textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

A cold front continues to slowly drop south down the Delmarva.

KEY MESSAGES

1. Cold front continues to sink south today, bringing gradually improving conditions with high pressure overhead Saturday and the first half of Sunday.

2. Unsettled weather returns Sunday into Monday.

DISCUSSION

KEY MESSAGE 1...Cold front continues to sink south today, bringing gradually improving conditions with high pressure overhead Saturday and the first half of Sunday.

The stationary front that has lingered across the region the last few days will continue to sink south as a cold front today, allowing gradually improving conditions to filter in. Cooler temperatures and lower humidity will begin to arrive today, and especially Saturday.

Some showers are possible late today into tonight as the cold front finally shifts far south enough to allow surface high pressure to be overhead Saturday and into early Sunday. The forecast Saturday looks to be the driest, coolest, and least humid day through the weekend. That being said highs on Saturday are only a few degrees lower than Friday, so it won't be dramatically cooler.

High pressure will not last, however, and shift offshore Sunday, bringing return flow back across the region, allowing temperatures to creep back up into the mid to upper 80s and the humidity to return. The morning will be largely dry with chances for rain returning by the afternoon.

KEY MESSAGE 2...Unsettled weather returns Sunday into Monday.

The next storm system arrives Sunday into Monday as a surface low moves across the Great Lakes, Ohio Valley, and eventually the Mid-Atlantic, bringing the chance for showers and thunderstorms.

The biggest change with previous guidance is the spread in the timing of precipitation. There are some models that show precipitation moving in as early as 06Z to 12Z Sunday Morning. That being said, there are still a few models not showing rain move in until the afternoon. The net result of this uncertainty is that there are PoPs in place from late Saturday Night all the way through Monday. This long period of rain mention is an artifact of the differences between models in the timing, but I don't expect rain to occur for that entire period.

Regardless of the timing, the primary threat at this point is heavy rain leading to flash flooding. Global guidance shows PWAT values surging back into the 2" range, increasing the potential for heavy downpours within any thunderstorms that develop. As a result, WPC does have much of the Northeast and Mid-Atlantic in a Marginal risk in the Excessive Rainfall Outlook.

Depending on the timing of the storms, some instability may be in place to support a severe potential. Latest guidance is a bit more robust with upper-level forcing for ascent as well as shear, probably linked to some convectively enhanced features of the incoming trough which are tough to resolve a few days out. At this point, it remains too early to lock down any specifics for severe and flash flood, but we will need to monitor the potential over the next several days.

AVIATION /09Z FRIDAY THROUGH TUESDAY/

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

Today...VFR. W wind around 10 knots. Sea breeze for ILG, ACY, MIV during the afternoon (SE winds).

Tonight...Mainly VFR. Light NE winds. Sct tstm possible.

Outlook...

Saturday through Sunday Morning...VFR. No significant weather.

Sunday Afternoon through Monday...Sub-VFR conditions likely (60-70%) at times with widespread showers and a few embedded thunderstorms anticipated.

Tuesday and Wednesday...VFR conditions expected.

MARINE

No prevailing marine hazards expected through today.

Outlook...

Today through Sunday...No marine headlines expected.

Sunday Night through Monday...SCA conditions possible (30-40%) as seas near 5 feet and southerly wind gusts near 25 kt.

Tuesday...No marine headlines expected.

Rip Currents...

Generally tranquil conditions expected at the beaches through Saturday. Winds turn onshore but will be light - under 10 MPH. An 8 to 10 second period swell will also be present, but breaking waves should be 2 feet or lower. This will result in a LOW risk for rip currents at all beaches for Friday and Saturday.

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

PHI WATCHES/WARNINGS/ADVISORIES

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