textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated Aviation section for 18Z TAFs.

KEY MESSAGES

1. Scattered severe thunderstorms are likely late this afternoon into this evening.

2. Widespread minor coastal flooding continues with the evening high tide cycles tonight along the Atlantic coast and Delaware Bay. A new Coastal Flood Advisory was issued for tonight's high tide.

3. Some severe thunderstorms possible Thursday afternoon and evening.

DISCUSSION

KEY MESSAGE 1...Scattered severe thunderstorms are likely late this afternoon into this evening.

For today, an expansive upper-level trough across much of eastern Canada will gradually shift eastward. Some stronger shortwave energy within the southern portion of the trough will slide across our area. At the surface, a cold front will be tied to this feature, which is forecast to cross through this evening. There is also indications of a pre-frontal trough as well. The low to mid level flow will increase some in response to these features, which will increase both the warm air and low-level moisture advection.

At this point we are still fairly confident that showers and thunderstorms will be around portions of the area late this afternoon and evening. As stronger synoptic forcing arrives, guidance continues to indicate bulk shear values in excess of 40 kt, which will aid storm organization. Looking at other parameters, MLCAPE in the area should max out around 1000-1500 J/kg with decent low-level shear around 25-30 kt. Low-level lapse rates will be in excess of 8 C/km, DCAPE will top out near 1000 J/kg, and PWATs will be near 1.8 inches, which support wet microbursts, indicating that damaging winds gusts are likely with any storms. Additionally, some guidance has continued to depict a localized corridor of backed surface winds, primarily over southwestern portions of the area, so an isolated tornado cannot be ruled out. One fly in the ointment though, is some forecast soundings (more notably the NAM) indicate an area of mid-level warming between 700-850 mb, which may keep the area largely capped through the afternoon. In fact the trend has been for a slightly later timing for the convection.

Given that there will likely be a prefrontal trough ahead of the main cold front, convection could start to fire along the trough as early as the 21-23z timeframe over Delmarva into portions of SE PA and adjacent portions of southern NJ. This initial round of convection would tend to favor clusters of storms with the potential for some embedded supercells. Storm motion will be to the northeast with the main uncertainty being the magnitude and coverage. Following this initial round, another round of storms will be possible by later in the evening into the early part of the overnight along and just ahead of the cold front. This second round would favor a more linear configuration. A forecast challenge continues to be that the instability should be a bit higher with the initial round but the forcing won't be as strong and this could be a limiting factor along with the warm layer aloft noted above. Stronger forcing will arrive with the cold front later in the evening however instability could be more limited by this time. For these reasons, the Storm Prediction Center is maintaining the SLIGHT risk (level 2/5). Finally, given that PWATs are abnormally high, we also cannot rule out some localized flash flooding, especially near/over the urban corridor. The limiting factor, however, will be rapid storm motion.

The cold front and associated convection should be largely out of the area by 2 AM Monday though the severe threat will be decreasing by around midnight.

KEY MESSAGE 2...Widespread minor coastal flooding continues with the evening high tide cycles tonight along the Atlantic coast and Delaware Bay. A new Coastal Flood Advisory was issued for tonight's high tide.

Minor tidal flooding was observed on Saturday Night as a New Moon this weekend is resulting in higher than normal astronomical tides. Further rounds of tidal flooding are expected with the evening/nighttime high tide cycles tonight and potentially Monday for the Atlantic Coast and Delaware Bay.

The tidal Delaware River and its tributaries could experience some spotty minor tidal flooding (non-advisory) with tonight's high tide but generally these astronomical tide driven flooding events produce more noteworthy impacts for the Atlantic coasts and Delaware Bay. The highest of the New Moon tides will be tonight and Monday night. A new Coastal Flood Advisory was issued for this evening for the same area it was last night (Jersey Shore/Delaware Beaches + Delaware Bay Communities + Middlesex County for the Raritan Bay).

Tidal flooding is not currently expected along the eastern shore of Chesapeake Bay.

KEY MESSAGE 3...Some severe thunderstorms possible Thursday afternoon and evening.

An upper-level trough centered over south-central Canada is forecast to strengthen eastward and also southward across the Great Lakes. A strong shortwave rounding the base of this trough may result in a portion of it taking on a negative tilt across the eastern Great Lakes and toward New England during Thursday. This will also drive deepening surface low pressure up across the eastern Great Lakes and into adjacent Canada during Thursday. A strong cold front will then arrive into our area Thursday afternoon or Thursday evening.

