textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated Aviation Section for 00Z TAFs.

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.

2. 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 a risk for strong to severe thunderstorms as well as a marginal risk for localized heavy rainfall leading to flash flooding.

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 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. 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 on Monday will result in maximum heat indices mainly in the mid to upper 90s, though could get near 100 degrees in Delmarva. 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.

From this forecast update, there remains some moisture advection ahead of the cold front. However, the cold front timing looks a bit earlier Tuesday which has resulted in some lower temperatures and heat index values. Temperatures into the mid 80s to low 90s for many and heat index values peaking near or in the 90s for many. Once the cold front fully comes through, we will see a continued slight reduction in temperatures into Wednesday and Thursday. 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...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 a risk for strong to severe thunderstorms as well as a marginal risk for localized heavy rainfall leading to flash flooding.

Showers and storm threat has ceased for the evening. Weather should remain mostly tranquil this evening into the overnight with some areas of patchy fog possible.

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 west 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.

Recent forecast trends have resulted in lower instability compared to previous model runs on Tuesday given the timing of the cold front to be earlier. There does remain uncertainty around the specific timing details of the front at this time. However, with that said, there is still better storm chances on Tuesday compared to Monday. With the stronger mid level flow aloft, severe thunderstorms still look to be a greater threat compared to flash flooding, but these details continue to remain unclear.

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

AVIATION /00Z SUNDAY THROUGH THURSDAY/

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

Tonight...VFR expected, though some patchy fog may develop especially for terminals which received rainfall on Sunday afternoon. Southwest winds generally around 4-7 kt, though may become light and variable at times. Moderate-high confidence.

Sunday...VFR expected. A stray afternoon shower or thunderstorm is possible, but confidence is quite low. Southwest winds early will become westerly around 7-12 kt. Moderate-high confidence.

Outlook...

Sunday night...VFR conditions expected.

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

MARINE

No marine headlines are in effect through Sunday. South- southwest winds generally around 10-20 kt with seas around 2-4 feet. Fair weather anticipated.

Outlook...

Monday through Thursday...No prevailing marine hazards expected. Thunderstorms possible Tuesday afternoon into Tuesday night and again Wednesday night.

Rip Currents...

For Sunday...Southwest winds will prevail around 10 to 15 mph. Breaking wave heights generally around 1 to 2 feet, with some localized beaches up to 3 feet as well as a southerly 2 to 3 foot swell with a period around 6 to 8 seconds. Given these conditions, have opted to go with a LOW risk for the development of dangerous rip currents at both the Jersey Shore and Delaware Beaches.

For Monday...South-southwest winds will be around 5 to 10 mph in the morning, increasing to around 10 to 20 mph in the afternoon. Breaking wave heights will generally range between 1 to 2 feet, with multiple swell groups and periods. Given these conditions, have opted to go with a LOW risk for the development of dangerous rip currents at 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.