Update Fri @ 7:18 AM — The models have trended towards the line of the heaviest rain shearing off north and west of the city. While we still will be getting some substantial rain, the remaining line of storms may weaken as it passes through Philadelphia about 9 AM.

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Update Thu @ 5:03 PM — Based on Thursday afternoon’s models, it appears that the cold front will move through about 7 -9 AM Friday morning. There’s enough CAPE to produce some thunderstorms and enough precipitable water to produce some briefly heavy downpours. Wind gusts 30- 40 mph, according to the HRRR. Rain until about noon. Most of the models have clouds breaking for sun in the afternoon.

A period of showers possible early afternoon Saturday with the second cold front, as described below.

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Update Thu @ 10:26 AM — The forecast continues to clarify somewhat (based on the GFS), with rain from the first front moving through after midnight tonight (Thurs) and tapering and ending around noon on Friday. Gusty winds Friday morning. About 1 inch of rain possible, more to the north and west. Partial clearing possible Friday afternoon. The model blend (NBM) is moving closer to this GFS forecast of rain ending earlier on Friday.

The morning HIRESW models have joined the GFS with rain ending late morning Friday. Any showers mid-day Saturday and Saturday afternoon with the second front appear to be light.

The first front stalls to our south as low pressure develops along and moves up towards us on Saturday. The GFS has additional showers developing around noontime on Saturday as this stronger cold front moves through. Gusty winds about 30 mph. It should be noted that the NBM still keeps us dry on Saturday, indicating there is a lack of model consensus with the GFS’s forecast but the latest HIRESW-FV3 (closer to the GFS in data input and structure) has considerable rain for Saturday afternoon.

The model blend (NBM) tends to be a lagging forecast, by its very nature and design. When there are changes in the forecast, the NBM can be slow to reflect those changes.

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Update Wed @ 8:28 PM — The GFS continues to forecast the rain ending late Friday morning with the passage of a cold front with partial clearing possible in the afternoon. The model blend (NBM) continues to prolong the rain through much of Friday afternoon. I’m leaning towards the GFS model at this time.

The GFS consistently shows the colder air hanging back on Friday and then moving through Saturday with a second cold front that is swept through by low pressure that develops along the first cold front. So Saturday look like it will have some rain for at least the first part of the day. The model blend (NBM) has us dry. Clearly, there are uncertainties with the forecast.

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Update Wed @ 7:56 AM — Forecast differences between the GFS (which has the rain ending Friday morning) and the NAM / SREF which has the rain remaining through much of Friday. The model blend (NBM) sides with the NAM and lingering rain.

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Update Tue @ 8:55 PM — Temperatures today were higher than forecast. A weak front moves through before daybreak Wednesday so tomorrow’s high temps may be closer to the mid 60s.

The model trend is for Friday’s cold front to move through our area somewhat earlier than had been previously forecast. Showers may begin as early as the evening Thursday. It appears the front will move off the coast Friday morning, so Friday does not look as rainy as had been previously forecast.

This afternoon’s GFS shows that second low, mentioned in my post below, forming along the front bringing some rain Saturday morning, This will sweep a secondary cold front through Saturday. Not all models on-board with this GFS forecast.

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Initial Post Discussion—[post_published]

The beautiful weather (although chilly) we’ve been having is expected to continue through much of Thursday. Moderating temperatures expected through the week with highs reaching the low 60s. Average highs are 55º (Blue Bell) and 57º (Philadelphia).

Deep low pressure over the Great Lakes will pull a warm front, then a cold front through our area.

The warm front is forecast to move through after midnight Thursday with the cold front following during the morning on Friday. Windy conditions will accompany both fronts. About 1 inch of rain is forecast.

A second low may form along the front, prolonging the rain. Much colder weather is expected for the weekend and is forecast to last well into next week.

As mentioned earlier this week, a coastal system will form in the western Atlantic but the upper air flow will move it east of our area.

For our area, high pressure extending down along the east coast will both block the northern movement of the coastal low and provide continued somewhat colder than seasonal average temperatures for the weekend.

The coastal low will throw some moisture back towards us, resulting in some periods of cloudiness, mostly of the higher cirrus type, on Saturday and a bit more cloudiness on Sunday.

Average seasonal high temperatures are in the 57-58º range, but we’ll likely be a few degrees below that range.

So, a relatively quiet period weather-wise for the weekend and several days afterwards as moderating temperatures move back next week. Another cold outbreak the end of next week.

And, of course, it’s the end of Daylight Saving Time on Sunday. The only good thing I can think of about the change back to Standard Time is that the new weather models, particularly the GFS, are available before bedtime.

Update Tue 7:25 AM— The forecast models have all moved away from a coastal storm for Thurs night and the weekend. Any storm developing will move south and east, away from us.

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Update Mon 8:12 PM — Watching the low possibility of a coastal low this weekend.

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Cold air descending from Canada will cause a dip in the jet stream and bring colder, below seasonable average temperatures to our area from Tuesday into Friday. (Average seasonal highs are in the 58-59º range.)

The front that precedes that colder air will bring some showers on Tuesday.

Most models show that any possible low pressure development from interaction of this jet flow with a southern stream jet impulse will be weak and remain off-shore.

Of interest is the current SREF and ICON models, which show enough cold air to the north aloft to support snow (red line is the 540 thickness line) but surface temperatures currently forecast well above freezing. (Click on image for a larger view.)

Stay tuned.