Sun 05:25 PM Forecast Review — An overly optimistic forecast.  I can’t blame this on the models. I just bet on the wrong horse (HREF).     This is as close as the clearing got to us this afternoon  —

Visible Satellite at 4:45 PM

Areas of Chester and Delaware counties had clearing, but I use Blue Bell as the forecast focal point.  The clearing may make it a bit  further north as low pressure has begun to develop over the Delaware border .  Showers and possible thunderstorms still expected later this evening and tonight. 

Going with the latest HREF (High Resolution Ensemble Forecast) model forecast.

Here are the current conditions:

Current Conditions at noon  (Click on image for a larger view.)

HREF forecast: Showers taper off noon- 2 PM. Clouds thin and break for some sun between 3-5 PM.

HREF forecast showing clouds thinning by 3-4 PM with developing areas of vertical motion to our west with residual activity to our far north.  (Click on image for a larger view.)

More showers and thunderstorms between 9 and 11 PM


Forecast Updates Highlighted
Sat 10:40 PM: Updated Sunday Forecast below

Sat AM Update: No significant change. Low clouds linger this morning. Brightening skies through high level cloudiness Saturday afternoon. No rain. Sunday’s forecast below still remains on track.

As mentioned earlier in the week, the timing of showers this weekend will make a difference in the overall forecast and it was hoped that much of the shower activity will occur during the evening hours. I started preparing this forecast this morning. By this evening, things are less clear for Sunday. More about this below.

The omega block influencing our weather is slowly breaking down, as low pressure in the Midwest slowly moves eastward. The upper level ridge of high pressure will maintain its hold on us for Saturday, acting like a wall against the low pressure advance, but it will get squeezed out of the way by Sunday.

SREF model forecast for Saturday 8 PM showing the upper level high.  (Click on image for a larger view.)


Showers rotating around the low in the Midwest will move in Friday night but will exit early Saturday morning.

Saturday will be mostly cloudy in the morning as moist air moves from the east. There may be a break in the clouds and some sun Saturday afternoon. High temp 70.8º sd 2.5º (NBM model Blue Bell).

Winds light (see meteogram below)


Nonetheless, the precipitation in Virginia (see above graphic) will advance into the Philadelphia region later Saturday night as a warm front. We’ll have showers and maybe some thundershowers late Saturday night.

Sunday will start out cloudy with showers in the early morning, but some sun may break out by noon. Rain ending Sunday about noon to 1:30 PM.

The more sun we get (if any), the more unstable things will get and the Canadian, ICON and to some extent the GFS maintains the chance of scattered showers and thunderstorms in the afternoon.

There still is a question about the amount of sunshine in the afternoon and possibility of additional showers (the NAM and HRRR maintain significant clouds) but the pre-release version of the new HREF (used for much of the forecast this weekend) has clouds thinning for some sun, similar to today..

(The higher resolution models only forecast out to 48 hours and Sunday afternoon is currently beyond this range.) High Temp 74º ± 3º

Also uncertain is the high temperature. The latest NBM has moved the high temp down to 67º but it has the standard deviation very large at 4.6º, showing the uncertainty in cloud cover and position of the warm front.

A weak cold front moves through Sunday night. Showers and possibly thundershowers move through between 6-9 PM Sunday night. Thunderstorms again in the evening 6-11 PM.

Winds for Saturday and Sunday

NBM wind meteogram  (Click on image for a larger view.)

I expect things go clarify with tonight’s models. Check here for updates during the weekend.


Forecast Updated Friday 8:38 AM is highlighted.

The omega block mentioned in the previous post has brought persistent nice weather for much of this week.

The blocking pattern will slowly break down as low pressure in the middle of the country pushes eastward and spawns a secondary low off the Delaware coastline later Sunday (as previously forecast).

In terms of timing, we will luck out as much of precipitation that moves in during the weekend will occur in the nighttime Saturday and early morning hours of Sunday and again later on Sunday.

Saturday will be mostly cloudy but with little or no rain. High near 70º
It does appears that Saturday will be mostly cloudy but rain-free for most of the daytime hours.

Rain moves in late Saturday night. Models differ on timing, but much of the rain will end early morning Sunday. We may get some clearing mid day Sunday. High Sunday 72-75º

GFS showing secondary low off Delaware coast Sunday evening with Thundershowers in our area (Click on image for a larger view.)

A cold front moves through later on Sunday afternoon or early evening with rain and thunderstorms likely, enhanced by the secondary low pressure system developing to our south. Following rain in the morning, Sunday should be rain-free from late morning to very late afternoon/early evening.

I’ll be pinning down the timing of the rain with my regular Weekend Weather Forecasts

Weather… and other things "up in the air"

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