textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Flood Watch expanded to include the Lehigh Valley and Berks County.

Updated the aviation section for the 06Z TAFs.

KEY MESSAGES

1. A warm front on Thursday will bring the next chance for widespread rain.

2. A cold front is expected to follow close behind on Friday.

DISCUSSION

KEY MESSAGE 1...A warm front tomorrow will bring the next chance for widespread rain.

Early morning convection has not been initialized well by the CAMs with many of the 06Z guidance missing it entirely. Ironically, the previous 00Z and 18Z guidance hi res runs displayed the morning convection a bit earlier. Right now, the greatest threat from these storms looks to be localized flash flooding, but a severe thunderstorm cannot be fully ruled out.

High pressure moves offshore with southerly return flow behind a warm front will bring dew points into the low 70s. Showers and thunderstorms will start moving into western areas before daybreak and continue through the day. CAMS producing some organization to convective activity largely focused on the I-95 urban corridor.

As for the hazards, there is a SLIGHT (2 out of 4) risk for excessive rainfall. Precipitable water values increase to perhaps as high as 150% of normal for this time of year. The hires guidance suggests that heaviest precipitation will be generally along the I-95 corridor and points northwest. Hires guidance has also pointed to areas along the I-78 corridor potentially seeing heavy rainfall amounts as well. Generally around 1-2 inches of rain with localized QPF values in excess of 3 inches. Widespread flash flooding still seems unlikely, but the localized threat remains elevated. The Flood Watch was expanded to include the Lehigh Valley and Berks county and remains in effect for northern NJ and the I-95 corridor.

For severe hazards, latest CAMS and HREF guidance show SBCAPE values remaining around 1500-2500 J/kg with ensemble maximums in excess 3000 J/kg. Deep layer shear forecasts remain around 30kt with a strongly veering profile aloft. Boundary layer flow is not particularly strong but surface winds are more backed than previous forecasts so an isolated tornado is possible primarily along and west of I-95. That said, damaging wind gusts and hail will remain the greatest threat. Dominant mode should be multicell convection but a supercell can't be ruled out especially where cloud bearing shear is maximized. SPC has expanded the area of a slight risk (2 out of 5) to now include much of the area.

KEY MESSAGE 2...A cold front will cross the area Friday.

The timing of this cold front will be critical in the risk for thunderstorm hazards. There are differences between models in both the timing, and if this will be a single cold front, or an initial cold front with a reinforcing shot of cold air advection coming in behind it. Saturday and Sunday appear to be dry across most of the area with the exception of southern areas Saturday where low probabilities of showers and storms will exist.

AVIATION /12Z THURSDAY THROUGH MONDAY/

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

Thursday...Mainly VFR. Light south winds in the morning become more out of the south-southwest at around 10 knots during the afternoon with gusts to 20 knots for KMIV/KACY. For the Lehigh Valley and I-95 terminals, showers and potentially an embedded thunderstorm are possible in the morning which was handled with VCSH with a PROB30 for -TSRA. As the potential for showers and thunderstorms increases from midday into the evening, VCTS and PROB30 groups for TSRA for most terminals to account for this activity.

Thursday night...VFR as showers and storms exit, but patchy fog possible mainly across the I-95 corridor and RDG/ABE. Winds light and variable. Low confidence.

Outlook...

Friday...Periods of MVFR or lower are possible in showers and storms.

Saturday through Monday...Prevailing VFR conditions.

MARINE

No marine hazards expected. Southerly winds with gusts near 20 kt possible.

Outlook...

Thursday night and through Sunday...No marine hazards expected.

Rip Currents...

On Thursday, winds remain southerly at 10 to 15 mph with gusts to 20 to 25 mph. There is an east-southeasterly swell with a period of 8-9 seconds and breaking waves around 2-3 feet. As a result, there is a MODERATE risk for the development of dangerous and life threatening rip currents at the Jersey Shore and a LOW risk at Delaware Beaches.

For Friday, winds in the morning are more westerly at 5 to 10 mph before becoming more southwesterly in the afternoon at 5 to 10 mph. An east-southeasterly swell remains with a period of 6-7 seconds with breaking waves around 2-3 feet. As a result, there is a MODERATE risk for the development of dangerous and life threatening rip currents at the Jersey Shore and a LOW risk at Delaware Beaches.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...Flood Watch from noon EDT today through late tonight for PAZ060>062-070-071-101>106. NJ...Flood Watch from noon EDT today through late tonight for NJZ001-007>010-012-013-015-017>019. DE...Flood Watch from noon EDT today through late tonight for DEZ001. MD...None. MARINE...None.