textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

The severe thunderstorm risk Sunday afternoon/evening has increased some for portions of our area.

KEY MESSAGES

1. Patchy areas of dense fog possible early Sunday morning, primarily over southern New Jersey and the Delmarva.

2. Showers, potentially some severe thunderstorms, later Sunday afternoon and evening, then turning much cooler with gusty winds on Monday.

DISCUSSION

KEY MESSAGE 1...Patchy areas of dense fog possible early Sunday morning, primarily over southern New Jersey and the Delmarva.

Broad surface high pressure over New York State will shift off the New England coast tonight. As it does so, east to southeasterly return flow on the southern periphery of the high will allow some low-level moisture to advect toward the coast and potentially inland. Current HREF probabilities depict roughly a 30-60% chance of visibilities 1 mile or less across the Delmarva and southern New Jersey beginning in the pre-dawn hours on Sunday. Visibilities may approach one half mile or less at times. Depending on the evolution of fog development tonight, fog headlines may become warranted. Any fog should clear by mid-morning as a warm front lifts north of the region.

KEY MESSAGE 2...Showers, potentially some severe thunderstorms, later Sunday afternoon and evening, then turning much cooler with gusty winds on Monday.

An upper-level trough across central Canada is forecast to amplify some Sunday and Monday on its south side from the Great Lakes and Northeast. This will drive surface low pressure eastward from the Great Lakes which tracks near our area later Sunday. However, another weak low develops along the front and within the elongated pressure trough. This secondary weak low may become convectively enhanced as it slides east-southeast along the front late Sunday afternoon and Sunday night. A frontal zone will be to our north and this will keep our area within the warm sector with southwesterly winds during most of the day Sunday. However, a strong cold front will move southeastward across the area Sunday night bringing colder air across our area Monday through Tuesday. There remains some timing differences among the model guidance with the passage of the cold front. This will depend on where exactly the new weak low develops along the front and its track as this will pull the cold front through our area. Ahead of the front, high temperatures Sunday are forecast to reach the 60s to low/mid 70s for most of the region. A more southerly wind component though will keep it cooler along the coast due to the wind off the cold ocean.

A ribbon of stronger forcing for ascent ahead of and with the cold front and weak surface low results in an area of showers arriving in our northwest zones around mid Sunday afternoon then spreading southeastward through Sunday night into early Monday morning. The model forecast soundings indicate some instability developing within the warm sector, however this may end up being mostly elevated. Some soundings do show some weak surface-based instability developing especially near and north/west of I-95 as the low-level lapse rates steepen during peak heating and the cap/inversion is eroded. The timing will be key as earlier convection will have the potential to become locally severe with damaging winds the main threat, although some large hail is possible due to some steeper mid level lapse rates. Some of the guidance, including the HREF and HRRR, indicate potentially robust updrafts to our west that should make it into parts of our western zones toward early evening. The robust updrafts given the setup would favor large hail production, and this looks to be maximized just to our west as of now. There is plenty of shear, especially speed shear, as strong flow overspreads the region with 850 mb winds right around 50 knots.

Given the potential for some severe thunderstorms and factoring in the latest guidance, the SPC has expanded the Slight Risk area (level 2 out of 5) eastward and it now includes from about I-95 north and west), and this is surrounded by a Marginal Risk (level 1 out of 5). Convection is forecast to develop across north-central Pennsylvania close to the front and where the warm air advection is maximized. These will then shift east and southeastward. The storm mode may be discrete initially, however then tend to transition into some short line segments. This mode change may result in a large hail risk initially that becomes more of a locally damaging wind risk. The tornado risk looks rather low as of now unless the low-level moisture (surface dew points) ends up increasing more than currently forecast. Much of our area may be on the eastern edge of the better buoyancy, therefore the eastern extent of the severe thunderstorm risk is lower confidence. While still some uncertainty with the overall timing, the main severe thunderstorm threat looks to be between 4 PM and 10 PM.

