Category Archives: Commentary

Commentary

SNOWSTORM POST-MORTEM

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.

I’m ready for Spring!

Final Two day snow totals

 

Commentary

LITTLE KNOWN FACT ABOUT DAYLIGHT SAVING TIME & TV WEATHER FORECASTS

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 Eastern Daylight 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. )

Update: With GFS version 16 that became operational in March 2020, the increase in complexity of the model has the availability time even later, about 10:45 EST and 11:45 PM DST.  And that’s only for the 24 hour forecast.   The five day forecast is first available about 11:20 PM EST and 12:20 AM DST!

And that’s just the forecast for the first 24 hours!

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!