Update Tues 9:45PM: The latest NAM data suggests that a wave develops along the front, increasing the precipitation and delaying the end of the snow.
Current QPF values have increased to 0.35 inches water equivalent during a time when the precipitation type will be snow. Using my own method and accounting for melting and warmer ground temps, it seems that about 2 inches of snow are possible. Snow flurries and showers may linger until 10-11AM.
10 :15 PM Update: The NAM built-in snow algorithms show less than 1 inch of snow. The latest WRF models are closer to the NAM, so my 2 inch total isn’t supported by the built in algorithms. We’ll see what happens.
10:45 PM Update: Latest GFS doesn’t show the enhanced precipitation nor does it delay the snow ending. It maintains a snow total of about 1/2 inch.
I’ve had a chance to review the afternoon model runs. Both the GFS and the NAM still predict just a coating (0.30 and 0.50) of snow falling after midnight and ending about 7-9 AM, based on their built-in snow algorithms.
I’ve never been a big fan of the built-in snow depth algorithms.
Interestingly, this afternoon’s Canadian High Resolution Model (HRDPS) has been consistently predicting ~1.6 inches of snow.
The latest RAP (Rapid Refresh) just available shows 1 inch of snow and ends with light flurries about 9 AM.
So it will be interesting to see how things turn out. I’ll be looking at the NAM data which comes out about 9:15 and the NAM FOUS data which becomes available about 9:35 PM tonight. Check back later for an update.
As posted last night, the GFS, NAM models were leaning towards 0.40 to 0.8 inch of snow accumulation, ending between 7 and 8 AM Wednesday morning. It’s not unusual for these post-frontal passage snow events to trend lower on the snow forecasts, as cold air often scours out the moisture available for snow. The models are showing less wave development on the front and faster influx of cold air.
This morning’s high resolution NAM NEST has just a coating of snow. (0.27 to 0.45 inches of snow, basically nothing to a coating). The NAM has just a coating or snow showers, based on built-in snow algorithms.
If I use my time-tested, old-fashioned NAM FOUS data technique, I’m still at the 1 inch accumulation.
BTW, over the past year or so, I have developed software and scripts that automatically download real-time raw model data directly from the NCEP and the CMC. (You may have noticed the new model graphics I post here.)
I think I may be running into a situation where my forecasts are distracted by “too much information”.
The most recent HRRR from 7 AM today shows nothing accumulating, with the precip remaining a wet mix of rain and snow!
So the trend is for a minimal snow accumulation. Stay tuned.
Update Mon 10:15 pm: Tonight’s NAM, NAM Nest and the two WRF models have become available. I’m staying with the 1 maybe 2 inch snow totals, even though the built in snow algorithms are showing about 0.5 inches of wet snow. Snow ends 8-9 Am Wednesday. We’ll have a better handle tomorrow.
Update Mon 10:50 pm: Tonight’s GFS has also gone into the 0.5 inch snow accumulation range. So the trend is for less snow, less than an inch.
The models are in fairly good agreement that rain will change to snow sometime after midnight Tuesday and light wet snow will be falling until sometime between 7 and 9 AM Wednesday morning.
The NAM built-in snow algorithms have only a half inch of snow accumulating, although if I use my old NAM FOUS data technique, it’s probably about 1.5 inches.
The GFS, Canadian High Resolution and the European are in the 1.3-2.0 inch range of accumulation using their built-in algorithms.
We know that these sort of post frontal passage scenarios often change, usually in the direction of less snow.
Tonight’s model runs will fall into the range of the 36-48 hour short range higher resolution models, and I’ll do a quick update later.
After the snow stops, it will become sunny by Wednesday afternoon, but fairly cold. Treated roadways will be clear by afternoon.
Today’s models continue with the idea of weak low pressure developing along the front as it moves through early Wednesday.
There are differences in timing and snow amounts, but the statistical models, the GFS and the European have increased snow totals to 3 inches. We’ve seen these forecasts change drastically, but I’m letting you know that some amount of snow is expected during the morning rush hour on Wednesday. Stay tuned
On my previous forecast, I mentioned that I would keep an eye on the strong cold front moving through Tuesday night into Wednesday morning. I mentioned that snow showers were possible Wednesday morning.
The most recent model runs suggest a trend that the rain mixes with and changes to snow sometime after midnight Tuesday as the upper atmosphere chills down, supporting snow.
While strong cold fronts often scour out any residual moisture, the models are showing that some weak low pressure development occurs along this front as it moves through, with moisture lingering after the cold air moves in aloft.
The GFS has about 1 inch of wet snow for the Philadelphia area ending about 9-11 AM Wednesday morning The NAM about 0.8 inches. We know that things change. Stay tuned.