Sat 8 AM Update: Last night’s models have additional changes for Sunday. For Sunday, some clouds in the morning with a chance of a widely scattered shower very early, then hazy sunshine. High 85-86. There’s a chance of a widely scattered thunderstorm late afternoon, especially west of Philadelphia. Thunderstorms move through Sunday evening.
The forecast has changed a bit more for Sunday since I posted the outlook yesterday. More about that in a moment.
The cold front moved through this evening (Fri) accompanied by showers and thunderstorms. The high resolution models (WRF- and the NAM NEST) did well in predicting these.
For Saturday, high pressure builds in giving us a delightful day of sunny skies, low humidity and temperatures 76-78 degrees.
A warm front moves through before daybreak on Sunday. As mentioned, yesterday’s GFS and NAM makes it a dry front passage, but today’s High Resolution models show light scattered showers before daybreak on Sunday. Winds shift to the south and dewpoints and temperatures increase on Sunday.
The highly advertised extreme summer-like high near 90 doesn’t seem to be in the forecast right now. The NBM and EKDMOS have backed away from those temperatures. A mix of sun and some clouds for much of the day Sunday. High temperatures 82-84. Dew points not as high as previously predicted either.
A cold front moves through Sunday night with showers and thunderstorms moving through.
Fri 10:45 pm. Minor forecast updates highlighted below for Saturday.
The forecast for this weekend continues to evolve and has changed since I posted my “Weekend Outlook” yesterday. A dip in the jet stream, a stalled front to our south and a secondary coastal low will make for a less than stellar weekend.
A front will move through this evening with some showers. Most of the dynamics are moving to our north and the showers are not expected to be very heavy.
After the frontal passage which hangs to our south, skies had been expected to clear. However today’s NAM and GFS have upper air disturbances moving through our area that will cause significant cloudiness for Saturday, mostly in the afternoon. A northerly wind will bring cooler temperatures and the much of the day will be somewhat windy, breezy with winds diminishing later in the afternoon. High temperatures 66-68.
For Sunday, there is still disagreement between the GFS and NAM regarding the timing and amounts of rain during the daytime hours.
It’s based on the uncertainty regarding low pressure developing along the stalled front to our south and the transfer of energy to a coastal low expected to develop.
The NAM has been consistent forecasting heavy rain for much of Sunday. It is supported by the European model (ECMWF).
The GFS and the new FV3-GFS have the rain moving in before daybreak with a hiatus during the late morning and afternoon hours. So if the GFS and FV3-GFS are correct, Sunday mid-day won’t be a washout. Impossible to say which is correct, but it’s something to hope for. All models have it cloudy with an easterly cold wind and temperatures in the mid 50s!
Thurs 7PM: Still uncertainty with Sunday’s forecast, with the NAM predicting rain for much of the day, while the GFS and FV3-GFS have the rain now later in the day.
I often post an “outlook” forecast on Tuesdays or Wednesdays. I’m glad I waited – the forecast for the weekend has changed significantly over the past few days.
Following a cold front passage on Friday night, skies will clear on Saturday. Saturday looks to be a nice day with sunny skies and high temps near 70. (Details Friday evening)
Sunday had initially looked to be another nice day, BUT the latest models, (NAM, FV3-GFS, EKDMOS) have the front that moved through Saturday stalling to our south with low pressure moving along the front. An easterly flow will bring cooler temps (high in the low 60s) and rain is looking likely, especially for Sunday morning.
The GFS is an outlier this time around, with it holding off the rain until later in the day on Sunday. Last week, the FV3-GFS had the correct forecast and I suspect its rainier forecast is correct for Sunday.
Speaking of the FV3-GFS, the NWS will be upgrading the current GFS model (known as GFS v14) to the FV3-GFS (to be known as GFS v15.1) on June 12th. This is a major upgrade to the principal US weather model.
The FV3-GFS has been in development for many years and in comparison testing for over 3 years. Its forecasts have been shown to be equal or better than the current GFS.
I’ll update the weekend forecast on Friday evening as usual.
Latest HRRR has scattered showers mid morning Saturday.(the NAM keeps these showers further west.) :
Latest NAM has heavy rain between 8 am and 11 am on Sunday:
Some changes in the forecast have cropped up since this morning.
Here’s the revised summary from earlier today: A series of weak disturbances emanating from a low pressure system in the Midwest will pass through our area Saturday into Sunday.
The forecast for Saturday has changed significantly. Some rain and scattered showers are expected to develop before daybreak and linger into the morning hours, tapering during the early afternoon. Both the NAM and GFS show this, but there are huge differences in timing and QPF values. The NBM maintains a moderate chance of light on-off showers through the day. High temperatures 72
The main surface low pressure system approaches Saturday night with heavy rain developing after midnight.
For Sunday and the Broad Street Run, depending upon the model, the heavy rain exits about 8-9 AM (GFS) or lingers later into the morning (NBM, FV3-GFS). The NAM is somewhere in the middle. Either way, low clouds, and some light sprinkles or drizzle in an easterly flow will make for a drab day. It will be windy in the morning. High 64
With such a dramatic shift in the forecast for Saturday, I would expect that additional changes in the forecast are likely for Sunday. Stay tuned.