Update Tue @ 1:07 PM — The latest ECMWF, ICON, Canadian R-GEM and our RAP model have been consistent forecasting strong to severe storms to be mostly northwest of the city and they will significantly reduce in intensity as they approach Philadelphia and move eastward.

So there are large differences in the high resolution and global models. Considering the dry period we’ve been in, I’m leaning again towards the storms losing much of their punch (and moisture/rain) as they move into the immediate PHL area and into NJ.

Here’s the latest Canadian R-GEM with it’s accumulated rain forecast—

Update Tue @ 10:34 AM — The short range high resolution 12z models are becoming available. All models show the storms moving from northwest to southeast beginning 4-5 PM in far northwestern suburbs and 6-11 PM from Philadelphia and eastward. The models show:

• Strongest storms, possibly severe, northwest of the city.
• Storms may diminish in intensity after passing through Philadelphia into New Jersey.
• Storms coming possibly in two waves, 5-8 PM and around 10-11 PM

Updated Mon 11:02 PM —Tonight’s NAM-NEST has the storms falling apart before they make it into Philadelphia.  Despite the NWS indicating strong storms tomorrow evening, I think Philadelphia and east will get only the weak remnants of any storms that affect our northwest suburbs.  However, tonight’s HIRES-W has a strong line of storms moving through between 5 and 7 PM. Much uncertainty.

Updated Mon 9:57 PM —Tonight’s models show the storms, some strong, making it into Philadelphia tomorrow between 5 and 7 PM, but diminishing significantly as they enter into NJ.  

Updated Mon 7:50 PM —Today’s models continue to show a cold front will drop down through our area late tomorrow afternoon.

Today’s models are not in agreement with the likelihood of thunderstorms in all areas Tuesday afternoon with the cold front passage.

As discussed with last night’s graphic, the models continue to show a barrier of warm, stable air that any storms will have to overcome.

Most models have scattered, strong storms in the northwest suburbs between 4 and 8 PM.

But, several models show these storms pushing against this warm mound of air over Philadelphia and losing their punch as they try to move eastward. So areas, especially from Philadelphia and eastward may not have much more than 0.2-0.3 inches of rain. (We need the rain.)

The 18z HRRR and HIREF models from this afternoon are the most aggressive with the rain here. But the 21z RAP, the Canadian GEM, Canadian HRDPS, the ECMWF, and the ICON don’t show much rain for the immediate PHL area and areas eastward.

As has been the case many times this summer, these storms blossom again near Baltimore MD and southern Delaware.

Our relatively dry spell of weather may not broken with only a few tenths of an inch of rain tomorrow.

I’ll update Tuesday morning after 10:15 AM with the new models.

Previously Posted Sun 9:20 PM —

Perhaps you’ve noticed that we haven’t been getting all that much rain this season (especially compared to the last two year’s endless barrage of severe thunderstorms.)

I’ve been noticing that storms have been falling apart as they approached. (Areas to our south in Delaware and Maryland have been getting much more rain.)

Frankly, the way things have been unfolding reminds me of the drought that developed here in 2000-2001. I haven’t heard anybody use the “d” word and I’m probably jumping the gun on this, but things are getting awfully dry in some areas.

Speaking of lack of rain, this week may continue this trend.

A disturbance is moving through the upper Midwest tonight, will intensify as it’s joined by other energy in Canada—

The GEFS model shows that disturbance—

We’ll see increasing heat and humidity Monday and especially Tuesday. Dew points may reach 70+º Tuesday with temperatures over 90º

Despite the setup for significant thunderstorms late Tuesday afternoon and night, some current models suggest that the heaviest rain will develop south of our area. (The GFS shows over an inch of rain for the city (but very little just north and west) and it had over-predicted the rain Friday night into Saturday throughout our area.)

Will the dry spell continue? Updates during the week.

Update Fri @ 1:13 PM — Latest models have the rain likely ending in the immediate PHL area between 12 and 2 PM Saturday. Clouds break later in the afternoon. There’s still a lack of complete consensus regarding when the rain ends. A steep gradient with the rain— southern areas will receive much more rain than areas north of Philadelphia.

Updates this evening with the regular Weekend Weather Forecast feature.

Updated Thu 9:28 PM —The latest ECMWF just became available. It shows a very rainy Saturday, quite different than the GFS and NAM-NEST.

Update Thu @ 8:27 PM — Friday looks to be very warm and humid. Most models have us fairly rain-free during the day, but the model blend has widely scattered showers. High near 87º ± 2.2º

Rain moves in after midnight Friday. Heaviest rain south of Philadelphia.

There are large differences in timing and placement of this rain. Also uncertainty about when it clears up on Saturday. The NAM-NEST keeps the rain south of us and has it lingering The GFS has rain in our immediate area but tapers after 11 AM. The model blend (NBM) has considerable rain throughout the day, but I’ve seen the NBM overly prolong rain duration.

So uncertainty about clearing on Saturday, but I’m leaning towards the GFS. Updates Friday.

Updated Thursday Forecast, Wed 10:43 PM —The high resolution models are available again tonight. Several show an area of rain showers moving through western, southern suburbs and Philadelphia before daybreak and ending about 11AM Thursday.  (The HRRR keeps all the rain south of our area.)  The sun breaks out after noon.

