My previous post included the link for the official snow totals. I always like to evaluate my forecast to improve on future forecasts.
For the immediate PHL area, snowfall was between 7-11 inches and areas somewhat further north and west, totals approached 15 inches.
With my call of 4-7 inches with an emphasis on “7 most likely”, my forecast fell short. So what happened?
Official QPF measured at the airport yesterday was 1.06 water, somewhat higher than the 0.87 predicted by the NAM and higher than the GFS. But overall the models did well on QPF. (The actual snow total for just Wednesday was 6.7 inches at PHL airport, according to the NWS.)
My emphasis on solar insolation through clouds lead my forecast astray. Had it been January, I would have predicted 8-10, which I mentioned in several posts. So I learned yesterday that heavy snowfall rates trump solar effects, even in late March.
While the models did well on QPF, they did less well on wind. That was lucky for our region. Temperatures were predicted well.
Mesoscale banding set up northwest of the city and snowfall was heavier in those areas.
An interesting and little known thing happens with weather forecasts on the evening TV newscasts when Daylight Saving Time is in effect. (Eastern Time)
Hmm….you’re thinking, what could that possibly be?
So let me cut to the chase and then I’ll explain.
When you watch the 10PM or 11PM news/weather on TV and you’re on Eastern Daylight Saving Time, the latest major weather models (with the exception of the NAM) are not available until after the broadcast is over!
Essentially, we’re all getting forecasts that are still based on radiosonde measurement and global model forecasts that were done earlier that day! You’re not getting the latest at 11 to 11:30 PM simply because the major models (GFS, European, Canadian) aren’t done being calculated by the supercomputers!
It doesn’t make a difference whether you’re watching Accuweather on Channel 6 or whatever they call it on Channels 3 and Channel 10. It’s simply not available on Eastern Daylight Savings Time until after the broadcast.
This is a peculiarity resulting from EasternDaylight Time, (not Central or Pacific Daylight Saving Time) combined with how long it takes for the models to be computed.
More specifically, the earliest GFS data is available between 11:32 and 11:39 EDT. (Before we make the switch to Daylight Saving Time it’s available 10:32 and 10:39 EST. )
And that’s just the forecast for the first 24 hours! The latest “Five Day Forecast” isn’t available for another hour or so later!!
Here’s some more info: Most of the major models are run every 6 or 12 hours, starting with 00 UTC (previously called Greenwich Mean Time).
00 UTC = 7 PM Eastern Standard Time but it is 8 PM during Eastern Daylight Time.
Since the major models take a minimum of 3.5 hours to chew through the numbers, even on the supercomputers, it’s not until after 11:30 EDT when the first “24 hour products” of the major models first become available.
Even hourly short range models use the major models as starting points, so they’re affected too!
If you’re a weather nerd, here’s the site where you can see what time each of the model outputs are going to become available each day. (It’s sort of the on-time train schedule for US weather computer models.)
So during the warmer months on the east coast, when you hear one forecast on the late news and then wake up to hearing another forecast, this is one reason why!
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.
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.