textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Increased confidence in the timing of storms later today.

For Wednesday, models trending slightly colder for much of the region, limiting the instability with storm chances.

The chance for showers and thunderstorms shifted a bit north on Friday. Friday afternoon high temperatures also inched up a few degrees.

KEY MESSAGES

1. Scattered severe thunderstorms are forecast to develop late this afternoon and evening across portions of New Jersey and eastern Pennsylvania. Damaging wind gusts, large hail, and a tornado or two will be possible.

2. On Wednesday, a shortwave trough could spark another round of showers and storms, but it will be working against a relatively stable boundary layer.

3. Hot and humid conditions are forecast across the region on Thursday.

4. Unsettled conditions will continue on Thursday with the potential for severe thunderstorms. Scattered thunderstorms are again possible on Friday, mainly from Philadelphia south.

5. The Labor Day Weekend will feature mostly dry weather with at or below normal temperatures and lower humidity.

DISCUSSION

KEY MESSAGE 1...Scattered severe thunderstorms are forecast to develop late this afternoon and evening across portions of New Jersey and eastern Pennsylvania. Damaging wind gusts, large hail, and a tornado or two will be possible.

The now stationary front is north of our region now. An approaching mid level trough is expected to result in showers and storms developing along the front and then progressing southward through the evening. The main timing for this round of showers and storms appears to be from 6 PM through midnight. As far as the hazards with the storms, it still appears that the primary hazards are damaging winds and cloud-to-ground lightning, but there is potential also for large hail, heavy rain, and tornadoes.

Surface based CAPE values above 2000 J/kg will be possible as the storms approach, thanks in part to the breaks in the clouds that we have seen since the earlier line of showers moved south of the region. Effective shear still looks like it will be in the 30 - 40 kt range, even well south of the front. However, as mentioned by the previous shift, low level shear is limited for most of the region, but directional shear is sufficient for at least some tornado risk, particularly for central and northern Jersey. All that to say, damaging winds still appear to be the primary threat, with large hail and tornadoes less likely, but still a threat.

Storm motions look to be fast enough to limit the flooding threat, and there doesn't appear much risk for training storms. If there is heavy enough rain in a short amount of time, there could be some issues with the more flash flood vulnerable areas, such as highly urbanized areas, but that threat is limited.

As for the heat advisory, heat index values in the urban corridor are flirting with 100 degrees, so will keep the advisory going as is.

KEY MESSAGE 2...On Wednesday, a shortwave trough could spark another round of showers and storms, but it will be working against a relatively stable boundary layer.

In the wake of the storms Tuesday Night, the front is expected to sink back south of the region by Wednesday morning, leaving our region in the relatively cooler and more stable air mass north of the front. The net result is that our region should be in northeasterly flow, and relatively cloudy conditions for much of the day. As a result, surface-based instability is going to be almost non-existent, though model soundings do show elevated instability for parcels above 900 mb.

Consequently, the risk for severe storms is still quite conditional and uncertain. Given how sharp of a mid level short wave trough is approaching, there will be ample forcing, but it is a question of if the elevated instability will be enough to sustain storms in our region.

KEY MESSAGE 3...Hot and humid conditions are forecast across the region on Thursday.

After a cool down on Wednesday for most neighborhoods (5 to 10 degrees lower - less across Delmarva), hot and humid conditions are once again expected across the region on Thursday as the frontal boundary lifts north. With the front to the north, high temperatures are forecast to climb into the upper 80s to mid 90s for many areas (around 80 or low 80s far N and W). These temperatures combined with dewpoints in the low to mid 70s will again result in heat indices well into the 90s to low 100s south of I-78, perhaps even as high as 105 degrees for Delmarva.

Another Heat Advisory may be needed for portions of the area on Thursday if this trend continues.

KEY MESSAGE 4...Unsettled conditions will continue on Thursday with the potential for severe thunderstorms. Scattered thunderstorms are again possible on Friday, mainly from Philadelphia south.

Another round of severe thunderstorms are possible on Thursday. Friday could also experience a few severe storms across Delmarva, depending on how quickly the frontal boundary pushes south.

SPC has a Slight Risk (level 2 out of 5) for severe thunderstorms on Thursday from about Wilmington to Wildwood NJ southward and a Marginal Risk (level 1) to the north of this.

The northwest flow pattern will remain in place on Thursday and will continue to be supportive of all severe threats, including damaging winds, large hail, and a few tornadoes, depending on how all the mesoscale details pan out. The threat for flash flooding is typically lower with these patterns, but not zero. Training can still occur and a single locale could have multiple days with heavy rainfall.

Friday's SPC's 15% probabilities for severe thunderstorms have shifted just south of our Delmarva zones, though there is still some uncertainty with this. Our latest forecast has about a 30-40% chance of showers and thunderstorms south of about Philadelphia.

KEY MESSAGE 5...The Labor Day Weekend will feature mostly dry weather with at or below normal temperatures and lower humidity.

By Saturday, current model consensus indicates the frontal boundary will be pushing south of our region with Canadian high pressure building across the Northeast and Mid-Atlantic. This will largely end any shower and thunderstorm potential. Thus, Labor Day Weekend is still shaping up to feature nice weather, with temperatures trending at or slightly below normal and lower humidity. The only road bump at the moment, could be a cold front that approaches the forecast area very late on Monday.

AVIATION /19Z TUESDAY THROUGH SUNDAY/

Through Tonight...Expect a broken line of storms to move from north to south across the TAF sites between 22 and 04Z. Ahead of this line, expect VFR conditions. With and behind this line, expect MVFR conditions, especially with MVFR ceilings behind the line, to develop. Winds should be mostly out of the SW ahead of the storms. With and behind the line of storms, expect an abrupt shift to northeasterly winds, but wind speeds (outside of the vicinity of any thunderstorms) should stay near or below 10 kt.

Wednesday...Prevailing VFR. There is a 20 to 40% chance for showers and storms through the day time that could result in temporary MVFR or lower conditions, however, there is considerable uncertainty to the extent of storm development tomorrow. Winds primarily out of the northeast and east around 10 kt through the day.

Outlook...

Thursday and Friday...Sub-VFR conditions are possible in showers and thunderstorms each day (20-40% chance), mainly during the afternoon to evening hours.

Saturday and Sunday...Primarily VFR conditions expected.

MARINE

Winds and seas are expected to stay below Small Craft Advisory conditions. However, gusty and erratic winds are possible in the vicinity of any storms, particularly tonight.

Outlook...

Thursday through Sunday...Sub-SCA conditions are expected through the period. There is a chance (40 to 50%) for thunderstorms Thursday, mainly in the afternoon and evening hours. For Friday, the chance of thunderstorms (30 to 40%) shifts south a bit, basically extending from Manasquan Inlet, NJ down to Fenwick Island, DE.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Heat Advisory until 8 PM EDT this evening for PAZ070-071-102- 104-106. NJ...Heat Advisory until 8 PM EDT this evening for NJZ015-017>019. DE...Heat Advisory until 8 PM EDT this evening for DEZ001. MD...None. MARINE...None.