little to no accumulation in Philadelphia and the immediate surrounding areas.  

Anything that falls will quickly melt.

I’ll keep an eye on any further changes.

BTW, there’s a chance of rain showers/ flurries early tomorrow (Friday) morning.

CHANGE IN SNOW FORECAST

The 2 AM runs of the GFS and NAM show almost no snow for Philadelphia and north.   – The precipitation develops mostly to our south.

Thermal profiles in our area are too warm for snow for much of the time, so we will get mostly rain.     There may be a brief changeover to snow before ending but accumulations look minimal at this time.

Will continue to watch this.

SNOW FOR SATURDAY?

A front moved through today, associated with a dip in the jetstream that will allow cold air to move in for the next few days.

The frontal boundary stalled to our south and multiple models are showing the development of low pressure that will move along the front bringing significant precipitation.  Both the NAM and GFS show QPF values, as high as 0.98 inches of water.

There are currently differences between the timing and thermal profiles  of the models and those differences result in significant differences in the precipitation type (rain /sleet/snow).

The NAM is warmer and shows little snow until the afternoon.  The GFS is colder and faster, and shows as much as 4-6 inches of snow!

We also need to include consideration of sun angle in April and resultant solar insolation through clouds along with daytime vs nighttime Spring snows.   Too early to make a call, but interesting weather for April.  Stay tuned.

Wednesday 10:45 pm -Tonight’s NAM continues with a thermal profile that is too warm for snow during much of the storm around Philadelphia.

Upper Bucks and Montgomery County as well as western Chester county may have over 4 inches, as measured on grassy surfaces, but the immediate PHL area is mostly rain and some sleet, changing to snow late in the game.  A light coating possible.   Expect changes