Philadelphia (PHI) — the National Weather Service forecast in plain English · Plaincast

What has changed

There is increasing concern for a heavy rain/flash flood threat with showers and storms late Saturday into Sunday.

Increasing concern for a moderate risk for rip currents Saturday.

Messages

A cold front will continue to dig south through early this morning, bringing a round of showers.

A low approaching the area Saturday into Sunday will bring a warm front followed close behind by a cold front.

During the later half of next week, a stationary front may set up just north of our region.

Discussion

KEY MESSAGE 1. A cold front will continue to dig south through early this morning, bringing a round of showers.

A cold front continues to slide south over the region this morning, and a round of showers are sliding across Delmarva and southern New Jersey. There is still ample low level moisture as evidenced by the 70+ degree dew points ahead of the front, but lift is on the weaker side, and instability is quite limited. Given radar trends however, issued a quick update to increase the chance of rain quite a bit early this morning across Delmarva and across portions of southern New Jersey. Some moderate to briefly heavy downpours will occur at times. Also, increased the cloud cover for a time.

Following any morning showers, today will shape up to be a nice day with clouds breaking later in the day. Highs in the upper 70s to low 80s in most areas.

KEY MESSAGE 2. A low approaching the area Saturday into Sunday will bring a warm front followed close behind by a cold front.

Once the cold front this morning slides south of our region, our attention next turns to the Saturday/Sunday time period when first a warm front will lift through the region on Saturday, followed closely by a cold front Saturday Night into Sunday. The exact evolution of the mid level short wave trough and surface low or trough is still uncertain. Regardless, there should be plenty of lift, even just with the fronts, for widespread showers and thunderstorms, particularly ahead of the cold front Saturday Night into Sunday.

As for the hazards, the main threat remains heavy rain and flash flooding. Model soundings continue to indicate precipitable water values well above the 90th percentile and even at or exceeding many of the daily max values for surrounding sounding locations. Model soundings also show remarkably deep warm cloud layers (for mid September), further increasing the concern for heavy rain. The big questions at this point are how quickly are the fronts (and thus storms) going to move through our region, and where will the axis of heaviest rain be. Given that we've had at this point a week of little rain, rivers are in pretty good shape in terms of lower water levels limiting the concern for river flooding. The bigger concern with this event will be flash flooding, particularly if the heavier rains fall in our more flash flood prone urban areas, and flooding on smaller creeks and streams.