textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Marine Dense Fog Advisories have been extended until 6 AM Sunday for all Atlantic Coastal Waters including the Delaware Bay.

KEY MESSAGES

1. Localized dense fog possible tonight, especially along the NJ coast.

2. A couple of fronts will impact the region through Sunday with continued chances for showers and possibly some thunderstorms.

3. Warmer air will overspread the region early next week, with a couple chances for showers and fog.

DISCUSSION

KEY MESSAGE 1...Localized dense fog possible tonight, especially along the NJ coast.

A stable airmass remains entrenched over the entire forecast area with low stratus. While a strong warm air advection regime is expected to take place overnight, there remains a strong low- level inversion keeping moisture trapped at/near the surface. This is specifically enhanced near coastal areas where cold air off the chilly Atlantic waters steepens the inversion compared to further inland. HREF probabilities suggest there is roughly a 60%+ chance of visibilities one-quarter of a mile or less tonight for coastal areas of NJ. These probabilities then drop drastically to roughly 10-20% further inland.

With this being said, it is fairly confident there will be at least some dense fog near the NJ coast tonight. However, the overall duration is less certain with the timing of the warm air advection push and the overall extent inland. For this reason, have opted to let the evening shift evaluate and issue advisories as needed.

KEY MESSAGE 2...A couple of fronts will impact the region through Sunday with continued chances for showers and possibly some thunderstorms.

High pressure over eastern Canada continues to shift away this afternoon. Meanwhile, the back-door front that is currently situated over central PA down into central and eastern VA will begin to lift back north later today. This warm front is not progged to reach our area until at least this evening for the Delmarva, near Philly until around/after midnight, and across northern areas by the pre-dawn hours Sunday morning. A cold front will then be quickly on its heels tracking through early to mid-day on Sunday from northwest to southeast.

In terms of showers, the rest of today should be fairly dry. For tonight, showers and t-storms associated with a pre-frontal trough (bringing the severe weather to E OH and W PA today) will be tracking across central PA this evening. As this line progresses east overnight, the line should weaken and decay as it runs into the cooler and more stable airmass (denoted in Key Message 1). Although these showers may outrun the strongest forcing, there remains an elevated potential for instability, so cannot completely rule out a t-storm or two tonight. Best chances will be further north and west over Berks Co, the Lehigh Valley and Poconos, but no severe weather is expected. Elsewhere, just some showers are possible tonight. Temperatures tonight will start off in the 40s, but rise overnight into the 50s once the warm front clears. Come Sunday, better forcing arrives with the actual cold front. With much of the area in the warm-sector by this point, there may be some re-development along the front itself depending on how fast the front progresses through the area. Thus, for areas south and west of I-95, especially across the Delmarva, there may be a potential for additional showers and t- storms late Sunday morning into early afternoon. Again, no severe threat is expected and any showers/storms will move offshore by mid- afternoon. High temps will top out in the 60s for much of the area, with 50s in the Poconos and possibly touching 70 across southern DE.

KEY MESSAGE 3...Warmer air will overspread the region early next week, with a couple chances for showers and fog.

The region will be beneath a fairly benign upper pattern from Sunday night through Tuesday night. At the surface, high pressure will build into the area in the wake of Sunday's frontal passage. As the high gradually shifts eastward through Tuesday night, light return flow is expected to commence, bringing about a warming trend. Lows Sunday night will likely be in the upper 30s to mid 40s across the entire area. On Monday, with light southerly flow and at least some sunshine, temperatures are expected to rise into the 60s across most of the area. The warming trend will continue through Wednesday with highs warming into the low to mid 70s and lows in the 40s and 50s. Highs may be cooler in proximity to the ocean due to potential sea breeze development each day. In addition, there is the potential for fog each night. Relatively light winds in combination with warm and moist air overspreading the region with very cool ocean temperatures could lead to dense fog, particularly near the coasts.

