Tag Archives: Daylight Saving Time


#Phillywx #Philadelphia #weather #PAwx

There are comments, but they’re not necessarily showing. Click on “Post a Comment” to see comments if they’re not showing as a default. Possible bug in WordPress(?)

Sunday Forecast Update

Updated Sat 9:21 PM — No major change in the the forecast. Sunny skies. High 66° with temperatures around 63° with winds NNW around 7-10 mph for the start of the Eagles game.

Weather and Daylight Savings Time-> Standard Time

Twice a year, I feel compelled to talk about Daylight Saving Time and how it negatively affects timely access to updated evening weather model data before bedtime for those of us on the east coast.

Frankly, I love Daylight Saving Time and the extra hour of daylight we get in the late afternoon. The problem for us weather people is that major weather models are run worldwide at “00z” (Coordinated Universal Time or what used to be called Greenwich Mean Time). This corresponds to 7 PM Eastern Standard Time, but 8 PM Eastern Daylight Time. That hour makes a difference.

Since numerical weather models take 2-5 hours to ingest data, compute and process, many of the major models become available too late in the night to be any use for us on the East Coast for planning our activities the following day.

When you hear tune back in at 11″, you’re not going to see any change in the 5 day forecast at 11PM and likely to see little change in the next day forecast; new model data simply hasn’t become available yet.

So for us weather people, the return to Eastern Standard Time makes a big difference for last minute evening updates about snow storms.

Here’s some of the changes in some model availability (eastern time) :

Model Name (00z model run)Daylight TimeStandard Time
GFS 24 hour11:46 PM10:46 PM
GFS 5 day12:20 AM11:20 PM
NAM 24 hour forecast10:05 PM9:05 PM
NAM-NEST 24 hour forecast10:12 PM9:12 PM
HREF 48 hour forecast11:25 PM10:25 PM
Canadian HRDPS 24 hour forecast11:20 PM10:20 PM
Canadian RGEM 24 hour forecst11:10 PM10:10 PM
ECMWF*2:00 AM1:00 AM
ECMWF (free open data version)3:15 AM2:15 AM
* Partial runs of the ECMWF may be available earlier for paid subscriptions.

I’ve written extensively about Daylight Saving Time in past years—

Little Known Facts about Daylight Saving Time and TV Weather Forecasts

Daylight Saving Time and Weather Models

“Check Back at 11” — TV Weather and Daylight Saving Time

Oh, I just remembered, I have a forecast for Saturday and Sunday!

An upper air disturbance and a very weak trough will move through to our north on Saturday. The disturbance is visible in satellite water vapor imagery—

5 PM Friday satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), jet level wind streamlines (orange contour-arrows) potential vorticity (fine white contours) and superimposed MRMS radar. (Click on image for a larger view.)

Previously Posted Fri 8:27 PM —


A mix of mid level clouds with some breaks of sun possible and some bright spots. Somewhat thicker clouds in the early afternoon, clearing late afternoon towards sunset. No rain. (The Canadian HRDPS has less cloudiness than our NOAA models and my forecast reflects that.)

High temperature 59º ( NBM model— location Blue Bell, PA)
High temperature 61º ( NBM model— location Philadelphia, PA)
uncertainty ± 2.6º (somewhat more spread than average)


Mostly sunny. Light winds.

High temperature 62º ( NBM model— location Blue Bell, PA)
High temperature 64º ( NBM model— location Philadelphia, PA)
uncertainty ± 1.7º (less uncertain than average)


#weather #paweather #wx #pawx #philadelphia

Update Sat 3/11 8:54 PM — Sunday’s forecast below is unchanged. High temp 46.5° sd 1.4°

Update Sat 03/11 @ 10:08 AM — We had the wet snow here before 8AM. The balance of precip will be light rain. There’s a wide range of model opinion regarding clearing today.

After reviewing the models, it appears that a considerable amount of low cloudiness will remain, but there’s a chance of some short-lived breaks of sun in some areas this afternoon. Some additional showers possible.

The HRRR shows another band of light showers moving through about 3-5 PM or so—

This morning’s 12z HRRR shows clouds (black/grey) and showers at 4 PM today. (Click on image for a larger view.)

That area is visible on this morning’s hybrid radar—

This morning’s Hybrid Radar with RAP model Omega overlay. Light showers may rotate through about 3 PM (Click on image for a larger view.)

Update Fri 3/10 11:02 PM — Tonight’s models continue to show non-accumulating wet snow showers, from the just outside of the city and north and west, Saturday morning.

Previously Posted Fri 4:51 PM —

This weekend will mark the transition into a different weather pattern than we’ve seen in awhile. Two low pressure systems will move in and develop – one Friday into Saturday and another late Sunday into Monday. These systems will pull cold air down from Canada and the jet stream pattern will become cyclonic and somewhat amplified along the Eastern US.

This translates into below average temperatures, windy and quite unsettled conditions, starting today (Friday) and into much of next week.


Low pressure that brought rain Friday night will blossom in the western Atlantic. Upper level low pressure will merge with this storm. Wrap around moisture from this surface low will bring continued cloudy conditions and showers/snow showers on Saturday morning. It will be windy and cold. Some widely scattered showers may linger into the afternoon.

Here’s the GFS for Saturday—

GFS model forecast for Saturday 1 PM. Wrap around moisture from low pressure in the Atlantic will bring rain and snow showers. (Click on image for a larger view.)

Despite temperatures well above freezing, snow showers are expected to develop in some areas. It would not surprise me if the snow shower coverage is greater than predicted below—

NAM-NEST forecast Precipitation Type for 11AM Saturday. Blue: snow showers (Click on image for a larger view.)

High temperature 45.7º sd 1.5º ( NBM model location Blue Bell, PA)


Another low pressure system approaches during the day. Sunday starts sunny through increasing high cloudiness.

Thickening and lowering cloudiness in the afternoon.

Some precip moves in during Sunday evening, which most likely will be rain in most areas.

GFS forecast for Sunday at 7 PM. Low pressure moving in from the west and south. (Click on image for a larger view.)

High temperature 46.9º sd 1.5º ( NBM model location Blue Bell, PA)


NBM Wind Meteogram for Blue Bell, PA (Click on image for a larger view.)

For next week, very deep low pressure develops near Cape Cod. We’ll be affected by windy conditions and cold temperatures.

GFS for Tuesday shows deep upper and surface low pressure systems—

GFS forecast Tuesday. Windy cold and unsettled here. (Click on image for a larger view.)
ECMWF 300 mb wind forecast (with red 500-1000 540 thickness) for Tuesday night shows broad but deep upper air trough in the East with closed low over the Northeast. (Click on image for a larger view.)

Daylight Saving Time and Weather Forecasts

Sunday morning is the beginning of Daylight Saving Time.  I love the extra daylight at the end of the day, but for  “weather nerds”, Daylight Saving Time means the models come out an hour later.  For those of us on Eastern Time, the evening models become available inconveniently late for the next day’s forecasts!

I’ve written extensively about this in past years—

Little Known Facts about Daylight Saving Time and TV Weather Forecasts

Daylight Saving Time and Weather Models

“Check Back at 11” — TV Weather and Daylight Saving Time