textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Combined previous Key Messages 1 and 2 into Key Message 1.

KEY MESSAGES

1. Low pressure passes through the region this morning, followed by a cold front this afternoon and evening.

2. A cold front is expected to cross through the region Wednesday night into Thursday resulting in more shower and thunderstorm chances.

DISCUSSION

KEY MESSAGE 1...Low pressure passes through the region this morning, followed by a cold front this afternoon and evening.

Low pressure will pass through the region later this morning. Showers and thunderstorms associated with it will taper off in the pre-dawn hours. Heavy rain and localized flash flooding developed with these showers, but with shortwave energy moving offshore, the showers will lose their potency and additional flash flooding will not be likely. Stratus and patchy fog will develop, and will gradually lift and dissipate as the morning progresses as northwest winds increase to near 10 mph by midday.

A cold front will approach later today and move across the region this afternoon and evening. Surface dew points will rise into the 70s today, and PWATs will be in excess of 2 inches. SBCAPE values will increase to 1000 to 1500 J/kg as well with high temperatures in the mid to upper 80s. Although there is the potential for some brief, locally heavy rain, shear will be limited and organized convection is not likely. PoPs will be capped at chance.

Behind the cold front, high temperatures on Tuesday will be similar to Monday, but dew points will drop from the low to mid 70s to the mid 60s. While warm, it should feel more comfortable on Tuesday compared to Monday.

KEY MESSAGE 2...A cold front is expected to cross through the region Wednesday night into Thursday resulting in more shower and thunderstorm chances.

In the wake of Monday night's cold front, weak high pressure will briefly build into the region for Tuesday and much of Wednesday. An upper-low will traverse portions of Ontario and Quebec during this time, passing north of the region Wednesday night. To the south of the upper-low, broad troughing will be in place across much of the Northeast, with a few embedded shortwaves pivoting around the base of the trough. At the surface, a cold front will begin to approach the area late Wednesday, and cross through the region Wednesday night into Thursday.

Timing discrepancies remain with the frontal passage amongst model guidance. Some guidance indicates the front passing through the region primarily during the overnight hours Wednesday night and clearing the region Thursday morning, while there is still some guidance to suggest slower timing. At this point, timing uncertainties limit confidence in whether or not there will be any notable severe weather or flash flooding concerns, but this will continue to bear watching, particularly if the slower solutions come to fruition. For now, PoPs peak in the 40-60% range across most of the area on Thursday.

There are also some indications that the pattern may remain unsettled into the weekend as the front could get hung up just south of the region with a couple of areas of low pressure riding along it and mid-level troughing remaining in place. PoPs are currently in the 20-40% range each day due to the present uncertainties in timing and placement of these features.

In terms of temperatures, Tuesday and Wednesday look to be fairly warm with modestly increasing humidity. Highs look to be in the upper 80s to lower 90s. Temperatures currently look a bit cooler in the wake of the front, with highs in the low to mid 80s and lows in the upper 50s to mid 60s late this week and into the weekend.

AVIATION /06Z MONDAY THROUGH FRIDAY/

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

Early this morning...SHRA will continue to pass through the terminals through around 09Z. Conds will vary from VFR to MVFR to IFR, eventually settling on IFR/LIFR after rain ends and in the pre-dawn hours. VRB winds 5 kt or less. Moderate confidence.

Today...Conds gradually lift to VFR by 18Z. A round of SHRA, perhaps a TSRA, may pass across the terminals in the late afternoon/early evening with brief sub-VFR conds. Will include PROB30 groups to handle this potential. NW winds 5 to 10 kt. Moderate confidence.

Tonight...Lingering SHRA possible through 06Z or so. Generally VFR outside of any SHRA. N-NW winds 5 to 10 kt. Moderate confidence.

Outlook...

Tuesday through Wednesday...VFR. No significant weather expected.

Wednesday night through Friday...Periods of scattered showers and perhaps storms are likely, with the greatest chance currently appearing to be late Wednesday night into Thursday. Sub-VFR conditions are possible with any showers or storms.

MARINE

Wind gusts on Delaware Bay have diminished to less than 25 kt. Therefore, the Small Craft Advisory is no longer in effect for Delaware Bay. On the ocean, south winds continue to range from 15 to 20 kt with gusts up to 25 kt. Winds will turn west later this morning and will diminish to 10 to 15 kt. Seas around 5 feet will subside to 2 to 4 feet as well. The SCA remains in effect for the ocean until 11 am.

Sub-SCA conditions then expected for tonight.

VSBY restrictions in showers and isolated thunderstorms early this morning. Another round of showers and thunderstorms possible this afternoon and early evening.

Outlook...

Tuesday through Friday...No marine headlines currently anticipated with winds under 25 kts and seas under 5 feet. Scattered showers and perhaps storms could lead to periods of visibility restrictions Wednesday night through Friday.

Rip Currents...

Today, southwest winds 15 to 20 mph with gusts up to 25 mph this morning will turn west and diminish to 10 to 15 mph for the afternoon. Breaking waves will average 1 to 2 feet. There will be an increasing southerly swell with a 5 to 7 second period. This results in a LOW risk for the development of dangerous and life threatening rip currents for the Jersey Shore and Delaware Beaches.

For Tuesday, north-northeast winds 5 to 10 mph in the morning will gradually turn to be more southerly at 5 to 10 mph in the afternoon. Breaking waves will average 1 to 2 feet. There will continue to be a southeasterly swell with a 6 to 8 second period. The result of these conditions is a LOW risk for the development of dangerous and life threatening rip currents for the Jersey Shore 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...Small Craft Advisory until 11 AM EDT this morning for ANZ450>455.