Above is a map of the current weather pattern in place and we see a large ridge of high pressure is in place from the southern Plains all the way to the western Atlantic Ocean, the center of the ridge is located just off the east coast which is leading to a southwesterly flow at all levels of the atmosphere and thus the second heat wave of the season, this ridge wont be leaving quickly either
More times than not in this type of pattern it would be hot and humid but dry , this go around there is potential for thunderstorms each afternoon/evening, and with each passing day storm chances will increase and become more widespread as we have some weak upper level disturbances early this week, then a cold front that will slowly approach later in the week
For highs over the next 3 days, expect upper 80s to mid 90s for most of the area, cooler in NE PA , with high humidity and heat indices in the mid 90s to around 100 degrees, for lows expect mid 60s to low 70s, for the most part clear to partly cloudy skies are expected but there is the chance for isolated to scattered thunderstorms each afternoon, it appears Tuesday has the best potential to be a dry day as there does not look to be much of a trigger for convective development.
For Thursday through Sunday with cloudier skies and more in the way of stormy conditions expect high temperatures to be mainly in the 80s area wide with very high humidity, even higher than it is now and heat indices in the lower to mid 90s and lows will continue to range from the mid 60s to lower 70s
Above is late Tuesday into Wednesday and we see the ridge remains in place but there are a few weaknesses in it which lead to the chance for thunderstorms to develop , Tuesday looks the dry day of the week as ridging will likely be dominant at all levels of the atmosphere,
Later Thursday and into Friday we will see a cold front approach from the west, this will lead to the most widespread thunderstorm activity this week and will likely lead to a very wet period lasting through the weekend, as the large ridge moves just off the coast it will create a parallel flow to the cold front, which will force the cold front to stall over or just west of the forecast area
We see on Sunday a large Bermuda high is in place, the cold front is hung up along the east coast and we continue to be hit by stormy conditions with high humidity although we wont see temperatures at the brutal levels in the 90s like we will see over the next few days, temperatures will likely drop down to near seasonable levels but with the humidity will feel much warmer.
The models hint at the east coast remaining on the western fringes of this Bermuda high as a trough cuts off over the Midwest and the pattern amplifies and becomes stagnant which would force this front to remain over us well into next week with heat, humidity and stormy conditions continuing , with the MJO in phases 6-7 moving towards phase 8 and factors like the stratosphere, there is strong support for this to continue over the next 7 days or so.