Beginning on Wednesday, the upper pattern will become much more unsettled as a potent trough over central portions of the US begins to work eastward towards our area. Broad ascent is likely to begin overspreading the region during this time, and will lead to a chance for scattered showers within the warm air advection regime. On Wednesday, shower activity appears most likely along and north of I- 95, with PoPs currently in the 20-40% range. Wednesday night, showers may have a tendency to become more widespread over the entire region.

The trough axis is expected to shift eastward across the region sometime on Thursday, with a surface low and trailing cold front moving through the area. Along and ahead of the cold front on Thursday, PoPs increase to 50-70% areawide. Most guidance features a line of showers moving through the region. Very strong wind fields aloft and at least modest instability are forecast to be present, so thunderstorms will be possible. After the frontal passage, some lingering rain or snow showers will be possible across the area as colder air filters into the region.

Cooler temperatures are expected Friday and into the first part of the weekend. Another weak system may glance the region Friday night, bringing a chance for light rain and perhaps a little snow in the Poconos, but it is possible that the system will pass to our north keeping the region dry. For now, will feature 20-30% PoPs during this time period, with the higher chances being across northern portions of the area.

AVIATION /20Z SATURDAY THROUGH THURSDAY/

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

Rest of Today...MVFR/IFR ceilings expected. Possible spotty mist and fog after 22Z so opted to include TEMPOs where needed. South winds around 5-10 kt. Moderate confidence overall.

Tonight...IFR/LIFR (possibly VLIFR at times) with low ceilings and visibility restrictions. Rain showers move in from west to east between 00-03Z for the Lehigh Valley terminals and I-95 terminals between 03-06Z. LLWS likely at all terminals between 00-10Z out of the southwest at around 40-50 kt. Possible clearing at MIV and ILG late, but confidence is low. Overall, moderate-low confidence.

Sunday...Lingering IFR/LIFR conditions early, should gradually improve to MVFR between 12-15Z with showers ending. The exception is MIV and ACY where showers may linger into the afternoon hours. Overall, a gradual improvement is expected with VFR conditions returning by the afternoon from northwest to southeast. West winds around 10-15 kt with occasional gusts up to 20 kt possible in the afternoon. Moderate confidence.

Outlook...

Sunday night through Tuesday night...Primarily VFR. Periods of sub- VFR conditions will be possible in fog each night.

Wednesday through Wednesday night...Sub-VFR conditions possible in rain showers.

Thursday...Sub-VFR conditions expected in widespread showers. Some thunder is possible.

MARINE

Marine Dense Fog Advisories have been extended until 6 AM Sunday for all Atlantic Coastal waters including the Delaware Bay. Small Craft Advisories remain in effect from 7 PM this evening until 6 PM Sunday for the Atlantic Coastal waters only.

South-southwest winds steadily increase tonight with gusts up to 25 kt possible. Southwest winds will then remain around 15-20 kt through Sunday afternoon before shifting west around 10-15 kt in wake of frontal passage. Seas around 4-6 feet.

A chance of showers tonight, becoming likely on Sunday with a slight chance for thunderstorms. Dense marine fog likely tonight, possibly lingering into Sunday as well.

Outlook...

Sunday night...SCA conditions may linger into the evening hours in the form of elevated seas, but should quickly become sub-SCA with seas 2-4 feet. Marine fog is possible.

Monday through Tuesday night...Sub-SCA conditions expected with winds under 25 kt and seas under 5 feet. Marine fog is possible Monday and Tuesday night.

Wednesday through Thursday...SCA conditions expected. Gales are possible on Thursday. Southerly winds 10-15 kt with gusts to 25 kt are expected Wednesday into Wednesday night. Southwesterly winds 20- 25 kt with gusts 30-35 kt expected Thursday, before becoming west- northwesterly behind a cold front Thursday afternoon. Seas 4-7 feet on Wednesday will increase to 5-9 feet Wednesday night into Thursday. Showers are likely, particularly on Thursday.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Dense Fog Advisory until 6 AM EST Sunday for ANZ430-431- 450>455. Small Craft Advisory from 7 PM this evening to 6 PM EST Sunday for ANZ450>455.