The precipitable water is forecast to increase to 1+ inches by Sunday night. This along with some potential for a time of training convection could result in some corridors of heavier rainfall (1+ inches). If heavy rainfall rates occur over the more urban areas, localized flash flooding is possible. Otherwise, the rainfall is much needed.

As cold air advection develops late Sunday night and especially during Monday in the wake of the cold front, high temperatures Monday occur during the morning to perhaps midday hours before leveling off and then falling some during the afternoon. A pressure gradient tightens in the wake of low pressure departing to our east and northeast Monday and high pressure building toward the Great Lakes. This combined with stronger cold air advection will steepen the low-level lapse rates and result in deeper mixing. A gusty northwest wind results during Monday especially in the afternoon into the evening, with peak wind gusts of 25-35 mph. There will be less wind on Tuesday, however colder/below average temperatures are forecast.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

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

Tonight and Sunday... VFR for most of the night before stratus and some fog develops. Thinking the worst/lowest conditions will be closest to the coast, where IFR/LIFR conditions are likely (50-70%), especially around daybreak and continuing through the early morning. KACY/KMIV were the only TAF sites where fog was included in the TAFs, between 11z-14/15z. Conditions gradually improve after 15z, but expecting some lingering MVFR/IFR restrictions to occur through 17z-19z. VFR after. Winds out of the southeast around 5 kt tonight and becoming more south/southwesterly tomorrow morning and for the afternoon around 5-10 kt.

For the I-95 terminals, thinking fog stays off to the south and east and currently expecting MVFR/IFR stratus after 09z-10z and continuing through the late morning or early afternoon (16z-18z). Cannot rule out some fog developing, but would put the probability around 15-25%. Winds east/southeast tonight and tomorrow morning around 5 kt, becoming more southerly/southwesterly late morning and afternoon, around 10 kt.

For the Lehigh Valley terminals (KRDG and KABE), currently have higher confidence in stratus rather than fog, though some fog appears possible (40-50%) at KRDG. However, there will be some MVFR stratus likely developing around 08z-10z and continuing through the late morning (14z-16z). Winds out of the east/southeast tonight around 5 kt, becoming more south/southwesterly late morning and continuing for the afternoon, around 10 kt.

Overall, confidence is low in the TAF forecast for tomorrow morning on when and if fog and/or stratus develops. Guidance tends to struggle with these setups. However, would put the highest probability for the worst conditions (IFR/LIFR) at KACY/KMIV, with gradually decreasing chances the further north and west you go, up to KABE/KRDG.

Sunday Night...Sub-VFR conditions likely (60-70%) as scattered showers and thunderstorms move through. Kept thunder out of the KPHL TAF for now given the probability of thunderstorms is too low to include thunder at this time. Winds become northerly once showers and thunderstorms move through. Low confidence.

Outlook...

Monday..Improving conditions Monday afternoon and at night. Northwest wind gusts 20-30 knots on Monday, before diminishing Monday night.

Tuesday...VFR with less wind.

Wednesday...Slight chance of rain showers mainly at night, otherwise mostly VFR.

Thursday...Periods of sub-VFR conditions possible with some showers, especially at night.

MARINE

No marine headlines are in effect through Sunday. North winds around 5-10 kt will shift out of the east late this afternoon and become southeast around 10-15 kt tonight. Seas around 2-3 feet. On Sunday, south-southwest winds will increase to 15-20 kt with gusts up to 22- 23 kt. Seas around 3-4 feet.

Patchy dense marine fog is possible across all waters beginning early Sunday morning and continuing through mid-day. A slight chance of thunderstorms are also possible on the northern ocean waters late Sunday afternoon.

Outlook...

Sunday night and Monday...Small Craft Advisory conditions probable Sunday night and especially Monday, then conditions subside Monday night.

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

Thursday...Small Craft Advisory conditions may develop, mainly at night.

PHI WATCHES/WARNINGS/ADVISORIES

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