textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

A Heat Advisory has been issued for portions of the region, which is in effect for Thursday through Friday.

Aviation section updated for the 06Z TAFs.

A Small Craft Advisory has been issued for the Atlantic coastal nearshore waters.

KEY MESSAGES

1. Scattered showers and thunderstorms will develop this afternoon and early evening, with a Marginal Risk of severe thunderstorms.

2. Dangerous heat and humidity is expected for portions of the region on Thursday and Friday.

3. Chances for thunderstorms continuing Thursday through Friday, including an increasing threat of severe thunderstorms both days. The weekend has trended drier.

DISCUSSION

KEY MESSAGE 1...Scattered showers and thunderstorms will develop this afternoon and early evening, with a Marginal Risk of severe thunderstorms.

A weak shortwave trough will propagate through a broader ridge aloft this afternoon and evening. At the surface, a warm front will lift north of the area today with high pressure offshore, allowing for deep moisture return amid strengthening warm advection.

Following some isolated morning showers, dewpoints will surge to near 70 degrees by this afternoon as the warm front lifts north. Considerable cloud cover will keep temperatures at bay though, with highs mainly in the mid 80s for most areas. The dewpoints will still make for an unpleasantly muggy day.

The high humidity will result in an unstable airmass later today despite the cloud cover. Scattered showers and thunderstorms are forecast to develop by mid to late afternoon and early evening aided by forcing aloft from the shortwave trough. Instability won't be overly impressive, mainly around 1000-1500 J/kg. More importantly, deep layer shear is weak, only around 10-20 kts. So anticipate the convection will be largely disorganized and single to multi-cellular in nature. DCAPE is modest at best, so cold pools won't be too strong especially given a shallow mixed layer today.

Nevertheless, cannot completely rule out a few stronger storms producing marginally severe, damaging wind gusts from water loaded downdrafts. If a cluster of storms can congeal into a broken line of sorts, this could be a focus for locally damaging wind gusts. PWats will be quite high near 2", so brief blinding downpours will also be possible with any stronger storms today which could cause some urban or poor drainage flooding. The main timing for this activity looks to be from around 4-10 PM. SPC maintains a Marginal Risk (1 out of 5) for severe storms for the entire forecast are today.

Warm and muggy tonight with lows in the mid 60s to low 70s. Some fog or low clouds may develop late, especially near any areas that get a decent amount of rain during the day.

KEY MESSAGE 2...Dangerous heat and humidity is expected for portions of the region on Thursday and Friday.

In general, mid-level ridging is expected to build into the region during the middle and latter part of the week. However, guidance has been trending southward with placement of an upper- low over Canada and associated troughing over the CONUS, and therefore featuring a slightly weaker ridge axis through our region. At the surface, the region will largely be under the influence of high pressure centered over portions of the Southeast.

Despite this, dangerous heat is still expected across the area on Thursday and Friday. Temperatures on both days look to climb into the low to mid 90s across most of the area, though highs may be a smidge warmer on Friday compared to Thursday. Dew points will start out in the low to mid 70s, but with afternoon heating and mixing, should mix down to the upper 60s and low 70s. This still results in max heat index values from 95 to 104 degrees across the majority of the area, outside of the Poconos. The greatest threat for heat indicies over 100 degrees will be found across southeast PA, central and southern NJ, and Delmarva. This level of heat and humidity can become dangerous quickly for sensitive individuals and those outdoors for extended periods of time. A Heat Advisory has been issued to highlight this threat, which is in effect from Thursday through Friday. The exception is for coastal Monmouth County, which is only in effect for Thursday.

A cold front is forecast to move through the region late Friday, which will is expected to knock temperatures and dewpoints down several degrees. Though it will still be hot with highs upper 80s to low 90s over the weekend, dewpoints will return to more comfortable levels, minimizing the threat for additional heat headlines.

KEY MESSAGE 3...Chances for thunderstorms continuing Thursday through Friday, including an increasing threat of severe thunderstorms both days. The weekend has trended drier.

The pattern in place will continue to support convection chances across the region Thursday through Friday. Much of this activity looks to be in the form of diurnally driven convection, though better synoptic forcing will arrive on Friday.

