An interesting  weekend weather forecast as a dip in the jet stream and an associated cold front moves through Friday night while deep low pressure winds up over the Canadian maritime area.  Complicating the picture is a tropical depression far off the southeastern coastline  which is expected to become brought into the circulation of the deepening low in Canada.

As is often the case, a double barrel low pressure system is frequently poorly modeled and combining the situation with a tropical system is a double whammy.

That said, the models have been reasonably consistent with the frontal passage timing for Friday night. Where the errors may occur is the amount of rain forecast and the temperatures/wind for Saturday and Sunday.

Much of the rain is expected to miss us on Friday, as energy is expected to be transferred to our north.

As it stands right now, Saturday will start damp, windy and noticeably chilly but some breaks of sun are expected for the afternoon.  High 58, but it wouldn’t surprise me if it’s cooler.

Sunday will still be windy and chilly, with a mix of clouds and some sun, as a secondary front moves through late Sunday.  High 56.

Expect changes in this forecast.  I’ll update Saturday morning.

The models are in good agreement with the thunderstorms and showers expected today as a result of the approaching cold front. With the breaks of sunshine that we’re getting, a little more thermodynamic ‘fuel’ will be available to help with the development.

Expect a good chance of thunderstorms, as early as 3 PM and most likely between  4-6 PM.  The models show the precip moving through with a primary line followed by a secondary line of showers early evening.

Yesterday, the short range models did well; the NAM over-stated the chance of showers.    In the immediate PHL area, there were a few widely scattered thunderstorms, mostly far north and west.

Despite strong lifted indexes, vertical velocities and even some low level convergence, the atmosphere was “capped” with hot air  aloft, basically negating any instability in the lower levels.

Today, the chance of thunderstorms is on par with the usual 30 percent background level seen in hot humid air masses.

The HRRR shows a weakening line of shower and thunderstorms moving through between 5 and 7 PM.  Not sure if this will hold together.

The models keep the temps in the low-mid 90s with dew points in the low to mid 70s.