Area Forecast Discussion

WHAT HAS CHANGED

Updated aviation section for 6Z TAFs.

KEY MESSAGES

1. Much less humid on Saturday.

2. Showers and some thunderstorms return Sunday afternoon and Sunday night, with some severe thunderstorm risk. Cooler and less humid for the first half of next week.

3. Widespread minor coastal flooding with the evening high tide cycles Saturday and Sunday along the Atlantic coast, Delaware Bay, and possibly on the tidal Delaware River.

DISCUSSION

KEY MESSAGE 1...Much less humid on Saturday.

A cold front is forecast to move through the region overnight, which will knock the temperatures down a few degrees and allow dew points to return to more comfortable levels for the weekend. Although it will still be hot with high temperatures in the upper 80s to low 90s over the weekend, dew points will mainly be in the upper 50s to low 60s. Therefore, heat indicies will not be far removed from the actual air temperature, thus no heat headlines are anticipated over the weekend.

KEY MESSAGE 2...Showers and some thunderstorms return Sunday afternoon and Sunday night, with some severe thunderstorm risk. Cooler and less humid for the first half of next week.

An expansive upper-level trough across much of eastern Canada Sunday will gradually shift eastward into early next week. Some stronger shortwave energy within the southern portion of the trough will slide across our area later Sunday. A cold front will also be tied to this feature and that will cross our area later Sunday afternoon and Sunday evening. The low to mid level flow will increase some in response to this incoming shortwave and cold front, which will also increase the warm air advection and also the low-level moisture advection. At least some of the guidance shows a rapid moisture return in the lower levels as the air mass recovers from the substantial drying on Saturday. This will result in it feeling more humid Sunday, however it is not expected to be at the levels we currently are experiencing.

The main forecast challenge is the timing, coverage and intensity of convection with this incoming system. It appears to be later in the afternoon and at night regarding the timing. There will be a period of stronger synoptic forcing arriving, and with the low to mid level flow increasing some the shear will increase some as well. The amount of instability should be sufficient enough to support some thunderstorms, however the magnitude of this will determine the intensity of the thunderstorms. If sufficient instability can be realized, then some severe thunderstorms will be possible with damaging winds the main severe weather threat. Given mostly unidirectional flow ahead of the cold front, convection should tend to develop into a couple of clusters or a line, with this potentially more favorable from about the Philadelphia metro on southward. Some model forecast soundings show steep low-level lapse rates with some dry air in the mid levels. The steep low-level lapse rates would result in higher DCAPE values, which is supportive of an enhanced wind threat with any stronger thunderstorms.

In the wake of the cold front, a less humid and cooler air mass overspreads our area through the first half of next week. An upper- level trough starts to approach on Tuesday and that may result in a few showers Tuesday and Wednesday, with a greater chance for some showers and thunderstorms during Thursday.

KEY MESSAGE 3...Widespread minor coastal flooding with the evening high tide cycles Saturday and Sunday along the Atlantic coast, Delaware Bay, and possibly on the tidal Delaware River.

A New Moon this weekend will result in higher than normal astronomical tides. Astronomical tides will generally be 1/2 foot less than minor coastal flooding thresholds, and tidal departures will be around 1 foot or so. This will result in widespread minor coastal flooding with the evening and nighttime high tide cycles Saturday and Sunday, and potentially Monday as well on the Atlantic coast and Delaware Bay. The tidal Delaware River and its tributaries could experience some spotty minor tidal flooding by Sunday night's high tide, and potentially Monday night too, but generally these astronomical tide driven flooding events produce more noteworthy impacts for the Atlantic coasts and Delaware Bay. The highest of the New Moon tides will be Sunday night and Monday night. Tidal flooding is not currently expected along the eastern shore of Chesapeake Bay.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

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

Overnight...VFR. Winds northwest around 5-10 kt. High confidence.

Saturday...VFR/SKC. West-northwest winds around 5-10 kt. High confidence.

Saturday night...VFR.

Outlook...

Sunday...Sub-VFR conditions probable for a time due to showers and some thunderstorms, especially in the afternoon and evening.

Monday...VFR. Northwesterly wind gusts to 20 knots possible.

Tuesday and Wednesday...Some sub-VFR conditions with some showers possible.

MARINE

No marine headlines are in effect through Saturday. Winds shift northwest tonight in wake of a frontal passage. Northwest winds will continue through Saturday morning, before transitioning back to southwest by Saturday afternoon. Seas generally around 2-3 feet, locally up to 4 feet.

Isolated thunderstorms diminishing this evening. Fair weather is expected on Saturday.

Outlook...

Saturday night...No marine hazards anticipated.

Sunday...A brief period of Small Craft Advisory conditions are possible in the afternoon and evening due to wind gusts to 25 knots.

Monday through Wednesday...No marine hazards anticipated.

Rip Currents...

For Saturday, a medium period 8 second swell, offshore winds, and breaking waves 2 feet or less will result in a LOW risk for development of dangerous rip currents at the Jersey Shore and Delaware Beaches.

For Sunday, winds become southerly and increase to around 15-20 MPH. However, with a medium period swell around 8 seconds continue, breaking waves will be again 2 feet or less, for most beaches. The exception will be at southern facing beaches in Cape May and Atlantic counties where breaking waves in the surf zone will be up to 3 feet as the winds will be most directly onshore here. Therefore, the risk for the development of dangerous rip currents will be higher for these beaches, and a MODERATE risk is currently forecast for these two counties. For the remainder of the Jersey Shore and the Delaware Beaches, a LOW risk for the development of dangerous rip currents is forecast.

Ocean water temperatures are mostly in the mid to upper 60s.

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

CLIMATE

Hot air mass is moving out, but some record highs were set. We've shared the list in the RER products and on social media. The following record warm low temperatures could still be set if temperatures remain this high through 1 AM EST (12 midnight standard time).

Location 6/12 Record Warmest Low/Year

Allentown 70/1942 Trenton 72/2015 & 2017 Atlantic City Airport 73/1973 & 2015 Georgetown 73/1947, 1973, 1986, & 2017

PHI WATCHES / WARNINGS / ADVISORIES

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