The latest NAM data is coming in as I type this. Looking at the trends-
QPF values have stabilized at about 1.00 inches water.
Temperatures at critical levels levels of the atmosphere decrease enough to support a changeover to snow about 9-11am in Philadelphia, earlier north and west of the city.
Prior to the changeover to snow, expect continued sleet and freezing rain. So snow changeover may be a bit later than previously thought .
Based on the actual QPF expected to fall as snow and factoring in March sun angle through clouds, I’m thinking that about 6-9 inches fall in Philadelphia and immediate suburbs, 10-12 in far northwest suburbs.
Winds will be high at 30 -35 mph with higher gusts.
So, the snow transition starts a bit later here and QPF values have decreased somewhat and winds remain a big problem.
important update 11:30pm. Correct winds to 20-25 mph with higher gusts. Reduce snowfall to 4-7 inches in Philadelphia based on GFS NAM blend
The models have been advertising another coastal snow storm development for Tuesday night into Wednesday. Not all of the major models are totally on-board with this scenario, but it still appears more than likely. The wind intensity of this storm will NOT be anything like this past Friday, although snowfall rates may look impressive during the day.
Incredibly, the current range is anywhere from 6-10 inches of snow for Philadelphia , based on the GFS (6″) and the NAM (10″).
Current timing- light snow develops during the evening Tuesday and increases in intensity after daybreak Wednesday, as the secondary coastal low develops and intensifies.
There’s still some thought that warm air might mix in, but right now the NAM and GFS critical temperatures all support snow.
I’ve given much thought about this past storm I’m not going to get into the March sun angle, solar insolation and ground temperature effects, since it made giving an accurate accumulation forecast impossible. I guess in March, it’s really about snowfall rate, not accumulation since accumulations mean much less in March— the snow can start melting as soon as it falls.
I’ll keep an eye on this one and will update tomorrow!
Sunday 10pm: The latest NAM data shows a significant snowstorm for Philadelphia on Wednesday. QPF values are almost two inches water. Critical temperatures show it falling as heavy wet snow. Still too early to be sure, but this could be a major snowfall.
11pm: Tonight’s GFS confirms a major snowstorm likely for PHL. We’ll be waking up to heavy snow falling on Wednesday, snowfall increases for the rest of the day. Current models suggest as much as 15 inches! Too soon to hang our hat on this.