Thu 10:30 PM — Tonight’s models just becoming available shows heavy thunderstorms developing Friday between 4 PM and 8 PM and continuing into late evening as the cold front mentioned in my Wednesday update tries to make further progress southward.
(The HIRESW-ARW has storms as early as 2 PM)

High CAPE values and high PWAT will make for strong storms and heavy rain.
Saturday looks rainy according to the latest NBM. My regular weekend weather forecast will cover the details.

---

---

Thu 6:50 AM — The GFS, RAP and HRRR have backed away from numerous showers/thunderstorms this evening. Still some scattered around. The HI Resolution models and NAM still have showers as shown in last night’s graphic..

---

Wed 8:02 PM— Thursday’s forecast has come into somewhat better focus as a weak front will try to get its toe in the door late Thursday with little success as it’s pushing against strong upper level high pressure entrenched over much of central and southeastern US. Today’s models have some showers and possible thunderstorms in the Philadelphia area late Thursday afternoon as the front tries to move through. Most, but not all, of the activity is is currently forecast to be north and northeast of Philadelphia.

A slight change in wind direction on Friday; the front will make another attempt to move through Friday evening with more showers and thunderstorms, likely more prevalent.

The weekend weather outlook still looks uncertain, but a mix of clouds, some sun and showers/thunderstorms is the most likely scenario for Saturday as the front hangs up somewhere near us. Sunday looks to be the better day.

Also on the horizon is another tropical storm that the GFS is forecasting to become a hurricane in the Gulf and approach the US coastline, possibly Louisiana, as early as Monday.

---

Tue 9:16 PM— After the ‘exciting’ weather from Henri, this week’s weather will be relatively unexciting, as a large high pressure ridge in the upper atmosphere continues with the high heat and the humidity—

There really isn’t any sign that crisp cold air from Canada will descend upon us any time soon.

Wednesday through Friday appear to be hot and humid (although without the mid 70s degree dew points we have recently had). It will still feel very hot.

A meteogram is worth a thousand words. Here’s the NBM Temp/Dew point/Heat Index (“Apparent Temperature) for Blue Bell this week—

While the general pattern is for continuation of hot/warm weather, small ripples in the upper air flow will result in possible thunderstorms Thursday late afternoon (especially far north) and Friday evening.

Following a cold front Friday evening, high pressure will try to move in over the weekend. Unfortunately, it appears that the main high pressure system will remain in Canada and we’ll get some easterly winds as the front stalls somewhere just to our south. That means somewhat cooler weather, but it also can mean some clouds and some showers.

The weekend forecast is too uncertain at this time to even try.

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…

---

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.

---

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.

---

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.

---

.

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.

---

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.

---

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.

---

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.

---

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.