textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Unsettled conditions possible on Wednesday.

Elevated tide levels possible through the middle of this week.

KEY MESSAGES

1. A warm front lifts north on Wednesday, and may touch off scattered afternoon and evening showers and thunderstorms.

2. Severe thunderstorms possible Thursday afternoon and evening along with breezy and hot conditions.

3. Tide levels will remain elevated through the middle of this week with spotty minor tidal flooding.

DISCUSSION

KEY MESSAGE 1...A warm front lifts north on Wednesday, and may touch off scattered afternoon and evening showers and thunderstorms.

A stationary front lies south of the area, and as low pressure tracks through the Great Lakes on Wednesday, it will pull that front north as a warm front. Meanwhile, though zonal flow will be in place aloft, some strong midlevel shortwave energy will pass through the region during this time. With southerly flow and increasing low level moisture, temperatures will creep up into the upper 70s and low 80s with dew points in the 50s to low 60s. Showers will become likely with scattered thunderstorms, mainly north and west of Philadelphia, and scattered showers and isolated thunderstorms will be possible across southern New Jersey, the Delaware Valley, and Delmarva. SPC Day 2 Outlook has a General Risk for thunderstorms for portions of southeast Pennsylvania and northern New Jersey.

KEY MESSAGE 2...Severe thunderstorms possible Thursday afternoon and evening along with breezy and hot conditions.

An upper-level trough centered over south-central Canada is forecast to strengthen eastward and also southward across the Great Lakes to end the week. A strong shortwave rounding the base of this trough may result in a portion of it taking on a negative tilt across the eastern Great Lakes and toward New England late Wednesday and Thursday. A strong mid level zonal jet will develop across the region. A deepening surface low will accompany it and track across the eastern Great Lakes and into adjacent Canada on Thursday. A strong cold front will then arrive into our area Thursday afternoon or Thursday evening.

As the synoptic pattern evolves, unseasonably strong southwest flow will develop. This will advect deeper moisture northward, and in tandem with strengthening warm air advection will lead to increased instability during the day Thursday. The biggest question with this setup regarding severe thunderstorm potential will be how much moisture advection (and thus how much instability) can occur before the front arrives, as well as the timing of the front itself. While the core of the 500 mb jet (80-100 knots) is forecast to remain mainly northwest of our area, 40-70 knots of flow at 500 mb is still forecast across our area Thursday afternoon into the evening. This increased mid level and low level flow will result in rather high shear magnitudes by mid June standards, upwards of 50+ kts. There will be at least some modest instability, around 1,000 J/kg or so, which isn't overly impressive, but the forcing and shear should be sufficient enough to compensate for the relatively unimpressive instability to yield a severe thunderstorm threat. The wind fields will be fairly unidirectional, so hodographs will probably be quite straight. This should ultimately support primarily a damaging wind threat, especially if the storm mode is primarily linear. However if shear and instability ends up being even greater, than all severe thunderstorm hazards may come into play especially if the mode is discrete for a time.

The details remain much less certain at this time range given the typical uncertainty with instability and the thermodynamic profiles. The forecast synoptic setup however with a deepening surface low tracking to our northwest with strong flow within a warmer and more moist environment will be supportive of severe thunderstorms. In addition, the main trough may end up arriving on Friday with some additional chances for anafrontal rain/showers and some thunderstorms.

The I-95 corridor and the coastal plain (away from the immediate coast) looks to be the hottest on Thursday ahead of the convection, high temperatures and heat indices in the low to mid 90s. Another thing to keep an eye on will be the rather breezy conditions developing as the day progresses amid the strengthening low level wind fields. Current model consensus indicates low levels winds in the boundary layer increasing into the 30-35 kt range, which would translate to peak gusts of 35-40 mph outside of any thunderstorms by Thursday afternoon/evening ahead of the cold front. The NBM 50th percentile peak gust for PHL is 34 kts (39 mph), which further adds confidence to the very breezy conditions for Thursday. Sustained southwest winds will increase to 15-25 mph. While these winds and gusts will likely be safely below Wind Advisory criteria, it will be an unusually breezy day for this time of year.

KEY MESSAGE 3...Tide levels will remain elevated through the middle of this week with spotty minor tidal flooding.

After the New Moon on Sunday, astronomical tides will continue to subside. However, spotty minor coastal flooding may continue to affect portions of the Atlantic Coast and along Delaware Bay during the evening high tides through Wednesday.

AVIATION /06Z TUESDAY THROUGH SATURDAY/

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

Early this morning...VFR. Light NW winds. High confidence.

Today...VFR. NW winds less than 10 kt in the morning, backing to the SW by this afternoon. High confidence.

Tonight...Generally VFR. VSBY restrictions possible in BR late. Light S winds. Low confidence.

Outlook...

Wednesday...Prevailing VFR, but some brief restrictions possible with a few showers or a thunderstorm possible.

Thursday...Prevailing VFR, but period of restrictions is probable in the afternoon and evening as showers and some thunderstorms increase in coverage. Southwest winds gusting 25-35 kts outside of any thunderstorms.

Friday...A few showers and thunderstorms may linger with some restrictions possible at times, but VFR should generally prevail.

Saturday...VFR. No significant weather.

MARINE

Sub-Small Craft Advisory conditions today and tonight. Northwest winds around 10 kt will become south 10 to 15 kt with gusts up to 20 kt this afternoon. Winds become south to southeast around 10 kt tonight. Seas will average 2 to 3 feet.

Outlook...

Wednesday...No marine hazards expected. VSBY restrictions possible in showers and possible thunderstorms in the afternoon.

Thursday...A period of Small Craft Advisory conditions is likely by the afternoon and evening hours. Southerly wind gusts could near gale force near the coast for a time late Thursday. Some gusty thunderstorms late in the afternoon and evening possible. Seas building 4-7 feet.

Friday through Saturday...No marine hazards expected.

Rip Currents...

Today, northwest winds 5 to 10 mph will become south 10 to 15 mph with gusts up to 20 mph in the afternoon. Breaking waves will be around 2 feet with a light southeast swell around 7 seconds. As a result, there is a LOW risk for the development of dangerous and life threatening rip currents at the Jersey Shore and Delaware Beaches.

On Wednesday, southeast winds will be 10 to 15 mph with a light southeast swell at 6 to 8 seconds and breaking waves of 2 to 3 feet. As a result, there is a LOW risk for the development of dangerous and life threatening rip currents at 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...None.