Updated Thu 10:35 PM — Tonight’s models have moved away from widely scattered showers for Friday. Additionally, they’ve moved away from Friday’s cloudiness as well. Looking at the humidity fields, I wouldn’t be surprised if there’s some periods of low clouds on Friday. I guess we’ll see.

Updated Thu 7:30 PM — Friday’s weather forecast will be interesting. Many models show sunshine and fair weather clouds. The higher resolution models show sunshine with periods of clouds and very widely scattered sprinkles from mid morning into the afternoon, especially northern areas.

Update Thu @ 11:40 AM — The moderate rainfall we’re getting Thursday morning will taper off about 2-3 PM Thursday afternoon. Clouds linger with a few very widely scattered showers.

Things improve considerably Friday through Sunday as the low pressure system that has been persistent will move away. High pressure will dominate through most of Sunday. A southwesterly flow of more summer-like air will move in.

Update Wed @ 7:58 PM — The most recent models show rain lingering into early Thursday afternoon before ending. We may see some light rain as late as 2-3 PM. Considerable lingering cloudiness. Some heavier rain expected in the morning Thursday; here’s the latest RAP model accumulated rain—

Updated Wed 6:02 PM — Current radar and water vapor imagery around 5:40 PM looks pretty close to the 06z HREF forecast (2 AM run) from this morning—

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Update Wed @ 7:42 AM — With the backdrop of low predictability, today’s weather: partly sunny with increasing cloudiness during the afternoon. Most of the shower activity holds off considerably in our area, as we’ll be in the middle of two areas of showers. Any showers we get will be light and scattered before 6 PM and any evening showers look light and scattered as well.

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Updated Tue 6:59 PM — The lack of precise predictability of the upper level low’s position off the New England coastline is already affecting today’s forecast— the narrow band of showers expected to move in here this evening will likely pass to our east.

The same upper low is now expected to spawn a surface low near the Carolina coast; the path of this low is expected to move northward. As you can tell, this is a complex scenario.

As best as we can predict, the next band of showers will set up in central Pennsylvania on Wednesday and will mostly stay to our west, although some shower activity may affect our area, especially late afternoon and evening on Wednesday.

The shower activity late Wednesday will be with us Thursday morning, but things dry out by Thursday afternoon.

The upper low is expected to reform in the same general position as it is in now. Forecast uncertainty will remain.

Currently the weekend looks good.

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Update Tue @ 9:30 AM — This week’s forecasts are totally dependent upon the movement of an upper level low off the New England coast. Last night’s models show this low moving southwestward back towards us on Wednesday.

In the meantime, a warm front in western PA will attempt to move closer to us this afternoon with showers and possibly thundershowers breaking out this afternoon between 5 and 9 PM.

The difficult to forecast position of the upper low and the difficult to time disturbances moving down from the north will determine our weather over the next few days.

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Updated Mon 10:41 PM — Tonight’s models have clouds with some breaks on Tuesday. Much reduced chance of showers during the day, but several models have showers moving in between 4 and 8 PM.

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Update Mon @ 4:59 PM — The models continue to keep the very hot weather in the center of the country with a upper low pressure trough in the northwest Atlantic.

For a change of pace, we’ll look at the ICON model which captures the current pattern and the outlook—

Tuesday’s forecast has already changed from what it was a day earlier. While much of Tuesday will be cloudy, the trend has been for any showers to be very light and widely scattered.

However, there is an increased chance of showers, possibly thunderstorms, about 5 PM on Tuesday.

Wednesday looks to have more rain than Tuesday.

Thursday may continue to have showers., especially east into NJ. Again, this week’s forecast is unusually low confidence.

Stay tuned.

Previously Posted Sun 9:51 PM —

The persistent pattern of an upper ridge in the mid section of the country and an upper trough over our area will shift a bit over the week, but we’ll still be in the upper trough to some extent. Temperatures in our area will be near or below average.

As the caption above explains, there’s uncertainty in the forecast for us.

So far, the models are showing several periods of showers/rain: Tuesday, late Wednesday into Thursday and maybe even a period on Friday.

Needless to say, this past weekend wasn’t typical weather for June and the coming week won’t be either.

No sign of the usual summertime Bermuda High effect; it’s well out to our east.

I expect changes in the forecast as the week unfolds. Much depends on the position of that upper low. Stay tuned.

Update Fri @ 9:51 AM —

Updated Thu 9:35 PM — The latest HRRR has backed off the severe storm threat for us. In fact, it’s showing no rain! Radar shows storms losing their punch as they move towards us. Wow, what a forecast change!

Update Thu @ 8:33 PM — As was the case last night, the “seed” for the storms tonight is visible on satellite water vapor imagery—

It should be noted that not all models are showing storms for the immediate PHL area. Some show only very scattered activity. The issue is that surface CAPE falls off very rapidly as one goes towards NJ.

