textproduct: Philadelphia/Mt Holly

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 tomorrow.

KEY MESSAGES

1. A cold front will continue to dig south through early this morning, bringing the potential for a round of showers and isolated thunderstorms.

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

3. 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 the potential for a round of showers and isolated thunderstorms.

The cold front continues to slide south over the region. We could still see a round of showers and maybe an isolated thunderstorm during the pre-dawn/dawn hours, but given upstream radar trends, it looks as if the main mode will be light rain showers. There is still ample low level moisture as evidenced by the 70+ degree dew points, but lift is meager, and instability is quite limited, especially the later this is before showers move into the region.

Following any morning showers, Friday 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.

Given the forecasted timing of this event, instability looks to be quite limited. Thus the threat for severe thunderstorm hazards (damaging winds, large hail, and tornadoes) is very limited.

In the wake of the cold front on Sunday, drier air should start to settle in to the area. A few models are depicting a secondary cold front moving through the region Sunday Night. That could bring some isolated showers to the region, but given the drier air in place, the vast majority of the region should stay dry.

KEY MESSAGE 3...During the later half of next week, a stationary front may set up just north of our region.

Once we get through this weekend, attention will turn to Thursday and Friday of next week. Models are depicting a stationary front setting up north of our region. The northern half of our region already has 40 to 60% chance for showers and storms through this period. However, of bigger note, the pattern has similarities to the Maddox frontal event heavy rain pattern. Given how it is currently depicted on most models, even if the similarities continue, the main area of heavy rain rain would be north of our region. However, if we see any trends of the front setting up further south, that could increase the risk for heavy rain within our region.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas.

Today...Prevailing VFR. Winds start north/northeasterly around 5 to 10 kt, eventually becoming light and variable after 18Z. A sea- breeze may develop resulting in more east/southeast winds at KACY/KMIV but winds should be rather light.

Tonight...VFR. Winds light and variable as they slowly settle out of the southeast.

Tomorrow...Prevailing VFR. Scattered SHRA and TSRA may temporarily lower conditions to MVFR or even IFR, particularly after 18Z, and particularly for KILG and KRDG.

Outlook...

Saturday Night through Sunday...Periods of sub-VFR conditions will be possible with scattered showers and storms.

Sunday night through Tuesday...VFR.

MARINE

Winds and seas should stay below Small Craft Advisory criteria through early next week. However, gusty and erratic winds will be possible in the vicinity of any showers and thunderstorms particularly Saturday Night into Sunday.

Outside of storms, a few wind shifts are expected over the next few days. First a gradual shift to southeasterly winds is expected tonight into tomorrow as a warm front lifts through the region. Second, a more abrupt wind shift is possible on Sunday Morning to southwesterly winds as a cold front moves through. Finally, another abrupt wind shift (this time to northwesterly winds) is possible with a secondary cold front on Sunday night.

With northwesterly winds on Monday, there is a potential for funneling effects down the Delware Bay, so locally higher wind gusts can be expected there. However, even with funneling effects, wind gusts should be less than 25 kt.

Rip Currents...

Although we are within a day of the new moon, the risk for the formation of dangerous rip currents today is expected to be LOW due to light winds, lower wave heights and mostly wind driven waves.

For tomorrow (Saturday), however, winds start to increase and will have more of an onshore component to them. Additionally, there could be an underlying 10 second swell (even if it is not the dominant period). Therefore the risk for the formation of dangerous rip currents is expected to be MODERATE tomorrow at both the Delaware and New Jersey coast.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...None.