textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Minor changes to Key Messages and Aviation sections.

KEY MESSAGES

1. Daily chances for showers and thunderstorms into the middle of the week. There is a risk for strong to severe thunderstorms, as well as a marginal risk for localized heavy rainfall leading to flash flooding.

2. Hot and humid conditions will persist into the middle of the week, but humidity will not be as oppressive as recent days.

3. There is potential for minor tidal flooding during the evening high tides this week along the Atlantic coast, Delaware Bay, and tidal Delaware River, especially on Tuesday and Wednesday.

DISCUSSION

KEY MESSAGE 1...Daily chances for showers and thunderstorms into the middle of the week. There is a risk for strong to severe thunderstorms, as well as a marginal risk for localized heavy rainfall leading to flash flooding.

A surface cold front will gradually sag southward across the northern tier of the region this afternoon into evening before stalling around the I-80 or I-78 corridors come Wednesday. During this time, a few upper level shortwave pulses will rendezvous with the surface cold front, bringing rounds of showers and storms through Wednesday morning. The first round of storms this afternoon and evening will be probably to the North/West across the southern Poconos and possibly into the Lehigh Valley. Other more scattered activity is possible across southern NJ. The upper vort maxs are difficult to track and the CAMs guidance is very inconsistent, so we'll continue with blended guidance as well as satellite/radar trends for our updates. That said, instability across the region does look to be limited to the 1000-2000 J/kg range with limited deep layer shear, which will, therefore, limit the severe weather potential today. SPC has expanded the Marginal Risk (level 1 out of 5) farther northeast now into the Lehigh Valley and central NJ, and the entire area remains in the Marginal Risk (level 1 out of 4) for excessive rainfall.

The latest guidance now suggests a second round of showers and perhaps some convective thunderstorms to arrive early Wednesday morning with the potential for heavy downpours during the Wednesday morning rush hour. While there remains a fair amount of uncertainty with this setup it appears that this round may be a result of a decaying upstream MCS from the Ohio Valley. This setup remains highly uncertain and fully dependent on how the volatile setup across the Ohio Valley eventually pans out through the overnight hours. Areas that are favored for higher pops and heavy rains will be Delmarva and south NJ, possibly into Philadelphia.

Additional rounds of upper level shortwaves look to ride along the quasi-stationary boundary which will support rounds of convection continuing at least through Wednesday night, and possibly into Thursday. The front looks to push through by late Thursday, and a much drier and cooler airmass will arrive by Friday and Saturday as a result. This will bring us a more pronounced break in convective potential and more pleasant weather conditions.

KEY MESSAGE 2...Hot and humid conditions will persist into the middle of the week, but humidity will not be as oppressive as recent days.

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 the week as Bermuda high pressure persists offshore. This will keep a very warm and moist airmass in place across the region through the period.

Hot and humid conditions will continue into the middle of the week, although slightly lower temperatures and dewpoints will arrive for Wednesday. Dewpoints are anticipated to mix out into the mid to upper 60s again this afternoon. This will give some slight reprieve from the excessive humidity we've experienced in recent days, though highs will still be in the upper 80s to mid 90s. The lower dewpoints will result in maximum heat indices mainly in the mid to upper 90s, though could get near 100 degrees in Delmarva and near the coast in New Jersey. Therefore, hot and humid, but not to the point where any heat headlines are needed.

By Friday and Saturday with the front finally having pushed through, a much drier and more cooler airmass will arrive, which will bring much more pleasant weather conditions. Dewpoints may drop into the 50s again for many areas with highs forecast to be in the low to mid 80s. Dewpoints look to start rebounding into the 60s again by Sunday.

KEY MESSAGE 3...There is potential for minor tidal flooding during the evening high tides this week along the Atlantic coast, Delaware Bay, and tidal Delaware River, especially on Tuesday and Wednesday.

As we approach the New Moon on Wednesday, astronomical tides for the evening high tide cycles will increase to around a half foot below minor flood stage in many areas this week along the Atlantic coasts of New Jersey and Delware, Delaware Bay, and the tidal Delaware River. With tide departures running around a half foot above average, this will raise a threat of minor flood stage being reached for these areas.

We will continue to monitor the tide departures in the coming total water level forecasts. Coastal Flood Advisories have been issued for areas along the Delaware Bay for the Tuesday evening high tide. Additional Coastal Flood Advisories may be needed over the coming days, but at least spotty minor tidal flooding should be anticipated around the times of the evening high tides this week. The highest astronomical tides will occur on Wednesday, though Tuesday's tides are of similar height. The threat of minor tidal flooding may linger through Thursday or Friday as well.

Tidal flooding is not currently forecast for the Eastern Shore of Chesapeake Bay.

AVIATION /15Z TUESDAY THROUGH SATURDAY/

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

This afternoon...VFR. A 20-40 percent chance of a shower or thunderstorm in the afternoon with the best chance north and west around ABE/RDG. North to Northwest winds around 15-20 kts decreasing and backing to West late. Low confidence regarding the shower/thunderstorm coverage/timing.

Tonight...VFR give way to MVFR restrictions with showers and thunderstorms arriving during the overnight hours, around 06-09Z, mainly across the southern terminals. Low confidence.

Outlook...

Wednesday through Thursday...Prevailing VFR. Restrictions possible with afternoon/evening showers and thunderstorms.

Friday and Saturday...VFR. No significant weather expected.

MARINE

No prevailing marine hazards expected through Tuesday night.

Outlook...

Wednesday through Friday...No prevailing marine hazards expected. Thunderstorms possible each afternoon and evening.

Saturday...No marine headlines are anticipated.

Rip Currents...

For Today...Light winds in the morning will settle out of the west-southwest in the afternoon between 5 to 15 mph. Breaking wave heights will remain around 2 feet or less with a southerly 2 foot swell around 5 to 7 seconds. Therefore, there is a LOW risk for dangerous rip currents at all beaches.

For Wednesday...Light winds around 5 to 10 mph will gradually turn from northwesterly in the morning to southeasterly in the late afternoon. Breaking wave heights will remain around 2 feet or less with a southerly 2 foot swell around 5 to 7 seconds. With little changes in overall conditions, will continue with a LOW risk for dangerous rip currents at all beaches.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...Coastal Flood Advisory from 9 PM this evening to 2 AM EDT Wednesday for NJZ016. Coastal Flood Advisory from 6 PM this evening to midnight EDT tonight for NJZ021-023-024. DE...Coastal Flood Advisory from 9 PM this evening to 2 AM EDT Wednesday for DEZ001. Coastal Flood Advisory from 6 PM this evening to midnight EDT tonight for DEZ002>004. MD...None. MARINE...None.