Fri 12:38 PM — Some of this morning’s models are beginning to show a more westward loop in Henri’s track, possibly affecting our area more directly as it loops westward. An evolving forecast…

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Fri 7:00 AM— Based on comparison of current water vapor imagery and model forecasts, it appears that the upper air low is further south than forecast by the models. This could have the effect of pulling Henri closer to the coastline. Just a conjecture. As for Friday as a ‘test forecast’, last night’s HRRR and NAM did the best with the radar location of this morning’s light showers.

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The coming weekend weather will be influenced by an upper air low pressure system, the residual moisture of Fred, and the moisture outflow of tropical storm (possible hurricane) Henri. The result will be a complex forecast situation based on an unstable atmosphere with periods of sun, clouds, showers and thunderstorms.

Sky conditions will be similar to today (Thursday) with periods of sun, periods of dark clouds. The big unknown will be the exact timing and placement of showers/thunderstorms.

With daytime heating and weak non-specific triggers, the best bet will be that most of the shower activity will be in the afternoon and evening, when thermal instability is at its highest.

PWAT values will be in the high 1.9” to 2.1” range, meaning that any slow moving storms can drop significant rainfall in a short period of time.

Friday will be a good test of the models with this setup. Friday, the upper air low will trigger the development of a weak surface low just to our south. The models have showers moving in mid to late morning and lasting into the early afternoon. (From Philadelphia and south) We’ll see how things play out tomorrow.

Henri may strengthen from a tropical storm to a hurricane but is still expected to stay far enough off the NJ coast to only bring indirect effects of surf/wave effects at the shore and outflow moisture enhancing the already unstable atmosphere in place here. Of interest is that the ICON model has been showing the possibility of a jog westward and looping track as it gets to the eastern end of Long Island.

Thu 7:00AM— Some areas got very heavy rain. The activity did not make it east of Philadelphia. The tornadic potential was luckily over-forecast.

Henri looks to remain far enough off the Jersey coast, affecting Long Island and Cape Cod later Sunday-Monday.

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Wed 10:05 PM—Latest models are looking less impressive with activity getting east of Philadephia, as current storms rotate up northward instead of eastward.

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Wed 7:30 PM— Latest HRRR has slowed down the passage of storms in the immediate PHL area. Current timing is between 11 PM and 2 AM. The HRRR also shows some areas of high helicity and gusty winds with the passage of these storms in our area.

Still watching Henri which may indirectly affect NJ on Sunday.

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Wed 1:20 PM— Latest GFS has the thunderstorms moving through our area somewhat earlier, starting about 5:30 PM. Strongest dynamics remain to our far northwest, closer to Allentown.

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Wed 7:14 AM- Last night’s models have moved the chance of tornadic activity a bit more westward into Lancaster, Berks and Lehigh Counties. Most likely time for thunderstorms in our area 8-11 PM tonight.

As for Henri, last night’s models have it encountering blocking high pressure and an upper low with it moving more westward, now impacting Long Island with indirect effects possible at the Jersey Shore on Sunday. This complex situation continues to result in a changing forecast.

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Tues 10:15 PM— Tonight’s models just becoming available show very strong dynamics ( specifically “helicity” and upward vertical motion ) between 7 and 10 PM Wednesday evening, especially west of the Delaware and specifically in Chester and Lehigh counties. Tornadic activity is possible.

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Tues 8:53 PM— An interesting and highly complex weather scenario is unfolding this week as the remnants of tropical storm Fred move up the Appalachians Wednesday into Thursday and a new tropical system, “Henri” moves up near the coastline over the weekend.

First, Fred. The moisture remnants of Fred will stay mostly west and northwest of our area with most of the heavy rain also staying west of Philadelphia and its immediate surrounding counties. The model forecast has been very consistent with this path.

Rain should move in during the evening hours Wednesday and depart by Thursday morning.

More interesting is a dramatic shift in the forecast path of Henri, developing in the Atlantic. It appears that an upper air closed low may draw Henri closer to the coastline.

While the major impact is currently forecast to affect Cape Cod and New England, the combination of moisture and upper dynamics may bring some significant rain thunderstorm activity to our area Saturday and Sunday. The ICON model is forecasting some very heavy rain on Saturday. (Although the ICON has been over-forecasting rain lately.)

It should be noted that if Henri stays a certain distance from us, Saturday may turn out to be exceptionally nice, as sinking air (subsidence) occurs a certain distance west of a tropical system.

This is a complex scenario and it’s likely that the current forecast will change significantly over the coming days. I’ll keep an eye on it.

Forecast Update Mon 9:59 PM in green box below

It amazes me how different the weather forecast for the current week looks compared to my forecast from last Wednesday.

What was going to be a simple transition to an upper level ridge (and just hot weather) now has a somewhat stuck upper air low pressure system off the NJ coastline which moves away Tuesday and gives way to a persistent jet ripple just east of the Appalachians.

This is something called a “leeward trough”, caused by the jet stream upper winds moving over the mountains. This, along with the heat and humidity, will cause the formation of thunderstorms each day, most likely west of the immediate Philadelphia area.

This upper low is expected to drift off to the NE, allowing a more southerly hot humid flow to move in.

There’s some question about whether we will see temperatures reach 96-97º (GFS model) , but but most models have us in at least the low to mid 90s with very high humidity (dew points near and above 70º)

“Apparent Temperatures” (heat index) will likely be over 100º Wednesday and Thursday.

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I’d like to thank everyone who provided feedback on my forecast updates. It looks like I’ll be keeping the updates visible and I’ll continue to explore how to make the update history less confusing.