textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated Rip Current Section.

KEY MESSAGES

1. A round of showers and some thunderstorms with areas of heavy rain could result in some flash flooding risk late Saturday into Sunday.

2. During the latter half of next week, a slow moving cold front looks to make its way southward across the area bringing the potential for showers and storms.

DISCUSSION

KEY MESSAGE 1...A round of showers and some thunderstorms with areas of heavy rain could result in some flash flooding risk late Saturday into Sunday.

A front to our south will lift back northward as a warm front later Saturday. As this occurs, an incoming shortwave will start to increase the forcing for ascent. It appears that the greatest lift occurs Saturday night and Sunday morning, which may become enhanced some by a weak surface low that develops in our vicinity. As a result of the latest timing, most of the area should be precipitation-free during the day Saturday. The precipitable water values increase from south to north later Saturday and Saturday night to 2.00-2.25 inches, and this will open the door to areas of heavier convective rainfall.

While the system does look to be progressive, times of excessive rainfall rates are certainly possible give the convective nature and high precipitable water values. The model forecast soundings also show a deep warm cloud layer, which would also increase the efficiency of the hourly rainfall rates. Despite the more progressive nature of this system, there continues to be some flash flooding risk especially for the urban I-95 corridor where excessive rainfall rates will be more prone to rapid runoff. It is the urbanized areas and small streams where flash flooding is more possible given any rapid runoff. Given the timing of this event, peaking overnight Saturday night and Sunday morning, and the model forecast soundings showing a deep moist profile, tends to result in very limited instability. As a result, thunder should be rather limited. Given some questions on where the heaviest rainfall amounts will occur and also the progressive nature of this, held off on a Flood Watch.

As a cold front moves across our area on Sunday, drier air will arrive from the northwest. The main forcing also peels off to our east and northeast, bringing an end to the showers/thunder. We may have to wait for a possible secondary cold front to arrive later Sunday for the main push of dry air, with it then turning noticeably less humid in its wake. This secondary front could have some convection tied to it back across the eastern Great Lakes, however these may weaken and/or dissipate by the time they reach our northwest zones late Sunday.

KEY MESSAGE 2...During the latter half of next week, a slow moving cold front looks to make its way southward across the area bringing the potential for showers and storms.

Once we get through this weekend, attention will turn to Thursday and Friday of next week. It should be first noted there is a lot of spread in the forecast guidance at this time given this is about a week away. But overall, the signal in guidance is for a slow moving cold front to move southward through the area late next Thursday into Friday bring the potential for showers and some storms. PWAT values along this front look to potentially be quite high (around 2 inches) but there remains some questions about how much instability there will be as well as the speed and timing of the front as it moves through. The upshot of all this is that while forecast confidence is low, this period will need to be monitored for heavy rain potential.

AVIATION /20Z FRIDAY THROUGH WEDNESDAY/

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

Rest of this afternoon...VFR. Northwest winds 5-10 knots (north- northeast at KMIV and KACY), becoming light and variable. A river/bay breeze may result in a wind shift to more southerly at KPHL and KILG late this afternoon. Moderate confidence.

Tonight...VFR. Winds becoming light and variable to locally calm. Moderate confidence.

Saturday...VFR with increasing clouds. Any afternoon shower or thunderstorm should remain south and west of KILG and KRDG. Northeast to east winds around 5 knots, becoming southeast 8-12 knots. Moderate confidence.

Outlook...

Saturday night and Sunday...A period of sub-VFR ceilings and visibilities along with showers and a few thunderstorms. The conditions improve Sunday afternoon.

Sunday night through Wednesday...VFR.

MARINE

The conditions are anticipated to remain below Small Craft Advisory criteria through Saturday. East to southeast winds increase however on Saturday with gusts up to 20 knots.

Outlook...

N/NW winds will increase later Sunday night into Monday and could briefly reach Small Craft Advisory levels. Winds will diminish by next Tuesday into Wednesday but seas look to increase to around 4-5 feet by this time.

Rip Currents...

For Saturday...Southeast winds will increase to around 10 to 20 mph with breaking wave heights ranging between 1 to 3 feet. There will also be a 1 to 2 foot southeast swell around 8 to 10 seconds. Given strengthening onshore flow, have opted to maintain a MODERATE risk for the development of dangerous rip currents at both the Jersey Shore and Delaware Beaches.

For Sunday...Southwest winds around 10 to 20 mph in the morning will diminish to around 5 to 10 mph in the afternoon. Breaking wave heights will range between 1 to 3 feet. An underlying southeast swell around 8 to 10 seconds will continue. Given these conditions, have opted to go with a MODERATE risk for rip currents for Ocean, Atlantic and Cape May Beaches with a LOW risk for Monmouth and Delaware Beaches.

For specific beach forecasts, visit weather.gov/beach/phi

PHI WATCHES/WARNINGS/ADVISORIES

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