Updated Sun 11:22 PM — Tonight’s models show thunderstorms of greater intensity as the front moves into New Jersey early afternoon.
Update Sun @ 7:38 PM — Most models show the timing of the front as described below. But, the latest RAP model and GFS have the main wave of the front moving through much earlier. The GFS has showers starting as early as 10-11 AM and the RAP as early as 8-10 AM.
Update Sun @ 5:44 PM — A cold front will move through our area Monday morning through early afternoon with showers and thunderstorms.
The front is likely to come through in two waves— an early period 6-8 AM with very widely scattered showers and storms. Another wave approaches about noon to 2 PM with a more distinct line of showers and thunderstorms. No severe storms expected here. Heavy rainfall (1-1.5 inches) in some locales will be the main impact.
Today’s 18zNAM forecast for 2 PM showing heavier rains (3 hour totals) north of the immediate PHL area as well as southward. Some areas may have 1.5″ rain, some may have less than 1/2 inch. (Click on image for a larger view.)
Since the front is coming through early, daytime heating effects will be minimized. This will reduce the likelihood of any severe weather. The energy with this front is distributed in two waves, also reducing severity. Possibly stronger storms will be nearer the Jersey shore and southward into Delaware.
Updated Sat 10:35 PM — Current Sunday forecast looks good. High temps 90-92°, dew points low 60s. The approaching cold front won’t affect us until Monday morning.
Showers and thundershowers Monday morning and early afternoon with the cold front likely will not be severe in our area. Some areas of heavy rain possible.
The upper air low pressure system will finally move out allowing high pressure and an upper air ridge to move in bringing much warmer conditions, above seasonable average temperatures. (average highs are 83º Blue Bell and 85º Philadelphia)
Today’s 18z GEFS shows the upper ridge (1) the upper low (2) and a cold front moving towards us (3) late Sunday. By the end of the week, the typical Bermuda high pressure (4) will move into its usual position bringing more heat and humidity. (Click on image for a larger view.)
Sunny and compared to recent conditions, very warm. Borderline humid with dew points in the mid 60s. High temperature 88.7º ± 2.0º NBM model, Blue Bell PA. There’s a chance of very widely scattered pop-up thundershowers, mostly far northwest of the city during the afternoon.
Sunday
Sunny and somewhat warmer. Lower dew points in the low 60s. High temperature 90.5º ± 3.5º NBM model, Blue Bell PA.
Today’s 19z NBM Model high temperatures Sunday. Black contours are 2 degree increments. (Click on image for a larger view.)
A cold front moves in late Sunday evening and overnight with showers and thunderstorms into Monday morning.
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.
GEFS (mode) forecast for 2 PM Sunday. Cold front in Ohio Valley moving eastward will arrive late Sunday or Monday. High pressure in western Atlantic (H) has replaced the upper low and will bring much warmer, summer-like temperatures.(Click on image for a larger view.)
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—
Today’s 21z RAP model accumulated rain forecast. (Click on image for a larger view.)
Updated Wed 6:02 PM — Current radar and water vapor imagery around 5:40 PM looks pretty close to the 06zHREF forecast (2 AM run) from this morning—
Radar/water vapor imager from 5:40 PM Wednesday shows the lack of rainfall in the immediate PHL area. The similarity with this morning’s HREF forecast (image below from 7:42 AM) is probably as good as it gets.
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.
This morning’s HREF forecast for 6 PM shows Philadelphia between two areas of rain. This feature is shown by several models. The heaviest rain remains far west of Philadelphia today and as it moves into our area from the west late tonight, much of the heavier activity diminishes as it moves towards us by Thursday morning.
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.
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.
Water Vapor image Tuesday morning: Warm front (red line) will attempt to move closer towards us as an upper air disturbance ( blue area with white arrow) moves over us late Tuesday afternoon, sparking showers and possibly thundershowers. The upper low (L) will move closer to us on Wednesday, impeding the movement of the warm front. (Click on image for a larger view.)ECMWF upper wind forecast (jet stream) for Wednesday evening shows the upper low (L) having moved southward and westward.
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.
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.
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—
Monday 12z ICON model forecast for Thursday: large hot ridge over Texas with impulses moving across the top (2). Upper low and highly amplified trough (1) in the western Atlantic. “Bermuda High” pressure(3) not really near Bermuda. The uncertainty around the position and configuration of (1) and the impulses (2) makes for a highly uncertain forecast for later in the week. (Click on image for a larger view.)
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.
Water Vapor imagery Monday afternoon shows disturbance ahead of warm air (orange-red). We’re on the eastern edge of a warm front boundary. Whether it moves west or east will depend upon the position of the trough in the northwestern Atlantic. This will determine both the amount and timing of any showers and storms this week. (Click on image for a larger view.)
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.
Originally 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.
ECMWF jet stream winds (250 mb) on Monday. The uncertainty for the week: in what position the upper low north of Maine will be later in the week. The strong accentuation of this eastern trough will diminish and there’s large uncertainty whether the low will cut off and move southward or even southwestward! Too much uncertainty between models. (Click on image for a larger view.)
As the caption above explains, there’s uncertainty in the forecast for us.
Tonight’s Water Vapor image shows the forecast dilemma. We are in the very dry flow from the north. If the jet position shifts slightly eastward, we’ll be in the path of numerous disturbances moving in from the Great Lakes. (Click on image for a larger view.)
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.
