textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Severe and flash flood potential has increased for Thursday.

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 on Thursday will bring the next chance for widespread rain.

By Thursday, high pressure moves offshore with southerly return flow behind a warm front will bring dew points into the low 70s. Showers and possibly some thunderstorms will start moving into western areas before daybreak and continue through the day. Latest hi res guidance is now in range of Thursday afternoon and are showing some better organization of convection.

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 hi res CAMs are suggesting that the heaviest precipitation will be generally along the I-95 corridor and points northwest. Generally around 0.5 to 1.0 inch of rain is expected across the region, but neighborhood probabilities are showing the risk for localized QPF values in excess of 2-3 inches. Widespread flash flooding still seems unlikely, but the localized threat has increased.

For severe hazards, latest CAM guidance has come in more bullish for convective development in the afternoon. HREF mean SBCAPE values are around 1000-2000 J/kg with ensemble maximums around 3000 J/kg. While deep layer shear is lacking a bit (around 25 kt), hodographs are showing veering winds aloft to support a mostly multicell threat. Furthermore, steep low-level lapse rates will support rapid updraft development. Will need to continue monitoring hi res trends to see if this threat holds. SPC has expanded the area of a slight risk to now include the Poconos, Lehigh Valley, and Berks County.

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 WEDNESDAY THROUGH SUNDAY/

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

Rest of tonight (through 12Z)...VFR. Winds light and variable. Some patchy fog is possible but was too low confidence to include in the TAFs at this time.

Wednesday...VFR. Light and variable winds in the morning become more out of the southwest at around 5 knots during the afternoon.

Wednesday night...Prevailing VFR but isolated showers and thunderstorms possible after 06Z.

Outlook...

Thursday...Scattered showers and thunderstorms in the afternoon could lead to temporary flight restrictions.

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

Saturday and Sunday...Prevailing VFR conditions.

MARINE

No marine hazards expected. Light winds and no restrictions to visibility expected Wednesday.

Outlook...

Thursday and Friday...Gusts near 20 kt are possible, particularly on Thursday with southerly winds behind a warm front.

Saturday and Sunday...No marine hazards expected.

Rip Currents...

On Wednesday, winds will shift southerly and increase to near 10 to 15 mph in the afternoon. A medium period swell will persist with breaking waves around 2 feet during much of the daytime. As a result, there is a LOW risk for the development of dangerous and life threatening rip currents at the Jersey Shore and at Delaware Beaches.

For Thursday, winds remain southerly at 10 to 15 mph with gusts to 20 mph. An east-southeasterly swell remains with a period of 8-9 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...None. NJ...None. DE...None. MD...None. MARINE...None.