textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Less likely to see isolated showers today. Higher confidence in the timing of the late week cold front.

KEY MESSAGES

1. High pressure will slowly build towards the region through Wednesday.

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

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

DISCUSSION

KEY MESSAGE 1...High pressure will slowly build towards the region through Wednesday.

High pressure is expected to slowly build towards the region. However, the main mid and upper level low is still relatively close - currently centered over Quebec. An embedded short wave trough is expected to pivot over the region through the afternoon this afternoon. However, unlike yesterday, model soundings show that there will be a stout mid-level inversion in place during this time, so the chance for isolated showers this afternoon has decreased from previous forecasts.

KEY MESSAGE 2...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, helping bring dew points back into the low-70s. Models came in better agreement with an earlier arrival. What that means is that showers will likely start moving in, at least to portions of the region, before daybreak. Chances for showers will continue through the day. I don't expect that any one location will see rain all day, rather will likely be off and on showers and storms.

As for the hazards, there is a marginal (1 out of 4) risk for heavy rain/flash flooding threat. Precipitable water values increase to around 1.7 or 1.8 in. That is close to the 90th percentile for this time of year. That being said, it is lower than what we have seen with some recent heavy rain events. Additionally, the warm cloud layer is thinner than what we have seen recently (generally less than 10k ft during the afternoon). Thus, will hold off on issuing any flood watch at this point.

For severe hazards, it will be something to monitor over the next few days, but forecasted instability and shear are modest (CAPE values generally expected to be less than 1000 J/kg, and bulk shear near or below 30kt). That being said, it always bears watching at this time of year, especially given the shallow layer of directional (veering) shear in the lowest levels.

KEY MESSAGE 3...A cold front is expected to follow close behind on Friday.

The timing of this cold front will be critical in the risk for thunderstorm hazards, and there is still differences both run- to-run and 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. The good news is that most guidance is trending drier on Saturday.

AVIATION /06Z TUESDAY THROUGH SATURDAY/

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

Through Today...Prevailing VFR conditions expected. Northwest winds up to 10 kts.

Tonight...Prevailing VFR conditions expected. Winds favoring westerly, but could become light and variable at all TAF sites.

Outlook...

Wednesday...Prevailing VFR.

Wednesday Night and Thursday...Prevailing VFR, but showers and storms after 06Z Thursday continuing at times through the day could lead to temporary flight restrictions.

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

Saturday...Prevailing VFR conditions.

MARINE

No marine hazards expected. Some gusts up to 20 kt possible through 12Z Tuesday on the NJ coastal waters.

Outlook...

Thursday and Friday...Gusts near 25 kt are possible, particularly on Thursday with southerly winds behind a warm front.

Saturday...No marine hazards expected.

Rip Currents...

On Tuesday, winds will shift more westerly around 10 to 15 mph with breaking waves still around 1 to 2 feet. There will be some medium to long period southeast swells, though seas will be mainly 2 feet or less with offshore winds. 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 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 near 1 to 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 specific beach forecasts, visit weather.gov/beach/phi

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...None.