textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated aviation section for 0z TAFs

KEY MESSAGES

1. Several rounds of showers and thunderstorms return Tuesday into Wednesday with an increasing potential for severe weather and flash flooding.

DISCUSSION

KEY MESSAGE 1...Several rounds of showers and thunderstorms return Tuesday into Wednesday with an increasing potential for severe weather and flash flooding.

Beginning early Tuesday morning, a warm front will lifts northward throughout the day. As this warm front lifts northward, winds turn more southerly with moisture being transported northward. PWAT values are forecast to surge upwards of 2.00-2.25 behind the front. As this front lifts north, hi res guidance continues to show isolated to scattered showers and thunderstorms to develop along the front, bringing periods of heavy downpours during the morning rush hour. Areas of greatest concern will be mainly around the I-78 and I- 80 corridors across northeast PA and northern NJ, some areas of which were hit hard with flash flooding just this past weekend. At some point, the warm front will stall nearby the NJ/NY/PA border, but it is uncertain where. The area of greatest flash flood risk will likely be where the front stalls. A general 1-2 inches of rain can be expected from this first of many rounds of precipitation with locally higher amounts. While this may not sound significant, many locations, especially in northern NJ, remain vulnerable to excessive rainfall after this past weekend. As such, rainfall rates of 1-2 inches per hour at times will be sufficient to result in scattered incidents of flash flooding during the critical morning rush hour Tuesday. Furthermore, there is a nonzero severe threat with these storms given the deep layer shear present along with the presence of the frontal boundary. All this said, high uncertainty remains with the timing and overall coverage of the morning convection.

As the day progresses into the late morning and the afternoon, some guidance continues to develop additional isolated showers and thunderstorms across the area. The second round of thunderstorms will be developing across PA during the afternoon hours, which carry the greatest severe potential with widespread coverage as an upper level trough moves through. With PWAT values near or over 2 inches along with instability building to around 2000-3000 J/kg or greater and deep layer shear in the 30-40 kt range, the environment will be favorable for severe thunderstorms with the primary hazard being damaging wind gusts, though a tornado or two cannot be ruled out. With that potential, the Storm Prediction Center has upgraded the region to an an ENHANCED (3 out of 5) risk for the entire region. The fly in the ointment for the severe potential will be how robust the morning convection ends up being. More thunderstorm coverage in the morning could mean less severe potential for the I-78 corridor and north should the atmosphere not recover in time. Regardless, the risk for severe weather remains with uncertainty remaining elevated as well. What is more certain, however, is the potential for torrential downpours from these thunderstorms. Rainfall rates around 2 inches per hour will be possible with this round of storms as well, bringing the risk of flash flooding to the rest of the region.

As this second round of storms move through the region, hi res guidance continues to suggest additional rounds of storms thereafter as the cold front pushes in from the west, in which a disorganized and chaotic storm mode could begin to take hold by the evening into overnight hours. The severe risk will begin to gradually diminish through the evening as the environment gets worked over, but flooding concerns will persist through at least 2AM with the continue risk for heavy downpours and the possibility of training storms given the messy and widespread coverage of thunderstorms expected over the urban corridor. Thunderstorm coverage could continue into the early morning hours of Wednesday, but hi res guidance suggests most storms will be concentrated along the coastal plains by that point. By the time we reach the overnight hours, an additional 1-2 inches could have fallen across the region with locally higher amounts.

Given the potential for flash flooding across the urban corridor, a Flash Flood Watch was issued from 6AM Tuesday through 2AM Wednesday morning.

On Wednesday, the SPC and WPC continue to have portions of Delmarva and southern NJ outlooked for severe weather and a risk for excessive rainfall, respectively. Part of this set-up will depend on the placement of the front and timing but latest CAM guidance continues to emphasize re-development of convection along the coastal plains of Delmarva and New Jersey through much of the day Wednesday. Primary threat with any severe storms is likely to be damaging wind gusts in addition to heavy rainfall leading to flash flooding, particularly in the more urbanized areas along the immediate coast.

Once the front moves away by late Wednesday, we are drier for Thursday and beyond into the weekend.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

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

Tonight...Mainly VFR through around 09Z. MVFR after 09Z with VCSH/VCTS. Southeast winds around 5 kt or less. High confidence.

Tuesday...Mainly MVFR cigs to start with iso/sct SHRA/TSRA in the morning around PHL and north. More widespread TSRA in the afternoon. Periods of sub-VFR in TSRA. Winds generally SSE early in the day veering to SSW by mid to late afternoon and increasing to around 10 gusting up to 20 knots. Moderate confidence in storm coverage, low confidence in storm timing.

Outlook...

Wednesday...Sub-VFR conditions likely at times in SHRA/TSRA.

Thursday through Saturday...VFR. No significant weather.

MARINE

Sub-advisory conditions will prevail tonight. Winds around 10 to 15 kts or less with seas of 2-3 feet.

Winds and seas reach SCA criteria by Tuesday afternoon and linger into the overnight. Winds will gust to 25-30 kts at times with seas building to 5-7 feet.

Outlook...

Wednesday...SCA conditions continue with seas in excess of 5 feet and could linger into Wednesday night. SCA remains in effect thru Wednesday afternoon for the ocean zones.

Thursday through Saturday...Conditions are expected to remain below SCA criteria.

Rip Currents...

For Tuesday, winds increase out of the south to around 15-20 MPH, with either a shore parallel or slight onshore flow expected depending on location. Breaking waves around 2 to 4 feet as well will result in more energy within the surf zone. As a result, elected to go with a MODERATE risk at all beaches.

For Wednesday, winds turn a bit more southwesterly but still around 10-20 MPH. However, breaking wave heights are expected to increase, with breaking waves as high as 5 feet possible. Still a 7 second period though which will mitigate the rip current threat some. However, thinking a MODERATE risk is sufficient for Wednesday's threat.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...Flood Watch from 6 AM EDT Tuesday through late Tuesday night for PAZ070-071-101>106. NJ...Flood Watch from 6 AM EDT Tuesday through late Tuesday night for NJZ008>010-012>015-017>019. DE...Flood Watch from 6 AM EDT Tuesday through late Tuesday night for DEZ001. MD...None. MARINE...Small Craft Advisory from noon Tuesday to midnight EDT Tuesday night for ANZ430-431. Small Craft Advisory from noon Tuesday to 6 PM EDT Wednesday for ANZ450>455.