Nonetheless, the latest HREF shows some heavy rain as early as 10 PM. I’m not sure we’ll see storms that early.

Update Thu @ 6:14 PM — A warm front was supposed to have moved through on two previous occasions Wednesday and Thursday. One didn’t have to be a meteorologist to know that it hadn’t moved through; our temperatures and dew points today relatively low. But tonight’s the night!

The models have the warm front moving through around midnight with more showers and thunderstorms. These storms may pack a greater punch than those that moved through this morning. Indeed, there’s the possibility of severe storms.

Update Thu @ 9:09 AM — Current models show that additional storms associated with a cold front late this evening will significantly diminish in strength as they approach Philadelphia.

Updated Wed 10:50 PM — Tonight’s early models have become available. A very interesting scenario is setting up .

First, the seed of these potential storms shows plainly on water vapor imagery—

All models show storms developing and moving through our area between 5 AM and 9 AM. The NAM-NEST forecast suggests a bow echo storm possibility.

This is an unusual setup. There’s almost no CAPE from Philadelphia eastward. Just to our west, there’s a sharp gradient to much higher CAPE. Yet the models show very impressive storm development and heavy rain, especially west. Additionally, there’s high helicity just to our west.

Based on the severity parameters, I would ordinarily discount any possibility of severe weather Thursday morning except far western suburbs, but the models still have very strong storms forming and moving east. Tomorrow will be a learning experience for me.

Update Wed @ 5:51 PM — This afternoon’s 2 PM (18z) models have become available. All models show a disturbance dropping down in the upper wind flow, similar to Monday morning’s setup.

The rain and thunderstorms are forecast to come through between 5 AM and 8 AM Thursday morning with some models having additional isolated storms popup around 10 AM.

There is the potential for strong storms specifically in the western sections of our area. It appears the largest hazard may be brief heavy rains, although the HRRR shows elevated hail parameters and cloud tops suggestive of severe storms (??).

The next major model runs will become available between 9:35 (HRRR) and 10:16 PM (NAM-NEST) I’ll update before 10:45 tonight.

Update Wed @ 9:55 AM — A persistent upper air pattern has been with us over the past week or so. Any change has been short-lived, as the same pattern reasserts itself. The upper ridge pattern in the middle of the country today is very similar to what it will be this weekend—

But before we get to the weekend, a warm front moves through late tonight and early Thursday morning with clouds and some showers/possible thunderstorms early Thursday morning—

Clouds clear for some very warm humid weather Thursday afternoon. A cold front moves through late Thursday night. Current forecasts have most of the activity to our north with this front.

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Updated Tue 9:40 PM — Uneventful weather through the weekend. A warm front moves through Thursday morning with showers. A cold front moves through Thursday night with some showers.

Beautiful weather for the weekend.

Update Tue @ 11:18 AM —

Forecast Review— My mantra “Never ignore the NAM” proved itself, despite different forecasts from the RAP, HIRESW and HRRR; my inclination last night was to ignore the NAM.

Anyhow, the heaviest activity did stay to our south, affecting the southern areas of the city, southern Chester County and southern Delaware County.

It seems that the models had trouble predicting the two centers of activity- the intense thunderstorm complex that entered West Virginia and the area that moved in around us.

Most models predicted only one center, with most latching onto the energy that went far south into West Virginia. The NAM and NAM-NEST latched onto the energy that affected our area.

Updated Tue 6:59 AM — The disturbance has evolved into two separate complexes not forecast by the models; one may hit Philadelphia shortly. One has a bow echo DERECHO appearance.

Updated Mon 10:54 PM — Of tonight’s early models, only the NAM and NAM-NEST have the rain and thunderstorms north enough to affect Philadelphia.

As shown in the previous graphics, tonight’s RAP, HRRR, HIRESW, HIRESW-MEM2, HIRESW-FV3, the afternoon HRDPS and Canadian GEM have the storms missing us and moving to our south

I’m leaning towards the storms missing us. (Or is this a scenario, “never ignore the NAM”?)

Updated Mon 9:42 PM — Tonight’s 00z HRRR just became available and supports the forecast that this disturbance will pass to our south—

Updated Mon 8:23 PM — The latest RAP model and Canadian High Resolution (HRDPS) has the disturbance moving far to our south on Tuesday, totally missing our area—

So there’s uncertainty whether we’ll see anything from this small but potent system that will move from northwest to southeast.

Previously Posted Mon 6:24 PM —

A strong upper level ridge is to our west. Riding down the ridge will be a disturbance that affects us on Tuesday morning—

A strong upper air disturbance will develop a weak low pressure system as it slides from northwest to southeast early Tuesday morning. CAPE values are expected to be low in our area.

On, Tuesday early showers (and thunderstorms to our south and east) will move through from the northwest, ending between 2PM and 4 PM. Any heavier activity will move south of us.