textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

The Dense Fog Advisory has been extended until 10 AM.

KEY MESSAGES

1. Areas of fog, some dense, will impact portions of the area into mid morning.

2. A cold front will bring a chance for showers and few thunderstorms to the area this afternoon through tonight.

3. Some additional opportunities for rainfall this week.

DISCUSSION

KEY MESSAGE 1...Areas of fog, some dense, will impact portions of the area into mid morning.

The dense fog has been slow to dissipate, so have extended the dense fog advisory to 10 AM. Calm winds, residual ground moisture, and a strong temperature inversion above the surface led to the development of fog earlier across much of eastern PA and northern NJ. Many observations across much of eastern PA and northern NJ have visibilities of a quarter mile or less. Areas where fog has already dissipated, no further fog development is expected this morning.

KEY MESSAGE 2...A cold front will bring a chance for showers and few thunderstorms to the area this afternoon through tonight.

A large scale trough centered over portions of Ontario and Quebec will shift slowly eastward through tonight, with the region embedded in enhanced (45-55 kt) southwesterly flow aloft downstream of the trough axis. At the surface, low pressure will track well to the north of the region with a cold front passing through the area this afternoon into this evening.

Ahead of the cold front, warm west-southwesterly flow will continue into this afternoon, with at least filtered sunshine. This will allow temperatures to climb into the mid 70s to low 80s across much of the area. As the cold front approaches this afternoon, scattered showers are expected to develop and track from northwest to southeast across the area. Some elevated instability will be present, with MUCAPE values generally around 500 J/kg, so a few thunderstorms will be possible. This appears most likely along and southeast of I-95. With the strong daytime heating, vertical mixing appears likely, limiting the extent of surface based instability that is able to be realized. Therefore, while gusty winds can't be ruled out with any stronger storm, severe weather is not currently anticipated.

After the cold front passes, the broad region of ascent from the trough will allow for continued scattered shower development, primarily along and southeast of I-95, through the overnight hours. QPF from this event looks to be low, generally around a tenth of an inch or less. Lows will be in the mid 40s to low 50s.

KEY MESSAGE 3...Some additional opportunities for rainfall this week.

An upper-level trough is forecast to be more present across much of the East through this week. Some guidance continues to be more potent with the trough amplification later this week with a closed low developing in the vicinity of our area and then moving off to the northeast. There are other models that sharpen this trough eastward, however keep it more progressive and not become closed off. The overall pattern that is forecast will offer some opportunities for rainfall through this week.

Our first system brings a cold front through the area into Monday morning. The front itself looks to shift farther south and east with it mostly offshore to start Monday. Mid level energy may result in just enough lift in the wake of the front for a time Monday to produce additional showers mainly south and east of I-95. A model trend has been to be weaker with this overall and thus decreased rainfall amounts/coverage. It turns chillier Monday night as the trough axis shifts offshore and surface high pressure begins to build in. There is some frost potential, mostly in the southern Poconos, later Monday night into early Tuesday morning.

The active pattern looks to reload as another upper-level trough quickly replaces the first one across much of the East. This feature may become closed off in the vicinity of our region late Wednesday through Friday before it lifts out during Saturday. The arrival of increased warm air advection ahead of this system and with a cold front should drive an increase in shower activity during Wednesday. The chance for thunder remains less certain as it will depend on how much destabilization can occur. If the system closes off as it arrives Thursday into Friday, then surface low pressure may end up developing off the northern Mid-Atlantic coast. This would increase the intensity and coverage of showers/rain and result in more of an onshore flow and therefore even cooler temperatures. This would also tend to result in a more stable air mass and thus little to no risk of thunder. The upper air pattern evolving during the second half of this week is less certain, as the amount and coverage of precipitation will depend on whether we end up with a closed low or just a sharper upper-level trough that is more progressive. As the trough or closed low moves away to start the weekend, a much warmer air mass will start to arrive.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

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

Today...Conditions look to improve to VFR by 14Z as fog dissipates at all terminals. Scattered showers are possible beginning 17-19Z. A few thunderstorms can't be ruled out for PNE, PHL, and ILG. Brief periods of visibility restrictions will be possible in heavier showers or storms. West- southwest winds increasing to around 10 kt with a few gusts 15-20 kt this afternoon, becoming northwest between 21-23Z at 5-10 kt. Moderate confidence.

Tonight...Primarily VFR. Light showers will be possible through the night, particularly for the I-95 terminals, MIV and ACY. Brief periods of MVFR visibilities can't be ruled out with any heavier showers. Northwest winds becoming north-northeasterly at around 5 kt. Moderate confidence.

Outlook...

Monday...Mostly VFR ceilings in the morning with some showers, especially south and east of PHL.

Tuesday...VFR.

Wednesday and Thursday...A period of sub-VFR conditions possible along with showers. A few thunderstorms possible Wednesday afternoon and evening.

MARINE

No marine headlines are anticipated through tonight. Southerly winds will increase to 10-15 kt with gusts around 20 kt on the waters this afternoon, shifting to northerly and eventually north-northeasterly tonight. Seas generally 2-4 feet through tonight. Showers will be possible beginning this evening and continuing through the overnight hours tonight.

Outlook...

Monday and Tuesday...The conditions are anticipated to be below Small Craft Advisory criteria.

Wednesday...A period of Small Craft Advisory conditions possible.

Thursday...The conditions are anticipated to be below Small Craft Advisory criteria.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Dense Fog Advisory until 10 AM EDT this morning for PAZ054-055- 060>062-071-103>106. NJ...Dense Fog Advisory until 10 AM EDT this morning for NJZ001- 007>010-012>015-018>020-026-027. DE...None. MD...None. MARINE...None.