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.