The forecast for the heavy rain and our weekend weather has improved considerably over the past 24 hours.
The heaviest rain is now expected to stay to our south and the low pressure system that had been expected to linger through the weekend is now forecast to move out to sea.
On Saturday, rain will end before 8 am. Skies are expected to be cloudy, but some sunny breaks are possible as high pressure moves in from the west. High temp near 80. It will be windy.
Sunday looks good with sunny skies and high temps near 85.
Friday Late Morning Update: Latest NAM model have the heaviest rain (2-3 inches) south of PHL. The NAM has the heavy rain accumulation tapering off rapidly as one goes to the north.
Below is the NAM 3KM forecast for accumulated precip by 8 AM Saturday.
Just a quick update. The models have been consistently advertising a very heavy rain event for Friday night into Saturday… until last night.
The most recent model runs from last night have shifted the very heavy rain to south of our area, into Delaware, MD and VA. It appears that the immediate PHL area will miss the heavy rain and areas north of PHL will be on the outer fringe of the rain shield.
This is not unlike the blizzards that get forecast, only to have the heavy snow occur elsewhere (or not at all).
The models are only so good in long time range- the general features of storm development are usually spot-on, but the details of moisture distribution and the location of the dynamics that go into heavy rain (or snow) formation often can’t be nailed down until 24 hours before the actual event.
Things may change again, but the current trend is for PHL to miss the heavy rain. I’ll update again later today.
BTW, I was looking at the models on Thursday and the maximum of heavy rain was very close to PHL….the models were suggesting 2-4 inches. What was strange, though, was that the statistical models were only showing 70% chance of rain. I couldn’t understand that, since with heavy rain like that being forecast, the statistical models usual show chances greater than 90%. Now it’s becoming clearer.
More sun today than I expected based on the GFS model forecast yesterday. (The GFS model was updated by the NWS this past Wednesday.)
Clouds start moving in 2-3 PM. Thunderstorms appearing likely as early as 5 PM and increasingly likely during the period from 5PM to 9 PM.
For Sunday, much of the day rain-free, but thunderstorms again appear likely late afternoon into evening.
A weak and poorly defined pattern typical of summer will make the specifics of the weekend forecast a bit more challenging to nail down.
A weak cold front is expected to sag south later Saturday and allow an area of showers and thunderstorms to move in late Saturday afternoon into the evening. It will be very humid during the day and high temperatures are predicted to be in the low 90s, although the GFS model is suggesting considerable cloudiness which may keep high temperatures somewhat lower.
Showers Saturday evening will taper off towards midnight.
The models are suggesting an unsettled Sunday with considerable cloudiness and a chance of showers, although some breaks of sun are possible. Timing of any showers is uncertain. Most of the day will be rain-free. High temperatures in the mid 80s. Remaining humid.
Heat is expected to return on Monday.