Using the high resolution NAM-NEST, the following are the probable snow totals for our area:
The last NAM data has just become available. QPF values have increased to 0.38 inches water, falling as snow. Snow starts about 3-4 AM and ends between 7 and 10 AM Friday. A fast burst of moderate snow. Total snow accumulation about 4 possibly 5 inches, based on this data.
One more thing….tonight’s NAM is forecasting another round of snow (2-3 inches) late Friday night ending Saturday morning. Tonight’s GFS is much warmer for Friday night and has rain.
Still too early to call the storm for Sunday evening, but temperatures appear too warm to support snow.
Look for an update on tonight’s snow about 9-9:15 PM with the new NAM data.
Forecast Update Thurs 4 PM
A quick update- Latest NAM shows a higher QPF- 0.23 inches water. Raising the snow total tonight to 2, possibly 3 inches of snow.
The larger storm on Sunday into Monday is looking too warm for snow at critical levels of the atmosphere.
Forecast Update Thurs 12 PM
This morning’s NAM maintains a QPF of 0.15 inches water falling as snow late tonight into early morning Friday. 1-2 inches possible, more towards the Delaware border.
Additional over-running of moisture over the stalled front and colder air may allow continuation of a light mix of non-accumulating rain mixed with sleet or snow through the day Friday.
The same process, somewhat more enhanced, will continue Friday night into Saturday with a change back to sleet and snow overnight Friday. Additional light accumulation by morning Saturday. This is a complex, poorly defined moisture over-running colder air event. Stay tuned.
from earlier this morning…
A very active and somewhat complex weather pattern is developing for the Philadelphia area through the weekend into Monday.
A stalled frontal boundary exists to our south. Waves of low pressure are expected to develop along the boundary and move rapidly west to east, on a track just to our south.
We had a surprise snow flurry last night that left a small accumulation in some areas from a weak unpredicted wave that developed.
A similar setup will allow a more enhanced wave of low pressure tonight, late Thursday night into Friday morning. The NAM has been predicting this for several days; Last night’s GFS finally joined the NAM with a QPF of ~ 0.10 inches water, more towards Delaware and South Jersey. About an inch of snow is expected.
This complex setup looks like another round of light precipitation Friday night into Saturday. Indeed, several waves may move through over the weekend, the timing is unclear right now. The largest wave is expected late Sunday into Monday.
A range of precipitation types from snow to sleet or a mix is possible with each wave, somewhat dependent upon whether the wave moves through at night when it is coldest, or during the daytime hours.
Stay tuned for clarification… This is an evolving and complex forecast.
Tonight’s NAM has reduced the QPF to 0.13 inches water. It’s typical, as pointed out last night, for the NAM forecast QPF to reduce as it did between last night and tonight. So we’re down to about an inch of snow, falling late Thursday night and ending early Friday morning. Despite the minimal QPF, the snow falling during nighttime will allow for accumulation. Higher amounts near Delaware. The GFS from this afternoon still showed no snow. Still a low confidence forecast.
Forecast Update Wed 6 PM
Today’s NAM continues with light snow for late Thursday night, ending daybreak Friday. QPF values have reduced to 0.17 inches water, or about 2 inches of snow. The GFS continues with no snow for us. Still a low confidence forecast.
Of increasing interest is a persistent GFS forecast for snow Sunday afternoon into Sunday night. The models have been quite inconsistent, as mentioned in previous posts this week. However, right now, the GFS is forecasting a 8-10 inch snowfall for us. Expect changes. Stay tuned.
As mentioned earlier today, there’s lack of continuity and agreement with the model forecasts for this week, but the past several model runs of the NAM show a weak disturbance moving just to our south Thursday night into Friday.
Tonight’s NAM just available shows a strengthened low which brings snow late Thursday night, ending early Friday morning. QPF values tonight approach 0.45 inches water, which would be a significant snowfall of 4-5 inches.
The NAM tends to overstate QPF early on and the GFS has been unimpressive with this low pressure development. But this needs to be watched. Stay tuned.
The medium range model forecast for the weekend sems to be in flux as details of the development of several systems seem to have changed over the past few days – a storm expected to develop Friday into Saturday is now forecast for Saturday into Sunday. It appears that we will get rain from this system. I would not be surprised if the timing changes again.
There’s also a small impulse forecast for this Thursday into Friday which keeps changing intensity and track.
So a very uncertain weather outlook for this weekend in Philadelphia.
The models are consistent in the appearance of a deep dive in the jet stream for much of next week caused by arctic high pressure. As a result, well below average temperatures are likely for much of next week. Storm development with this jet dip is expected to be off-shore, but that could change.