Forecast for Saturday and Sunday
After the precipitation ends about 6-7 AM Saturday morning, skies will be mostly cloudy with brightening skies and possibly some sun during the mid to late afternoon. High 40-42.
Latest models are showing snow for late Sunday.
For Sunday, increasing clouds. Snow begins in the mid afternoon and ends Monday morning. Current trends is for snow for Philadelphia and north and west. Accumulations 3-5 inches, based on current QPF.
Prior Forecast for Friday night…
The early NAM data has become available. QPF values are 0.39 inches water. Precipitation is just moving into the Philadelphia area now.
There are some changes– temperatures at critical levels in the atmosphere appear to remain too warm for snow in Philadelphia and the immediate suburban areas for much of this precipitation.
Areas far north and west (Doylestown and north and Pottstown and west) will have rain/sleet changing to snow about 2-3 AM. A few hours of snow are possible before ending between 6 and 7 AM. 2 or so inches possible.
For the immediate suburbs of Philadelphia- mostly rain or rain with some sleet….cold air does move in aloft towards the end the storm and there may be a transition to snow with some light accumulation before ending between 6 and 7 AM. Nothing significant, as it’s March and anything that falls will melt soon enough tomorrow.
As we can see, the forecast can change for these borderline thermal profile events.
I’ll be checking the GFS later this evening when it becomes available.
There’s still uncertainty with the Sunday storm. Stay tuned.
A very difficult forecast for tonight.
Yet another wave of low pressure will move along the stalled boundary to our south. The GFS and NAM have similar QPF values, the NAM having a greater QPF (as almost always) than the GFS. The approximate QPF is 0.35 inches water.
The big forecast issues are borderline thermal profiles, both aloft and at the surface. (Actually, temperatures at the “surface” are really defined and measured at 2 meters (6feet) above ground.)
The GFS is warmer at the surface and colder at critical levels. The NAM is colder at the surface and borderline warm for snow at the onset.
Here’s a video of the NAM NEST high resolution change in temperature at critical levels- the white line/area and indicates the snow – sleet/rain changeover line:
Both models have similar onset of precipitation – 9 -10 PM tonight and both have the precip ending by 7 – 8 AM Saturday.
I’m going to lean towards the NAM with a rain/sleet/snow mix at the onset, changing slowly to all snow overnight. At times the precip will mix back to sleet and rain.
Areas to the north of the white areas in the video will be all snow. 2-4 inches possible in far northwest. Closer to the city, 1-2 soggy inches, mostly grassy surfaces.
This forecast is as good as I can do it.
BTW, the Sunday evening storm is still too much in flux to call. The GFS is warmer and the NAM is colder. It’s likely mostly rain, but that forecast can change. I’ll focus on this over the weekend.
I’ll be updating this forecast tonight, probably with the NAM early data around 9:15 PM.
Updated today’s QPF…
According to the NWS Snow Spotters Page we had a range of snow totals, about 2.5 inches in Montgomery county and 3.5 or so in Delaware county and 3.5-3.8 in parts of Chester county.
So the NAM (and my forecast) overstated the QPF a bit, but still did better than the GFS which hadn’t picked up on this snow until yesterday and was still under-predicting. Sometimes the average QPF works best, but one never knows when. BTW, it’s still snowing lightly here in Montgomery county at 8:30AM
For today, Friday, the NAM continues to spit out 0.09 inches QPF of precipitation for this morning through this afternoon. Not clear if it will mix with and change to rain, but that changeover is likely. For tonight, QPF values are significant (0.90 inches water) but warmer thermal profiles are going to make the forecast prediction even more difficult.
I’ll be updating later this morning, based on the newer NAM and GFS data.