textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated Aviation section for 06z TAFs.

KEY MESSAGES

1. Surge of warmth and humidity Wednesday ahead of a cold front.

2. A strong cold front passage on Friday will bring windy conditions, showers and potential for some thunderstorms, and colder temperatures into the weekend.

3. There is potential for widespread frost and freeze conditions Saturday night.

DISCUSSION

KEY MESSAGE 1...Surge of warmth and humidity Wednesday ahead of a cold front.

A frontal zone draped to our north will result in surface low pressure riding along it through Wednesday, pulling a cold front across our area by late Wednesday. Ahead of this feature, southerly flow will boost the low-level warm air advection regime, resulting in much warmer air surging into our area. Wednesday's highs are forecast to mainly range in the low to mid 80s, with even some upper 80s to near 90 degrees possible over southern Delaware.

In addition, dew points will increase back through the 60s into Wednesday. This will result in a much more humid feel over the next couple days, with it being the most humid on Wednesday (heat index values should approach 90F for areas from about Philadelphia south and east; away from the coast). There does not appear to be much in the way of forcing, therefore just a slight chance (less than 25%) for showers is expected. After the front moves through, cooler air will briefly return on Thursday.

KEY MESSAGE 2...A strong cold front passage on Friday will bring windy conditions, showers and potential for some thunderstorms, and colder temperatures into the weekend.

A sharp upper-level trough is forecast to arrive across the East Coast on Friday, then gradually lift out over the weekend. This will drive rapidly strengthening surface low pressure just north of our area Friday, with an associated strong cold front sweeping through. Guidance has come into consensus that the fast moving cold frontal passage will occur between the mid afternoon to evening hours likely moving from our western most zones and offshore within about 3 hours or so.

A warm and increasingly moist airmass ahead of this system will provide modest instability and decent boundary layer mixing as the strong cold front approaches on Friday. Large scale forcing for ascent will increase with the arrival of the robust upper- level trough and cold front, therefore showers will increase in coverage and intensity Thursday night into Friday. However, the exact details regarding the convective evolution remain unclear. Some thunderstorms are possible given the strength of the system and instability, however this will ultimately depend on the magnitude of the instability and timing of the cold front. Cannot rule out some severe thunderstorm potential, especially if any more robust convection can materialize along the cold front.

While it will be mostly cloudy, strong warm advection will boost temperatures up well into the 70s and low 80s ahead of the front. As surface pressure falls rapidly amid a strengthening southwesterly gradient, breezy conditions will likely develop outside of any showers and storms with gusts of 25-35 mph possible.

During and after cold frontal passage, strong cold air advection will occur as the sharp trough aloft sweeps through. This will support efficient mixing of winds aloft down toward the surface in the form of frequent wind gusts as temperatures plummet into the 50s then 40s overnight. Boundary layer winds could be as high as 40-50 mph for a few hours in the wake of the front, which could warrant a Wind Advisory. The timing of the peak winds appears to be focused during the evening and overnight hours.

The coldest airmass so far this season will settle into the area Saturday into Saturday night. High temperatures Saturday are only in the low to mid 50s in most areas (40s north of I-78). Breezy conditions will continue, with northwest winds gusting 30-40 mph possible in the morning before gradually diminishing later in the day.

KEY MESSAGE 3...There is potential for widespread frost and freeze conditions Saturday night.

As strong surface high pressure settles nearly overhead into Saturday night, temperatures will cool significantly with radiational cooling. This should result in widespread low temperatures in the 30s Saturday night into Sunday morning, and potentially into the upper 20s north of I-78. This will introduce our first widespread frost and freeze potential of the season. A freeze appears unlikely south of I-78, but the usual cold spots could drop to near the freezing mark. Otherwise, areas of frost are possible.

The airmass will start to modify Sunday and into early next week. Highs Sunday should rebound into the mid 50s to near 60 degrees in most areas with winds 5-10 mph making for a much more pleasant day compared to Saturday. There could be some frost potential Sunday night, but lows are likely to rebound into the upper 30s to low 40s in most areas. Temperatures will then rebound to near normal values, with highs well into the 60s early next week and dry weather expected.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

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

Rest of tonight...VFR with mostly clear skies. A period of LLWS is likely through 09Z. West-southwest winds 4-8 kt. High confidence.

Wednesday...VFR with increasing clouds after daybreak. West winds around 5-10 kt early, increasing to around 10 gusting upwards of 15-20 knots by the afternoon. A period of showers is possible from west to east, but confidence is not high enough to include in TAFs at this time. Moderate-high confidence overall.

Wednesday night...VFR. Slight chance for brief restrictions in showers, but confidence and coverage is low for impacts.

Outlook...

Thursday and Friday...Periods of restrictions probable during this period, especially on Friday in showers. Potential for thunderstorms as well. Southwest winds Friday shifting northwest Friday evening with gusts up to 40 kts possible.

Saturday through Sunday...VFR. Northwest winds gusting up to 30 kts on Saturday.

MARINE

No marine headlines are in effect through Wednesday night. West winds around 10-15 kt on Wednesday, turning north and gusting to 15-20 kts in the evening and overnights following a cold frontal passage. Seas around 2-4 feet. Fair weather expected outside of gusty winds.

Outlook...

Thursday...No marine hazards are anticipated.

Friday and Saturday...Small Craft Advisory conditions developing during the daytime Friday, then gale force winds likely Friday night into Saturday as a strong cold front pushes offshore Friday evening. Seas building over 5 feet.

Saturday night through Sunday...Lingering SCA conditions early Saturday night. Otherwise, no marine hazards are anticipated.

PHI WATCHES/WARNINGS/ADVISORIES

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