textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

The Monday system has trended slower meaning showers and thunderstorms may last into Tuesday.

The risk for excessive rainfall and severe thunderstorms Monday has increased some for portions of our area.

KEY MESSAGES

1. Mostly tranquil tonight and Sunday.

2. A system moves in Monday into Monday night bringing the threat for heavy rainfall with an associated flooding risk along with at least some threat for severe weather.

DISCUSSION

KEY MESSAGE 1...Mostly tranquil tonight and Sunday.

High pressure well to the south/west across the lower Ohio Valley will continue to weaken tonight and Sunday. Its associated ridge will remain across our area thru the period. It will provide mostly dry weather and comfortable temperatures tonight and Sunday. There are a few weak shortwaves in the West/Northwest flow aloft and these may trigger a few showers this evening and again Sunday. Pops are in the 10% to 20% range for these showers.

KEY MESSAGE 2...A system moves in Monday into Monday night bringing the threat for heavy rainfall with an associated flooding risk along with at least some threat for severe weather.

Forecast guidance continues to indicate an unsettled start to the week with low pressure impacting the area but there remain differences in the details of how all this will play out. The main question continues to be the exact track of the low, which will have notable consequence on specific impacts.

The main takeaway though is that is that most areas will see at least some showers and thunderstorms Monday afternoon into the evening, potentially even lasting into the overnight. Consensus guidance shows PWAT values (1.75- 2.00") near the climatological maxima, setting up the risk for heavy rainfall. NBM probabilities for 1+ inches of rain with the system are in the 60-80 percent range for areas along and north of the I-95 corridor and generally in the 25 to 50 percent range farther south. For 2+ inches, these probabilities are as high as 30 to 50 percent over NE PA into NW NJ, generally around 10 to 25 percent near the I-95 corridor, and less than 10 percent farther south. It is worth noting though that forecast models notoriously struggle with QPF, especially when it involves convection. Looking at the different deterministic models, there are indications that some parts of our area could see 2-3+ inches of rain but the question is not just if this occurs but if so, where. If the low tracks farther north, this would tend to keep the heaviest rain generally favored for NE PA into NW NJ with more of a severe weather threat south of these areas. However a track farther south near Delmarva / SE PA / southern NJ could unfortunately mean that the heaviest rain would be targeted near the I-95 corridor. We're still in a region-wide drought though so that could mitigate the flood threat some. At this time the Weather Prediction Center has areas from the I-95 corridor northward in a SLIGHT risk (level 2 out of 4) for excessive rainfall meaning scattered instances of flash flooding will be possible. Farther south, the risk is MARGINAL (level 1 out of 4) meaning any flash flooding will be more isolated if it occurs. The Storm Prediction Center has added a SLIGHT risk (level 2 out of 5) for areas south of Philadelphia with a MARGINAL risk (level 1 out of 5) for areas north of there regarding the potential for severe thunderstorms. As of now, damaging wind gusts are the primary threat however will need to watch the track of the low and warm front as shear profiles will be stronger near it, although instability is still more uncertain.

In terms of timing, the trend is for the system to be a bit slower meaning showers/storms may not really move in until the afternoon but would linger through the evening and potentially overnight as well. The severe weather threat should be diminishing through the evening but the flood thread may last into the overnight.

Tuesday now looks like it could see some additional showers and storms, more scattered in nature, due to the system's slower movement with a final piece of the upper level trough moving through. This should be focused near and S/E of the urban corridor with the severe weather and flash flooding threat looking low by this time.

AVIATION /19Z SATURDAY THROUGH THURSDAY/

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

Tonight...VFR expected. A brief shower is possible this evening (most probable KRDG/KABE). West to Northwest winds 10 to 15 kts with gusts around 20 kts then decreasing after 00Z/01Z. High confid.

Sunday...VFR expected. A small chance for a shower during the afternoon. West winds 5 to 10 knots much of the time. High confid.

Outlook...

Sunday night...VFR. No significant weather.

Monday and Monday night...Sub-VFR conditions with showers and some thunderstorms likely mainly for the afternoon and evening periods.

Tuesday...Scattered showers/storms could bring brief restrictions to the I-95 TAF sites and points S/E of there. VFR should prevail farther north.

Wednesday...VFR. No significant weather.

Thursday...Chances for shower/storms return (30 to 50 percent chances) and these could bring brief restrictions.

MARINE

The Small Craft Advisory was taken down as gusts have decreased back below 25 kts at the northern NJ marine zones. Seas at the adjacent buoy 44025 reached 5 ft earlier but are back to 4 ft attm. Mostly fair weather expected tonight and Sunday. A brief shower possible.

Outlook...

Sunday night...No marine headlines expected.

Monday through Monday night...SCA conditions expected with southerly wind gusts increasing to around 30 kt with seas 4 to 7 feet.

Tuesday...SCA conditions could linger early.

Wednesday through Thursday...Sub SCA conditions.

Rip Currents...

Rest of today, West winds around 10-15 mph are expected. However, the onshore swells begin to weaken, decreasing to around 2 to 4 feet at around a 7 second period. Breaking waves in the surf zone should be around 2 feet or less as a result. Therefore, a LOW risk for the development of dangerous rip currents is forecast for all beaches on Saturday.

For Sunday, westerly winds diminish to around 10 mph. Onshore swells will remain weak around 1 to 2 feet with a 7 second period. Breaking waves in the surf zone will be around 1 foot or less. Therefore, a LOW risk for the development of dangerous rip currents is forecast for all beaches on Sunday.

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

PHI WATCHES/WARNINGS/ADVISORIES

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