textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

A Marginal Risk for severe thunderstorms is in place for southern DelMarVa.

KEY MESSAGES

1. Scattered showers and thunderstorms associated with a warm front will be possible overnight tonight through Wednesday.

2. Severe thunderstorms possible Thursday afternoon and evening along with breezy and hot conditions.

3. Tide levels will remain elevated through the middle of this week with spotty minor tidal flooding.

DISCUSSION

KEY MESSAGE 1...A warm front lifts north on Wednesday, and may touch off scattered afternoon and evening showers and thunderstorms.

A stationary front lies south of the area, and as low pressure tracks through the Great Lakes on Wednesday, it will pull that front north as a warm front. As the warm front first pulls north, guidance does suggest that some overnight convection will be possible across portions of DelMarVa. While any convection while be limited in terms of instability, straightline hodographs and modest shear profiles are suggestive that damaging winds will be possible in any of the stronger storms.

Heading into the daylight hours zonal flow will be in place aloft, some strong midlevel shortwave energy will pass through the region during this time. With southerly flow and increasing low level moisture, temperatures will creep up into the upper 70s and low 80s with dew points in the 50s to low 60s. Showers will become likely with scattered thunderstorms, mainly north and west of Philadelphia, and scattered showers and isolated thunderstorms will be possible across southern New Jersey, the Delaware Valley, and Delmarva.

KEY MESSAGE 2...Severe thunderstorms possible Thursday afternoon and evening along with breezy and hot conditions.

A closed upper-low over portions of Manitoba and Ontario will continue to meander in that region, with large scale troughing in place over much of the Northern CONUS. On Thursday, a strong shortwave still appears poised to round the base of the trough, ejecting through southern portions of Ontario and Quebec southward into New England, taking on an increasingly negative tilt as it does so. A belt of enhanced flow aloft will overspread the region as the shortwave passes to the north. At the surface, a warm front looks to lift through the area during the early morning hours. Low pressure looks to track north of the region with a tailing cold front passing through the region during the afternoon and into the evening hours.

Showers and perhaps a few elevated thunderstorms appear likely during the morning hours across primarily east central PA and northern NJ. Severe weather appears unlikely with this initial round of activity. Southeastern PA, central and southern NJ, and the Delmarva will likely remain dry through the morning and into the afternoon.

As the shortwave and cold front begin to approach the region Thursday, unseasonably strong mid and upper-level flow will overspread the region. Guidance suggests H5 flow on the order of 60 kt or greater. This will lead to deep layer shear on the order of 50- 55 kt. Ahead of the cold front, temperatures appear likely to surge into the upper 80s to lower 90s with strong southwesterly low-level flow. In addition, moisture will continue surging into the area, with dewpoints rising into the upper 60s to near lower 70s, though at least modest mixing does appear probable with the strong surface heating. The result will be at least some destabilization, with guidance generally depicting MLCAPE values around 1000 J/kg.

Latest guidance has trended a little faster with the shortwave, with both the GFS and NAM showing it passing north of the area mid- morning into early afternoon. The ECM continues to feature a slightly slower northeastward progression of the wave, with it passing north of the region during the afternoon. This will be an important detail to monitor over the next couple of days as it will play a role in storm coverage along and ahead of the cold front. If the wave does indeed pass through earlier in the day, despite the flow remaining enhanced aloft, subsidence in the wake of the wave could make storm development a little more difficult. Nevertheless, the strong cold front should still be able to serve as a focus for isolated to scattered storm development. Given the destabilization and impressive kinematics, any storms that do develop will have the potential to be severe. The primary threats appear to be damaging winds and perhaps some instances of hail given the strong yet unidirectional shear.

Outside of the severe thunderstorm potential, it will also be a hot and very breezy day. With temperatures in the upper 80s to lower 90s and dewpoints in the upper 60s to lower 70s, heat indices in the low to mid 90s are likely for the I-95 corridor and points south and east, with drier and slightly cooler conditions to the northwest. The surface low that is expected to track north of the area will be fairly deep, likely around 990 mb or perhaps a bit deeper. This will lead to a strong mass response with intensifying southwesterly flow into the afternoon. As of right now, it appears wind gusts of 25-35 mph will be common areawide. It still appears unlikely that Wind Advisory criteria will be met, but it will be noticeably breezy.

KEY MESSAGE 3...Tide levels will remain elevated through the middle of this week with spotty minor tidal flooding.

After the New Moon on Sunday, astronomical tides will continue to subside. However, spotty minor coastal flooding may continue to affect portions of the Atlantic Coast and along Delaware Bay during the evening high tides through Wednesday.

AVIATION /19Z TUESDAY THROUGH SUNDAY/

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

Today...VFR. SW winds generally 8-10 kts this with occasional gusts to 15kts at MIV/ACY this afternoon. High confidence.

Tonight...Generally VFR. VSBY restrictions possible in BR late. Light S winds. Low confidence.

Outlook...

Wednesday...Prevailing VFR, but some brief restrictions possible with a few showers or a thunderstorm possible.

Thursday...Prevailing VFR, but period of restrictions is probable in the afternoon and evening as showers and some thunderstorms increase in coverage. Southwest winds gusting 25-35 kts outside of any thunderstorms.

Friday...A few showers and thunderstorms may linger with some restrictions possible at times, but VFR should generally prevail.

Saturday...VFR. No significant weather.

MARINE

Sub-Small Craft Advisory conditions today and tonight. Southerly winds 10 to 15 kts with gusts up to 20 kt this afternoon will weaken slightly overnight. Seas will average 2 to 3 feet.

A passing wave this evening may bring some showers to the offshore waters of Delaware during the late overnight hours with possible VSBY restrictions and higher winds/waves. That threat continues into the late afternoon as another round of showers and possible thunderstorms in the afternoon moves through.

Stronger winds develop Thursday as Small Craft Advisory conditions become likely in the afternoon and evening hours. Brief southerly wind gusts could be close to gale force within 5 miles of the coast on Thursday. Showers and thunderstorms will again be possible in the late in the afternoon and evening bringing a lightning threat and hazardous seas with significant wave heights building 4-7 feet through the overnight hours.

Heading towards the weekend, winds and seas drop below SCA with no marine hazards anticipated.

Rip Currents...

Today, South winds 10 to 15 mph with gusts up to 20 mph in the afternoon. Breaking waves will be around 2 feet with a light southeast swell around 7 seconds. As a result, there is a LOW risk for the development of dangerous and life threatening rip currents at the Jersey Shore and Delaware Beaches.

On Wednesday, southeast winds will be 10 to 15 mph with a light southeast swell at 6 to 8 seconds and breaking waves of 2 to 3 feet. As a result, there is a LOW risk for the development of dangerous and life threatening rip currents at the Jersey Shore and Delaware Beaches.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...None.