textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated Aviation section for the 12Z TAFs.

KEY MESSAGES

1. A warm front lifts north through the region tonight.

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

3. Widespread frost and freeze conditions are increasingly likely Saturday night.

DISCUSSION

KEY MESSAGE 1...A warm front lifts north through the region tonight.

Northerly winds will usher a cooler and drier airmass into the region. Surface dew points will fall into the 40s and 50s throughout the area by this afternoon, and high temperatures today will be much cooler than Wednesday. Highs today will be some 15 to 20 degrees cooler than Wednesday, topping off in the low to mid 60s.

Low pressure then develops over the Midwest today and lifts to the northeast into the Great Lakes and Ohio Valley by this evening, then towards the southern Province of Ontario by Friday morning. This will pull a warm front north through the region. Clouds will lower and thicken, and rain begins to develop late.

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

A sharp trough will begin shifting southeastward towards the region from the Upper Midwest late this week. Slow height falls aloft will begin across the area late Thursday night, with rapid height falls through the day on Friday. The trough axis currently looks to pass through the region late Friday night or early Saturday morning. At the surface, a warm front will lift northward through the region Thursday night into Friday morning, with a strong cold front passing through Friday evening into Friday night.

Guidance continues to indicate a slower system which continues to delay the timing of the cold frontal passage somewhat. Currently, it appears likely that isentropic ascent will lead to a broad area of mostly light showers north of the warm front late Thursday night into the first part of the day Friday. Once the region is in the warm sector, additional showers will likely mostly hold off until the cold front approaches.

In the warm sector, warm and moist southwesterly flow will be taking place with temperatures surging into the upper 70s to low 80s with dewpoints in the mid to upper 60s. Some destabilization is likely to take place, although the window will likely only be a few hours long. Guidance currently indicates a narrow corridor ahead of the front of around 500 J/kg MLCAPE. While instability will be modest, wind fields aloft will be quite strong, with 500 mb flow exceeding 60 kt. Scattered to widespread showers and some storms are likely to form along and immediately ahead of the cold front. It remains a possibility that a few storms could become severe, with damaging wind gusts being the primary hazard.

The cold front and accompanying showers and storms will clear the area from west to east Friday evening into Friday night. In the wake of the cold front, much cooler and drier air is expected to infiltrate the region. In addition, breezy northwest winds look to impact the area overnight Friday night and through much of the day Saturday. Right now, it looks like there could be an extended period of wind gusts in the 30-40 mph range, but it is not out of the question that winds could be a bit higher and the need for wind headlines still cannot be ruled out. Either way, it will be a much cooler day on Saturday, with highs only topping out in the 40s across the Poconos and far northern NJ, and low to mid 50s areawide.

KEY MESSAGE 3...Widespread frost and freeze conditions are increasingly likely Saturday night.

Saturday night as the trough axis shifts east away from the region and high pressure builds in at the surface, the coldest air so far this season is expected across the entire area. Skies will likely be mostly clear, with winds quickly diminishing. This could set the stage for very good radiational cooling conditions. As a result, low temperatures are expected to fall below freezing across the Poconos, potentially southward into portions of the Lehigh Valley, and adjacent areas of far northern NJ. For much of the rest of the area, lows in the mid 30s to near 40 are expected. Rural areas even across southeastern PA and southern NJ cannot entirely rule out seeing their first freeze of the year. Regardless, a fairly widespread frost is expected across eastern PA and most of NJ, and some patchy frost can't be ruled out farther south into Delmarva either.

Trends will continue to be monitored, but frost and/or freeze headlines appear increasingly likely to be needed in upcoming forecast cycles.

AVIATION /12Z THURSDAY THROUGH MONDAY/

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

Today...VFR. N winds around 10 kt, veering to the E this afternoon, then SE late. High confidence.

Tonight...VFR initially, then MVFR, and IFR CIGs develop late. Potential for MVFR VSBYs in RA. SE winds 5 to 10 kt, becoming S late. Moderate confidence.

Outlook...

Friday through Friday night...Sub-VFR conditions likely in showers and a few thunderstorms. Showers and storms will clear from west to east Friday night, though gusty northwest winds will set in.

Saturday...VFR. Gusty northwest winds.

Saturday night through Monday...VFR. No significant weather.

MARINE

Small Craft Advisories remain in effect for for the Atlantic coastal waters from Cape May to Fenwick Island until 11 am.

Behind a cold front, north winds will average 10 to 15 kt with gusts up to 20 kt, except up to 25 kt for the Delaware ocean waters this morning. Winds become east by this afternoon, then southeast late. Southeast winds 10 to 15 kt turn south late tonight.

Outlook...

Friday through Friday night...Advisory conditions likely to develop by late afternoon Friday as southwesterly winds pick up to 15-20 kt with gusts to 25-30 kt. Friday night, winds will shift to northwesterly with a cold frontal passage and increase further. Gales will be possible. Seas increasing to 4-6 feet. Showers and a few storms will likely lead to visibility restrictions.

Saturday through Sunday...Advisory conditions likely to continue through Saturday and even Saturday night with northwesterly winds potentially gusting to near gale force through the daytime hours Saturday before beginning to decrease Saturday night. Winds and seas will likely subside to below advisory criteria through the day Sunday. Sunday night through Monday...No marine headlines currently anticipated with winds below 25 kt and seas below 5 feet.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Watch from Friday evening through Saturday afternoon for ANZ430-431-450>455-480>483-485. Small Craft Advisory until 11 AM EDT this morning for ANZ454- 455.