textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Aviation section updated for 0Z TAFs.

KEY MESSAGES

1) Humidity returns on Thursday along with potential severe thunderstorms and flash flooding.

2) Additional rounds of showers and thunderstorms are expected on Friday which may linger into Saturday before drier conditions return on Sunday and into early next week.

DISCUSSION

KEY MESSAGE 1...Humidity returns on Thursday along with potential severe thunderstorms and flash flooding.

Weak high pressure over western New York and western Pennsylvania slides to the south and east this evening and will be off the Mid- Atlantic coast tonight. Return flow sets up behind the departing high, and a light southerly flow tonight will increase during the day Thursday. Dew points in the mid 60s this evening will increase into the upper 60s tonight, and then into the mid 70s on Thursday. Although temperatures will "only" be in the mid 80s or so, which is right around normal for this time of the year, the heat index will be in the low to mid 90s due to those high dew points.

The main concern on Thursday will be the potential for showers and thunderstorms in the afternoon and evening. With low pressure approaches from the west and tracks towards the Mid- Atlantic by Thursday evening. A mid-level trough will move into the Northeast with some shortwave energy passing through the Delaware Valley, Delmarva, and southern New Jersey.

SBCAPE values will be in excess of 1500 J/kg with PWATS from 2-2.5 inches, highest values from the Delaware Valley, including Philadelphia, south into Delmarva. 0-6 km shear will be from 25 to 30 kt. The two main threats from severe weather will be damaging wind gusts and heavy rain leading to localized flash flooding.

The Storm Prediction Center has a Slight Risk for severe weather (2 out of 5) for the Delaware Valley south of Philadelphia, Delmarva, and southern New Jersey, and a Marginal Risk (1 out of 5) from around Philadelphia north to Allentown. The Weather Prediction Center has a Slight Risk (2 out of 4) for Excessive Rainfall for southeast Pennsylvania, southern New Jersey, the eastern shores of Maryland, and Delaware, and a Marginal Risk (1 out of 4) for the southern Poconos and northern New Jersey.

Although some showers and thunderstorms will fire up over far western portions of the forecast area from noon-2pm, the main timing will be from 4pm-10pm across the Delaware Valley, including Philadelphia, the eastern shores of Maryland, Delaware, and southern New Jersey. While showers and thunderstorms capable of damaging winds and localized flash flooding are likely for the northern zones as well, the highest impacts in terms of potential severe weather and flash flooding should be focused generally south of Philadelphia.

A warm and muggy night with patchy fog developing Thursday night after showers and thunderstorms come to an end by midnight. Lows will be in the upper 60s to low 70s.

KEY MESSAGE 2...Additional rounds of showers and thunderstorms are expected on Friday which may linger into Saturday before drier conditions return on Sunday and into early next week.

On Friday, another round of showers and thunderstorms are expected to develop out ahead of a cold front moving southward into the area. Similarly to Thursday, moisture is anticipated to increase ahead of the front, where PWAT values are progged to be in excess of 1.8 inches. This supports that any given shower or storm will have the ability to produce heavy rainfall and flash flooding. As of now, the majority of our forecast area is highlighted in an MARGINAL risk (Level 1/4) for excessive rainfall. In terms of the severe weather threat, ML CAPES are expected to reach at least 1000 J/kg with bulk shear values around 30 kt. Given better frontal forcing, there looks to be at least another opportunity for severe weather. As of now, SPC has our entire area highlighted in just a general thunder risk, however some of the MLP guidance indicate higher probabilities especially in terms of damaging winds. It is also worth noting that Friday will be quite sticky outside with highs in the upper 80s to low 90s with dew points in the low to mid 70s. This will result in heat indicies in the mid to upper 90s. We may get close to Heat Advisory criteria in some spots, but basis the current forecast, heat indicies are expected to remain just below advisory criteria.

There remains a vast amount of uncertainty with how Saturday will pan out as guidance continues to struggle with how quickly the cold front clears the area. Some guidance slows the progression of the front, keeping more in the way of showers around on Saturday, whereas others show a faster progression, yielding mainly dry weather. As mentioned by the previous shift, opted to stay very close to NBM PoPs for this period, which keeps 10-30% PoPs north, with 30-60% PoPs mainly across south Jersey and the Delmarva. Temperatures will mainly be in the 80s.

By Sunday and into early next week, guidance is showing an expansive upper level ridge setting up over the northern High Plains. This will yield dry weather and mainly clear skies across the Mid- Atlantic. Temperatures will be around normal for Sunday and Monday, but will swing back above normal by Tuesday and Wednesday.

AVIATION /00Z THURSDAY THROUGH MONDAY/

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

Tonight...VFR initially. MVFR or lower CIGs in stratus developing near dawn. Light S to nearly calm winds. Low confidence.

Thursday through Thursday night...Any sub-VFR CIGs become VFR by 18Z. SHRA/TSRA develop after 18Z and may linger through 0Z. Sub- VFR conds in the heaviest SHRA/TSRA, and then sub-VFR conds in BR and/or stratus late Thursday night. Moderate confidence overall.

Outlook...

Friday through Friday night...Mainly VFR with periods of sub- VFR possible. A chance for showers and thunderstorms in the afternoon and evening hours.

Saturday though Saturday night...Mainly VFR with a slight chance of sub-VFR conditions. A chance of showers and thunderstorms in the afternoon.

Sunday through Monday...VFR. No significant weather expected.

MARINE

Sub-Small Craft Advisory conditions tonight through Thursday night. South to southwest winds around 10 kt, increasing to 10 to 15 kt with gusts up to 20 kt Thursday afternoon, becoming W 5 to 10 kt Thursday night. Seas 2 to 3 feet.

VSBY restrictions in fog possible tonight, then again Thursday night. Showers and thunderstorms producing heavy rain and gusty winds will impact the waters late Thursday afternoon and evening.

Outlook...

Friday through Monday...No marine headlines are expected at this time. Winds will largely remain between 5-15 kt through Sunday, with winds up to 20 kt possible on Monday. Seas around 2-4 feet. A chance for showers and thunderstorms are expected each afternoon and evening through Saturday, with fair weather returning for Sunday and Monday.

Rip Currents...

For Thursday, winds are more southerly, and the moderate swell also becomes more southeasterly. This combination gives a MODERATE risk for the northern NJ beaches, with a LOW risk further south.

For Friday, winds turn southwesterly, and the swell weakens. Thus, this should result in a LOW risk of rip currents for all beaches.

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

PHI WATCHES/WARNINGS/ADVISORIES

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