textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated aviation section for the 12z TAFs.

The timing for the severe thunderstorm risk today looks to be delayed some.

KEY MESSAGES

1. There continues to be an elevated risk for severe thunderstorms and flash flooding today and Wednesday, with the greater risk this afternoon and this evening.

DISCUSSION

KEY MESSAGE 1...There continues to be an elevated risk for severe thunderstorms and flash flooding today and Wednesday, with the greater risk this afternoon and this evening.

A strong upper-level trough is forecast to pivot into and across the Northeast and Mid-Atlantic later this afternoon and tonight, then remain in place through Wednesday. This will drive surface low pressure well to our north, however an associated warm front will lift north across our region today followed by a cold front late this afternoon and tonight.

An increase in the low-level warm air advection this morning will be enhanced by a southerly low-level jet of about 30 knots. This is providing lift within a moistening air mass to generate some showers and thunderstorms this morning, with this mainly focused north and west of I-95. There is some 850 mb convergence north of I-78 early this morning, and this is within an axis of increasing instability. This is likely driving the ongoing cluster of convection moving northeastward across our far northwest zones. These cells should be faster moving therefore the flash flood risk should remain limited. As a warm front advances northward, increasing dew points and at least some sunshine should promote plenty of instability this afternoon and evening. The northward movement of the warm front will determine the extent of the greater instability. The low-level flow is more backed through the morning before it starts to veer some this afternoon, therefore will need to watch for some rotation if stronger convective elements (more discrete) can become realized by early afternoon. As the main upper-level trough arrives during the afternoon and the surface cold front also starts to approach, strong mid to upper level flow will foster greater forcing for ascent. This is expected to result in an increase in convective development, especially to our west, with an initial focus along a pre- frontal trough. Convection should tend to organize into clusters or one or more line segments as the low- level flow veers some more by evening. This veering however will be delayed though if the warm front is slower to advance northward, therefore any stronger cells that especially interact with the warm front will need to be watched more closely for a tornado risk. Otherwise, tall initial CAPE profiles may tend to widen some into this evening and with steep low-level lapse rates the DCAPE will increase. Areas of damaging straight-line winds will be the main severe weather hazard, with this potentially becoming more enhanced if convection develops a cold pool with bowing segments. Some more robust convective cores could produce severe hail. The severe risk will continue for a time this evening as instability may remain favorable as the upper-level trough amplifies some into the region. Convection may tend to become messy with competing outflows which could also enhance the locally heavy rain/flash flooding risk.

Another risk is areas of excessive rainfall that could lead to flash flooding. As a warm front lifts northward, winds turn more southerly with moisture being transported northward. Precipitable water values are forecast to surge to 2.00-2.25 inches in the wake of the warm front. Within the warm air advection burst through this morning, some convection is anticipated especially north and west of I-95. While this activity could produce locally heavy rain, storm motions should limit this. The warm front may stall near the NJ/NY/PA border, but this is uncertain. As additional convection develops this afternoon and evening, deeper convection will likely increase the rainfall rates with the strong convective cores capable of producing 1-3 inches per hour. Some areas are saturated, especially in northern NJ and portions of the Philadelphia metro, which remain more vulnerable to excessive rainfall after this past several days. Excessive rainfall rates, especially for the urban areas, could lead to flash flooding. This risk will become enhanced where convection ends up being more focused. There will be some areas that could get 2-4 inches of rain while some areas get much less. Given the continued risk for flash flooding across the urban corridor, a Flash Flood Watch remains in effect for today through later tonight (the main time frame looks to be this afternoon and this evening).

On Wednesday, the cold front will sag farther south and east and depending on how quickly this clears our area there will be some showers and thunderstorms, especially as additional shortwaves move through the flow within the upper-level trough overhead. This looks to be more concentrated across southern New Jersey to Delmarva but could extend into the I-95 corridor for a time. There is some severe weather risk mainly near and south/east of the I-95 corridor with locally damaging wind gusts the main threat. The greater instability and shear looks to reside across Delmarva on southward, where a little higher severe thunderstorm risk is indicated. Some locally excessive rainfall is possible with some local flash flood risk, however this looks to be less compared to today given the shift farther south and more into the coastal plain. Once the front moves away by later Wednesday, it turns drier for Thursday/Friday and right into the weekend.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

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

Today...VFR/MVFR ceilings this morning, possibly localized IFR ceilings for a time. The MVFR ceilings then improve to VFR from south to north between about 15-18z. Some showers and a few thunderstorms this morning especially north and west of KPHL, then more widespread showers and thunderstorms after 18z from west to east with MVFR/IFR conditions at times. Locally strong winds probable with stronger thunderstorms. Southeast to south winds 5-10 knots early this morning, then south to southwest winds 10-15 knots later this morning and this afternoon with some local gusts to around 20 knots. Low confidence on specific timing details of thunderstorms.

Tonight...Times of MVFR/IFR conditions with showers and thunderstorms in the evening, then MVFR ceilings overnight as showers and thunderstorms taper off. South to southwest winds 10 knots or less. Low confidence.

Outlook...

Wednesday...Sub-VFR conditions at times along with some showers and thunderstorms especially near and south/east of KPHL.

Thursday through Saturday...VFR.

MARINE

Southerly flow increases today, and Small Craft Advisory conditions are expected to develop this afternoon and continue tonight. The conditions do subside though overnight on Delaware Bay. A Small Craft Advisory remains in effect starting this afternoon. Gusty thunderstorms probable mainly later this afternoon and this evening.

Outlook...

Wednesday...Small Craft Advisory remains in effect for the ocean zones.

Thursday through Saturday...The conditions are anticipated to be below Small Craft Advisory criteria.

Rip Currents...

For Today, southerly winds increase to 15-25 mph with either a shore parallel or slight onshore flow expected depending on location. Breaking waves of 2-4 feet will result in more energy within the surf zone. As a result, maintained a MODERATE risk for the development of dangerous rip currents at all beaches.

For Wednesday, winds turn more southwesterly but still 10-20 mph. However, breaking wave heights are expected to increase, with breaking waves in the surf zone as high as 5 feet possible. Still a 7 second period though which will mitigate the rip current threat some. However, will continue with a MODERATE risk for the development of dangerous rip currents at all beaches.

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

PHI WATCHES/WARNINGS/ADVISORIES

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