The next opportunity for storms following today comes Thursday. Large scale forcing and shear are expected to remain relatively weak, however we'll have a convectively enhanced shortwave from remnant convection upstream in the Midwest to contend with. This will help improve forcing at the mesoscale. Hotter temperatures and high dewpoints will yield steeper low level lapse rates. This is a pretty typical severe thunderstorm setup for our region, and will likely trigger at least some diurnally driven convection. Exact details remain unclear depending on how things evolve upstream today, but given the increasing risk, the Storm Prediction Center has upgraded the entire region in a SLIGHT risk. Damaging winds will likely be the main threat, but large hail and a brief tornado cannot be ruled out.

On Friday, the region is expected to begin feeling the influence of the upper trough to the west of the region, in addition to a cold front that will be approaching from the west. Scattered showers and storms may focus near and ahead of this boundary, and with strong destabilization and slightly better wind fields aloft, a severe threat will once again materialize. SPC has our entire area highlighted in a SLIGHT risk of severe thunderstorms. Details remain unclear about how Friday's convection may evolve.

Guidance has trended considerably drier for much of the weekend in wake of the cold front, so it now appears that much of the weekend will be dry. However, another front may approach towards the end of the weekend and into early next week, with more chances of showers and storms returning next week.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

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

Through 12Z...Generally a VFR forecast. A few isolated showers around, but no restrictions expected with this activity. South winds near 5 kts. High confidence.

Today...VFR initially. MVFR CIGs develop from west to east starting right after 12Z for KRDG/KABE, and MVFR CIGs spread to KTTN/KPNE/KPHL/KILG. One wave of SHRA may impact KRDG/KABE/KTTN/KPNE/KPHL and KILG in the morning, and will carry PROB30 groups for those terminals for potential SHRA and lower ceilings. Scattered thunderstorms will then impact the terminals in the afternoon, and will carry TEMPO groups for this. S to SW winds 5 to 10 kt in the morning, increasing to 10 to 15 kt with gusts up to 20 kt in the afternoon. Low confidence on timing and restrictions resulting from any SHRA and TSRA.

Tonight...There should be a period of VFR conditions before 06Z following the afternoon/evening storms, however some low stratus or fog may develop at some terminals later. Winds southwest near 5 kts. Low confidence.

Outlook...

Thursday through Friday night...Prevailing VFR expected through the period. Brief periods of sub-VFR conditions are possible, with isolated to scattered showers and thunderstorms during the afternoon and evening hours.

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

MARINE

Increasing southerly winds and building seas are forecast as the day progresses. For the Atlantic coastal waters, winds gusting up to 25 kts and seas near 4-5 feet are expected. A Small Craft Advisory was issued to highlight these conditions from 2 PM through midnight. Some isolated thunderstorms may impact the nearshore waters and Delaware Bay this evening. Winds and seas lessening overnight.

Outlook...

Thursday through Sunday...No marine headlines anticipated. Winds will generally remain between 5-15 kt through Saturday, with gusts up to 20 kt possible on Sunday. Seas generally around 2-3 feet. Isolated to scattered showers and thunderstorms possible on Thursday and Friday. Fair weather expected for Saturday and Sunday.

Rip Currents...

For today, southerly winds increase to 10 to 20 mph with gusts up to 25 mph, especially in the afternoon. Breaking waves will vary between 2 to 3 feet. While there will be a light easterly swell in the morning, a more dominant southerly swell around 7 to 8 seconds should propagate northward in the afternoon. Given these conditions, have opted to go with a MODERATE risk for the development of dangerous rip currents for all beaches.

For Thursday, southwesterly winds diminish to around 10 to 15 mph. Breaking waves around 1 to 2 feet with a southerly swell around 6 to 7 seconds. Given the lighter conditions, have opted to go with a LOW risk for the development of dangerous rip currents for all beaches.

Ocean water temperatures are gradually increasing, but remain mostly in the 60s.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...Heat Advisory from 11 AM Thursday to 8 PM EDT Friday for PAZ060>062-070-071-101>106. NJ...Heat Advisory from 11 AM Thursday to 8 PM EDT Friday for NJZ001-007>010-012-013-015-017>020-027. Heat Advisory from 11 AM to 8 PM EDT Thursday for NJZ014. DE...Heat Advisory from 11 AM Thursday to 8 PM EDT Friday for DEZ001. MD...None. MARINE...Small Craft Advisory from 2 PM this afternoon to midnight EDT tonight for ANZ450>455.