Away on vacation. The weather forecasts return after Labor Day.

I’m writing this forcast while a shower is raining down on the Phillies, unpredicted by the models.   Nuff said?

The weather pattern of the last two weeks will be replaced by a pattern more typical of late August, with a dip in the jet stream pushing a cold front through Sunday evening.

Saturday will be sunny and hot with dew points returning to the more uncomfortable upper 60s to 70.  Temperatures will approach 90.

Fri 11pm update- tonight’s NAM shows thunderstorms possible late Saturday afternoon.

On Sunday, considerable cloudiness is expected with warm and very humid conditions.  Highs near 87 with dew points in the low 70s.  There’s the likelihood of showers and thunderstorms ahead of the frontal passage starting about 5 pm. The models are suggesting that the heaviest storms will be far north of our area, but that can change.

The short range models (HRRR and LAMPS) show a lower probability of thunderstorms today compared to  Friday.

Over the past week, the NAM lifted index and vertical velocity parameters have been highly correlated with thunderstorm development.

The NAM model shows positive vertical velocities and very unstable lifted indexes for much of the day, all parameters  conducive to thunderstorm development.  These parameters are more conducive  to thunderstorms than yesterday.   The NAM cranks out some precip during the evening hours, after 8 PM.

So, if we go with the short range models, look for scattered thunderstorms between 5 and 7 PM with a probability of only 35%.

If we go with the NAM parameters, look for scattered thunderstorms possible anytime late morning through  afternoon and especially late evening.

Trying to nail down a specific predictive parameter has been a challenge with this air mass.