Category Archives: Commentary

Commentary, Winter Storm Outlook

SNOW FOR PHILLY ON WEDNESDAY- HERE WE GO AGAIN

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.

NAM Forecast Wednesday 1 PM

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!

[su_note note_color=”#ebf2d9″]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.

[/su_note]

 

Commentary

SO WHAT HAPPENED?

The snow totals exceeded everyone’s expectations, including my own.   So what happened?

I can only speak for myself— I was very reluctant to predict snow totals based only on the raw model numbers for a storm in March.  Indeed, I felt I over-estimated last night when I said 2-4 inches and reneged this morning.

But if I had gone by the raw model numbers, the forecast would have been more on target.   So I can’t say the models got it wrong, except for perhaps surface temperatures, which showed to be colder than forecast. They apparently didn’t get the ‘dynamic cooling’ factored in at the surface.

Indeed, as said in my blog post yesterday, if this had been  a storm in January, I would have predicted 7+ inches.   But, I felt that warm surface temperatures and radiant energy through clouds would severely reduce accumulations and over-ride the raw numbers.

But the raw model numbers (at least based on the NAM FOUS) had enough QPF and low enough temperatures to account for what we have received.   So the models did well, based on my usual criteria.  I just didn’t trust it explicitly for the forecast.

I use the NAM FOUS data, an odd, tabular set of numbers that often provides everything I need for a good snow forecast; I rarely use the preconfigured snow totals of some of the model outputs.  

BTW, the NAM from this afternoon shows snow ending shortly after 7PM, although current radar would suggest otherwise.   We’ll see.