textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No significant changes to the forecast.

KEY MESSAGES

1. Hot and humid conditions are expected to persist through Tuesday of next week, though an improvement in humidity is expected Sunday into Monday. A Heat Advisory is in effect for portions of the area through today.

2. Isolated to scattered showers and thunderstorms are forecast to develop this afternoon and evening. Sunday still looks to be largely dry, then daily chances for showers and thunderstorms will return Monday and into the middle of next week. There is at least a marginal risk for localized heavy rainfall leading to flash flooding and locally damaging wind gusts with any stronger thunderstorms that develop.

DISCUSSION

KEY MESSAGE 1...Hot and humid conditions are expected to persist through Tuesday of next week, though an improvement in humidity is expected Sunday into Monday. A Heat Advisory is in effect for portions of the area through today.

A relatively stagnant synoptic pattern will remain in place across our region. Quasi zonal mid and upper level flow will be in place through the middle of next week as Bermuda high pressure persists offshore. This will keep a steady stream of very warm and moist air in place across the region through the period.

A prolonged period of hot and humid conditions will continue for the next several days. With dewpoints remaining in the low to mid 70s and highs in the upper 80s to low 90s today, heat indices will again be near 100 degrees this afternoon, mainly along and southeast of the I-95 corridor. A weak cold frontal passage Sunday will help usher in slightly drier air and allow dewpoints to mix out during the afternoon hours both Sunday and Monday. This will give some slight reprieve from the excessive humidity, though highs will still be in the upper 80s to low or even mid 90s. The lower dewpoints Sunday and Monday will result in maximum heat indices mainly in the mid to upper 90s, though could get near 100 degrees in Delmarva and far southern New Jersey. Since the forecast is decidedly below Heat Advisory criteria, we've opted not to extend it to cover additional days at this time, though it will still be hot and humid.

A Heat Advisory remains in effect through 8 PM this evening for portions of southeastern Pennsylvania, portions of New Jersey, and northern Delware.

Moisture advection will return again by Tuesday ahead of another cold front, and slightly more humid conditions should return for Tuesday. A somewhat stronger cold front looks to come through late Tuesday or Wednesday, which will help bring a slight reduction in temperatures. Highs by Wednesday and Thursday are mostly in the mid to upper 80s, which will be closer to average for early to mid August.

KEY MESSAGE 2...Isolated to scattered showers and thunderstorms are forecast to develop this afternoon and evening. Sunday still looks to be largely dry, then daily chances for showers and thunderstorms will return Monday and into the middle of next week. There is at least a marginal risk for localized heavy rainfall leading to flash flooding and locally damaging wind gusts with any stronger thunderstorms that develop.

The evolution of today's convection remains a little unclear still. A feature of interest will be the convective complex currently across the upper Ohio Valley early this morning. While that activity should weaken later this morning, it's remnants will likely become a focus for forcing diurnally driven convection in our region later today as it drifts eastward. CAMs still suggesting isolated to scattered thunderstorms developing across the area this afternoon and evening. Synoptic forcing isn't as strong as what we had Friday, though the remnant dynamics from the convection upstream could give us better forcing at the mesoscale level. So we could see more widespread thunderstorms than CAMs suggest, but not super confident on that at the moment. PWATs look slightly lower than today, but overall the thermodynamics appear similar to Friday (2000-3000 J/kg CAPE). Notably, mean flow is more westerly and a bit stronger than Friday, so deep layer wind shear is a little stronger in the 25-30 kt range. This will pose more of a large hail risk compared to Friday as well, but damaging winds will be the main threat with any stronger storms. Locally heavy rainfall leading to flash flooding is also possible, but thinking the stronger flow aloft should keep storms moving faster and pose slightly less of a flash flood risk.

We remain in a Marginal (level 1 out of 4) Risk for excessive rainfall from WPC. SPC has maintained the Marginal (level 1 out of 5) Risk for severe thunderstorms for most of our area.

We look to have a weak cold frontal passage by Sunday, ushering in slightly drier air. This combined with subtle subsidence aloft will help briefly limit convection potential on Sunday, though cannot completely rule out some isolated sea breeze convection in the afternoon.

Better support for forcing aloft will return by late Monday as mean flow shifts more westerly and mid level flow increases. At this point though, it appears the greater threat of storms will be to our northwest on Monday where SPC has a Marginal Risk highlighted. Still could get some isolated storms, but Monday is trending drier locally as the lingering lower dewpoints should limit our instability a bit. Better moisture and instability return by Tuesday though, along with better storm chances. With the stronger mid level flow aloft, severe thunderstorms still look to be a greater threat compared to flash flooding, but these details remain unclear.

A somewhat stronger cold front looks to come through late Tuesday or Wednesday, which will help break the excessive heat and humidity some, but the pattern aloft may still support diurnally driven convection.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

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

Today...Any restrictions in fog/stratus will clear by 13Z or so, then VFR. Isolated to scattered thunderstorms in the afternoon may cause localized restrictions, however confidence on where any of this may develop is low. Opted to include PROB30 groups for TSRA at all terminals. Southwest winds increasing to near 10 kts, with gusts near 18 kts by 18Z. High confidence in prevailing VFR, low confidence on shower and storm details.

Tonight...Prevailing VFR, though some patchy fog may develop. Some isolated showers and thunderstorms could linger through 06Z, but confidence on this is too low to include a TAF mention at this time. Southwest winds near 5 kts or less. Moderate confidence.

Outlook...

Sunday-Sunday night...VFR conditions expected.

Monday-Wednesday...Mostly VFR except restrictions possible with afternoon/evening showers and thunderstorms.

MARINE

No prevailing marine hazards are expected through tonight. Southwest to south winds around 10-20 kt with seas of 2-4 feet. A 20-40% chance of a thunderstorm this afternoon into tonight with briefly higher winds and seas possible.

Outlook...

Sunday through Wednesday...No prevailing marine hazards expected. Isolated storms could develop along the sea breeze Sunday, potentially impacting the back bays or nearshore waters. Thunderstorms possible Tuesday and Wednesday nights.

Rip Currents...

For today...Southwesterly winds continue and should increase some to 10-20 mph. Breaking waves in the surf zone are forecast to increase some, especially for much of the Jersey Shore, at 2 to 3 feet. Given the increasing surf and southerly winds, went with a MODERATE risk for the development of dangerous rip currents for most of the Jersey Shore. Breaking waves should be a little lower and winds will have more of an offshore or shore parallel component for the Delaware Beaches and coastal Monmouth County, therefore went with a LOW risk for the development of dangerous rip currents.

For Sunday...Similar conditions Sunday as today. Southwesterly winds continue around 10-15 mph. Breaking waves in the surf zone are forecast to remain 1 to 3 feet. Maintained the same risk of rip currents for all coastal areas.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...Heat Advisory until 8 PM EDT this evening for PAZ070-071- 101>106. NJ...Heat Advisory until 8 PM EDT this evening for NJZ008>010- 012>015-017>020-027. DE...Heat Advisory until 8 PM EDT this evening for DEZ001. MD...None. MARINE...None.