Sun 09:59 PM Forecast Review — The models did well with the forecast for the storms to diminish as they approached Philadephia Sunday evening.  
 

I’m trying to get past yesterday’s model forecast failure in the immediate Philadelphia area. Anthony Wood wrote a Philadelphia Inquirer article which mentions the forecast failure, but the expert he interviews provides an excuse, not any real answer, to explain the forecast failure. With so many models incorrectly forecasting heavy rain/storms in Philadelphia proper, the only thing I can postulate is there was some essential upper air data that was missing and/or systematically incorrect. But let’s move on…

Updated Saturday forecast, Fri 10:30 PM highlighted below
Updated Sunday forecast, Sat 8:30 AM highlighted below
Updated Sunday forecast, Sat 7:40 PM highlighted below
Updated Sunday forecast, Sat 11:40 AM highlighted below
Updated Sunday forecast, Sat 9:15 PM highlighted below

The front that moved through here Thursday is south of us, but the boundary isn’t that far south as shown on today’s water vapor image—

Saturday

The cool air that’s with us on Friday will remain through Saturday, but aloft, warm humid air will try to return northward. Some cloudiness will develop as this moisture tries to return and will affect areas from the city southward according to the GFS. Any cloudiness dissipates by the afternoon. (The NBM has very little cloudiness.) Still a very nice day with high temperatures 78.7º ± 2.2º Blue Bell (NBM) and very low humidity.

Sunday

Sunday’s weather will be influenced by a warm front, followed by a strong upper air disturbance with a cold front moving through Sunday evening.

The warm front will continue to push from the south aloft. This will result in cloudiness and some scattered showers Sunday. The NBM has considerable showers, the GFS not so much.

Update: Today’s GFS joins the other models with showers/thunderstorms for most of Sunday, starting during the morning and continuing through the afternoon as the warm front moves through. There may be a break in the action late afternoon. Additional showers/thunderstorms during the evening, about 9 PM.

Update: Most models have the line of thunderstorms Sunday evening weakening as it approaches Philadelphia from the west; they may not make it into the city or eastward.

At 9 15 PM, there hasn’t been much weakening of the line of storms. We’ll see what happens..

Update: The GFS maintains very little in the way of showers during the day. Any showers/thunderstorms in the evening will be mostly north and west of the city, less into Philadelphia. High temperatures 77.9º ± 3.3º Blue Bell (NBM) with higher humidity/dewpoints in low 60s.

As the cold front moves in Sunday evening, more showers and thunderstorms are likely. Current estimates are between 6-9 PM.

This upper air impulse (above graphic #1) will be the kicker for a very amplified jet flow next week. We’ll be on the cool side of things, but the amplification will spawn low pressure that, depending upon the exact track, may provide very unsettled and rainy conditions, especially by next Thursday.

Updated on Sat 9:12 AM highlighted below

The cooler, less humid air mass that moved in will continue to influence our weather on Saturday. Two weak fronts will move in Sunday.

Saturday

Saturday will be partly sunny. Return of moisture aloft may result in some cloudiness in the afternoon with a slight chance of an isolated shower far ahead of an approaching warm front. High temperature 86.0º ± 1.3º NBM model Blue Bell. Dew points in the upper 50s to near 60º.

Sunday

A weak warm front will move in before daybreak on Sunday. Some scattered showers before daybreak. Clouds early will break for sunshine by afternoon.

As a result of this warm front, Sunday will have noticeably higher humidity (dew points) than Saturday. Dew points will move back into the uncomfortable 70º range. High temperature 87.0º ± 2.2º NBM model Blue Bell.

A weak surface trough may move through on Sunday evening with scattered showers and maybe some thunderstorms especially in western suburbs.

Outlook

While we’ll be moving back into the 90º+ range by Tuesday, the extreme heat being forecast for the middle of the country does not appear to make it into our area because we’ll still be a slightly different upper air situation—

Updated Sunday forecast Sat, 7:30 PM highlighted below.

Update Sat 6:55 PM: We had some storms during the afternoon, mostly in western suburbs and Bucks county. This afternoon’s models (18z) are still advertising strong dynamics in Philadelphia as the main batch of storms develops and moves through between 7 and 11 PM

Updated Fri 6:30 PM with latest GFS timing highlighted below
Updated Fri 7:15 PM with latest RAP timing highlighted below
Updated Sat 7:15 AM with latest NBM meteogram highlighted below

Low pressure near the Great Lakes will send mid-level and upper air disturbances into our area Saturday and Sunday causing periods thunderstorms and plain rain.

So what is the most likely timing of the thunderstorms and rain on Saturday and Sunday?

Saturday

Based on the latest GFS and HRRR models, some light shower activity is possible around daybreak Saturday morning. Expect considerable sunshine with some building cumulus clouds for most of the day.

The HRRR shows some widely scattered thunderstorm activity possible as early as 1-2 PM.

Most models are on board with very heavy thunderstorms late afternoon and evening. Peak time 4-8 PM. Earlier in western suburbs, a later in NJ. This will be a wide line of thunderstorms that will move from west to east. Friday afternoon’s GFS shows thunderstorms beginning 7- 8 PM Saturday evening.

This afternoon’s RAP maintains the possibility of earlier storms, in line with the meteogram graphic of the NBM below.

Be advised that the upper air dynamics for these storms is considerable and impressive. Severe weather and heavy rain likely. CAPE values >3000 J/kg and Lifted Index less than -8. Precipitable water is about 2″. Friday afternoon’s RAP shows CAPE values as high as 4000 J/Kg!

High temperatures 93.8º ± 1.6º (NBM Blue Bell)

The trigger for these heavy thunderstorms will be a very intense upper air disturbance. The timing is best captured by a meteogram, showing probability of thunderstorms—

Updated Meteogram

Sunday

The procession of disturbances across our area will continue.

Following some early morning showers, considerable low cloudiness. Some sun will likely break out at times but any sun will heat things up to get some thunderstorms going.

Friday afternoon’s GFS shows very little rain on Sunday. The RAP shows afternoon thunderstorms, similar to the NBM forecast.

Updated Saturday 7:30 PM: The latest models have a party to mostly sunny Sunday. A slight chance of an afternoon scattered thundershower northern suburbs.

Most of the precipitation will be scattered rain showers. The model blend (NBM) paints a very cloudy and rainy day picture, but I think it’s likely we’ll get some sun at times, based on the NAM and HREF. High temp 83.7º ± 2.7º (NBM Blue Bell.)

Look for updates over the weekend.