Update Wed @ 8:58 PM — Several changes to the forecast. (FYI, there have been problems with NOAA data servers this afternoon. Many models were unavailable at their usual times. Some models are still giving errors during download. With so many models missing, I’m not going to attempt to retrieve them; this forecast is based heavily on the GFS, Canadian RGEM , German ICON and ECMWF models.)

For Thursday, except for the possibility of some light very widely scattered showers mostly in the morning, much of the area and the day will be dry.

For Friday, the front that was expected to bring rain will arrive much later in the day. Much of Friday will be rain-free.

Unfortunately for Saturday, the front will be slow to move south. Some showers and considerable clouds on Saturday into early afternoon, especially areas south and east of the city. Areas of Maryland and Delaware will have very heavy rain in the morning. Some clearing during the afternoon.

Sunday looks good.

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Update Wed @ 9:40 AM —There are changes in the forecast for Thursday through Saturday. Significant differences exist between the ECMWF and the GFS regarding the position of boundary that is to our south and the disturbances that will ride along it for tonight and Thursday.

It now appears that showers to our south will not make it into the general Philadelphia area tonight. It’s unclear how much rain (if any) moves into our area on Thursday. The ECMWF is more aggressive with the rain Thursday than the the GFS. Will have to wait until more model runs come in.

Also unclear is the forecast for Saturday. Both the ECMWF and GFS have considerable clouds now for Saturday with a lingering chance of showers, especially in the afternoon and at the Jersey shore. So what appeared to be a clear cut forecast last night for the next few days will be not so clear cut. Updates this evening.

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Updated Tue 9:53 PM — A diffuse, stalled frontal boundary just to our south and will remain quasi-stationary through Friday. Upper air disturbances will move along this boundary to our south on Wednesday. Any showers will essentially stay to our south through Wednesday.  

It will be quite humid on Wednesday morning. Dew points above 70°, but will drop in the afternoon.

Thursday: This diffuse boundary will attempt to move back northward Wednesday night with some showers that will linger into Thursday morning. The boundary won’t make much progress, but with it somewhat further north, we’ll be in the path of additional upper air disturbances. Showers and thunderstorms likely late Thursday afternoon.

Friday: A stronger kicker will move through on Friday with showers and thunderstorms.  

The weekend looks good.

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Update Tue @ 5:48 PM — An unsettled week weather-wise coming up. No strong systems. Several chances of some showers or storms. The upcoming weekend weather looks good!

Tue @ 5:46 PM — Forecast Review— As expected, the storms moved off mainly to our south and north. Not much happening in the immediate PHL area. The showers stayed together in NJ more than forecast but no strong thunderstorms with such low CAPE values. The HREF forecast graphic below probably did the best.

Update Tue @ 3:47 PM — Latest HRRR continues with the strongest storms moving south of Philadelphia and another batch north of Allentown.

The lower resolution ECMWF does show some activity into Philadelphia. (I waited years to get some access to the ECMWF data. Adding the ECMWF to my forecasting toolkit this past 6 months hasn’t really improved my forecast accuracy, whether it be forecasting snow storms or thunderstorms.)

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Update Tue @ 11:34 AM — The morning high resolution models are showing any storms will be scattered in two groups and less impressive than even previously forecast. Many areas may not receive any rain.

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Update Tue @ 9:50 AM — This morning’s models continue with a forecast of a scattered group of storms affecting our area this afternoon and night. This likely will NOT be an organized solid line of storms.

As was the case last night, the high resolution models show two scattered groupings of storms, one that moves north (Allentown area) and another that moves mostly south of us.

The NAM-NEST shows the storms move through in two waves. One about 5 PM and another about 11 PM. They also move through in two general groupings, north and south of PHL.

The latest HRRR shows a somewhat similar scenario for 6 PM—

As discussed just below, CAPE values diminish significantly from west to east. Storms will diminish as they move into NJ.

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Previously Posted Mon 9:09 PM —

A complex disturbance will affect our area late Tuesday afternoon or evening. There are large differences in the model forecasts, with a number of models having any storms either weaken and/or move off to our north.

The overall jet flow is depicted here —

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The jet flow complexity over our area is shown in the current water vapor imagery—

The models have one thing in common: they’re forecasting CAPE values that are high west of Philadelphia but rapidly diminish east of the city.

Here’s the HRDPS which captures this—

The trajectory of the impulse visible on water vapor (1 above) varies with each model. The NAM-NEST has strong storms north of our area, near Allentown.

The latest RAP and HRDPS show strong thunderstorms about 4-6 PM moving into the city, but dissipating as they move east. The ICON has the storms closer to 7-9 PM and the ECMWF has most of the strongest activity moving to our north, like the NAM-NEST.

The impulse expected to trigger these storms is quite a distance away at this moment. I expect significant changes in the forecast by tomorrow morning. A couple of things worth noting: CAPE values to our west are high and vertical shear is also forecast to be highly elevated. Helicity values also elevated, especially northern Bucks county. Precipitable water (PWAT) values also very high (2.2-2.4″) indicating potential for heavy rains.

Check back for updates tomorrow.