textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Aviation section updated for the 00Z TAF issuance.

There is increasing concern for heat impacts during the later half of the week.

KEY MESSAGES

1. Low pressure south of the area will push a warm front north through the region Tuesday night through Wednesday.

2. For the later half of the week, we will have a hot, humid, and at times stormy pattern in place.

DISCUSSION

KEY MESSAGE 1...Low pressure south of the area will push a warm front north through the region Tuesday night through Wednesday.

A cold front currently working its way through the region will wash out over the area on Tuesday. High pressure builds in from the north and west on Tuesday before departing Tuesday night. A pleasant day on tap for Tuesday with highs in the low to mid 80s and dew points generally in the low 60s.

A warm front then lifts north through the region Tuesday night, allowing showers and isolated thunderstorms to develop late Tuesday night and into Wednesday morning. Low pressure developing over the Southeast lifts north along the Eastern Seaboard, but there is considerable uncertainty as to its track. 12Z GFS takes it out to sea with minimal impacts to the area, while the 12Z ECMWF as it inland across Pennsylvania, where the heaviest rain will fall, and the 12Z NAM brings it right over the I-95 corridor. Heavy rain will result in flash flooding, but it is uncertain as to where that heavy rain will fall. The Weather Prediction Center has a Marginal Risk for Excessive Rainfall, which is a 1 out of 4. For now, will follow the NBM and carry high chance to low end likely PoPs for most of the region. There is the potential for another 1/2 inch to 1 inch of rain Wednesday afternoon and evening.

KEY MESSAGE 3...For the later half of the week, we will have a hot, humid, and at times stormy pattern in place.

Expect a prolonged period of southwesterly flow as our region will be in between a Bermuda High well to our east, and a mid level long wave trough that will be slowly sliding east over the northern tier of the U.S./southern Canada. As such, heat and humidity will be building throughout the later half of the week. At this point, Friday and Saturday appear to be the warmest days, with highs ranging from the upper 70s to near 90. That in and of itself is pretty close to normal. However, with dewpoints in the mid 70s possible, we could get heat index values at or above 100 degrees for much of the area. All that being said, this still doesn't look nearly as impactful an extreme heat setup as what we saw earlier this summer during the first week of July.

The other aspect of this are the daily storm chances, primarily confined to the afternoon and evening with a typical diurnal pattern. Given the hot and humid conditions that will be in place, we can't rule out the more common thunderstorm hazards of damaging winds and heavy rain/flooding, but in general, I don't see any one day that has an especially high threat of any of these at this time. If an embedded short wave trough crosses our region as the larger trough axis approaches, that could increase the risk, particularly for severe threats, but trying to time that out at this point would be very difficult, given that we are still seeing some differences between the models with the larger pattern.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

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

Tonight...Primarily VFR. Winds generally north at around 5 kts, though they may become light and variable to calm for a time, mainly at RDG and ABE. There is a low chance (around 20%) of patchy fog formation, particularly if winds are light enough in areas that received substantial rainfall. Given the low probability, have left any mention of fog out of the TAFs at this time. Moderate confidence overall.

Tuesday...VFR. Northeast winds early, gradually becoming more east or east-southeast in the afternoon at 5-10 kts. High confidence.

Outlook...

Tuesday night...MVFR or lower possible late in SHRA.

Wednesday through Wednesday night...MVFR or lower possible in SHRA and TSRA.

Thursday through Saturday...Periods of MVFR or lower in afternoon and evening SHRA/TSRA.

MARINE

A Small Craft Advisory remains in effect until 6 am Tuesday, primarily for elevated seas on the ocean of 6 to 9 feet. Seas will subside this evening, and the SCA may be able to be cancelled prior to that. South winds generally 15 to 20 kt, however, gusts of 30 to 35 kt possible in thunderstorms. Winds veer to the northeast tonight at 10 to 15 kt, and then turn east on Tuesday.

Outlook...

Tuesday night through Saturday...Overall, a prolonged period of sub-SCA conditions, however there is a chance for 25+ kt wind gusts Thursday night and Friday night. Afternoon and evening showers and thunderstorms possible as well.

Rip Currents...

For Tuesday, surf heights should decrease some. However, there will be a northeast (onshore) breeze and southeasterly swells with breaking waves of 2-3 feet in most areas. This will result in a MODERATE risk for the formation of dangerous rip currents along the NJ Shore and Delaware Beaches.

For Wednesday, surf heights and swells will continue to lessen some, and surface winds will shift south to southeasterly. This will result in a MODERATE risk for the formation of dangerous rip currents along the NJ Shore, and a LOW Risk for the Delaware Beaches.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EDT Tuesday for ANZ450>455.