As the synoptic pattern evolves, the flow will increase out of the south and southwest. This will advect deeper moisture northward, and in tandem with strengthening warm air advection will lead to increased instability during the day Thursday. In addition, a strong mid to upper level jet streak is forecast to extend eastward from the eastern Great Lakes, with 40-70 knots of flow at 500 mb forecast across our area Thursday afternoon. This increased flow will result in greater shear magnitudes. While there are some timing differences among the guidance regarding the main features, the consensus is that enough instability should overlap with the stronger shear along with incoming stronger forcing. As a result, there is some severe thunderstorm potential Thursday afternoon and evening. As of now, damaging winds may be the primary threat especially if convection develops into a squall line, however if shear and instability ends up being even greater than all severe thunderstorm hazards may come into play. The details are much less certain at this time range though given the typical uncertainty with instability and the thermodynamic profiles. The forecast synoptic setup however with a deepening surface low tracking well to our northwest with stronger flow within a warmer and more moist environment suggests some risk for severe thunderstorms. Finally, it's possible the front ends up being slow moving which could result in showers/storms lingering into Friday (though there should be a lower threat of severe weather by Friday).

AVIATION /18Z SUNDAY THROUGH FRIDAY/

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

Rest of Today...VFR with increasing clouds. Outside of stray shower, any convective activity is expected to occur tonight (see below). South-southwest winds around 10-15 kt gusting up to 20-25 kt. High confidence.

Tonight...Prevailing VFR to start the night. Showers and thunderstorms are likely to move into the area around 23-02Z for western areas, and 02-05Z for eastern areas. Expect brief periods of MVR/IFR conditions with storms with a lingering period of some showers and MVFR conditions in wake of the storms. Improvement expected by 06-09Z. Southerly winds around 5-10 kt tonight, becoming northwesterly around 10-15 kt in wake of the front. Winds associated with evening convection could briefly gust to 40+ knots. Moderate confidence as a whole.

Monday...VFR with a few/scattered clouds in the afternoon. Northwest winds around 10-15 kt, occasionally gusting up to 20 kt. High confidence.

Outlook...

Tuesday...Mainly VFR.

Wednesday...Some sub-VFR conditions with a few showers or a thunderstorm possible.

Thursday...A period of sub-VFR conditions probable in the afternoon and evening as showers and some thunderstorms increase.

MARINE

Small Craft Advisories remain in effect for the Atlantic Coastal Waters from Sandy Hook NJ to Cape Henlopen DE from 1 PM this afternoon to 12 AM Monday. Small Craft Advisories have also been issued for the Delaware Bay from 1 PM to 8 PM today.

Southwest winds around 10-15 kt and seas of 2 feet or less are expected to continue through this morning. By this afternoon and into this evening, southerly winds are expected to increase to around 15-20 kt with gusts up to 25 kt. Seas build from 2-4, potentially up to 5 feet. Scattered thunderstorms possible this evening.

Outlook...

Monday through Wednesday...The conditions are anticipated to be below Small Craft Advisory criteria.

Thursday...A period of Small Craft Advisory conditions probable. Southerly wind gusts could near gale force for a time later Thursday. Some gusty thunderstorms late in the afternoon and evening possible.

Rip Currents...

For today, winds become southerly and increase to 10-15 MPH with some higher gusts. However, a medium period swell around 6-8 seconds will continue along with breaking waves once again 2 feet or less for most beaches. The exception will be at southern facing beaches in Cape May and Atlantic counties where breaking waves in the surf zone will be up to 3 feet as the winds will be most directly onshore here. Therefore, the risk for the development of dangerous rip currents will be higher for these beaches, and a MODERATE risk is currently forecast for these two counties. For the remainder of the Jersey Shore and the Delaware Beaches, a LOW risk for the development of dangerous rip currents is forecast.

For Monday, winds become northwest 10-15 mph with a continued medium southerly swell of 6-7 seconds. With breaking waves 2 feet or less, have gone with a LOW risk for all beaches.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT Monday for NJZ012>014-020>027. DE...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT Monday for DEZ002>004. MD...None. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ430- 431. Small Craft Advisory until midnight EDT tonight for ANZ450>454.