The nor’easter that we’ve watching for Monday evening is looking less and less an issue for Philadelphia.

The latest NAM data just out (an hour later due to Daylight Saving Time) has the storm developing and tracking too far east to pose a threat to Philadelphia.

QPF values are only 0.06  0.03 inches water in PHL, less to the west.  The biggest issue might be some icy spots early Tuesday morning and some winds on Tuesday during the day.

The GFS won’t be available until later. (See earlier post.)




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. )

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!


This morning’s models are in (an hour later due to Daylight Saving Time).   The development and track is just far enough east to minimally affect Philadelphia and its adjacent counties in Pennsylvania.   A bit more action in southeastern NJ where QPF’s are about 0.25 inches water, but warmer temperatures will impact accumulation.

At this time, expect just wet surfaces to just coating in most of our area. Freezing Monday night may make for some black ice situations in some areas on Tuesday morning.