textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Small Craft Advisories have been issued for the Atlantic coastal waters from Cape May to Fenwick Island for early Thursday morning.

Gale Watch issued for all Atlantic waters and Delaware Bay for Friday night through Saturday.

The wind forecast for Friday into Friday night has lessened some.

KEY MESSAGES

1. A cold front will sink south through the region this evening yielding cooler weather for Thursday.

2. A strong cold front 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 cold front will sink south through the region this evening yielding cooler weather for Thursday.

Low pressure tracking across northern New England will move into Nova Scotia tonight. Associated with this low is a frontal boundary that extends southwest into the Ohio Valley. This front will continue to slide south across the region this afternoon and into this evening. After a round of showers and some rumbles of thunder earlier today, temperatures have been knocked down into the 70s for many areas. Some recovery is possible for the rest of the afternoon with highs still forecast to top out in the low 80s for these areas. Elsewhere, temperatures have remained well into mid to upper 80s under mostly sunny skies. Some showers remain possible in southern areas through the early evening hours before diminishing.

After the front passes by this evening, cooler air will briefly return overnight tonight into Thursday. With skies clearing across northern areas overnight, lows in the 50s are expected for most areas, with 40s in the higher elevations. For southern areas, more in the way of cloud cover is expected to persist, which will keep overnight lows mostly in the 60s. For Thursday, much of the day is expected to be dry with a mix of sun and clouds. However, the front will begin to slowly work its way back north late in the day, so cannot rule out a few showers across the Delmarva as a result. Highs will be generally uniform in the low to mid 60s with onshore flow.

KEY MESSAGE 2...A strong cold front passage Friday evening 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. A consensus of guidance has slowed the passage of the front later in the day, most likely during the evening hours (4-8 PM), 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. It appears there will be a round of showers and a few thunderstorms overnight Thursday night into early Friday morning associated with the warm advection surge aloft. There should then be a break in convection by midday prior to the cold front's arrival late in the day and into the evening. A line or broken line of showers and storms should develop along or just ahead of the 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 20-30 mph possible.

During and after cold frontal passage, strong cold air advection will occur as the sharp trough aloft sweeps through. However, the timing of this now looks to trail behind the front until sometime after midnight. 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. However, the threat of winds of this magnitude have lessened some given the later timing of the system. The timing of the peak winds now appears to be focused during the overnight hours and into Saturday morning, with gusts of 30-40 mph likely.

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...Widespread frost and freeze conditions are increasingly likely 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 (including the Pine Barrens) could drop to near the freezing mark. Aside from the freeze potential, areas of frost are anticipated, especially across much of our NJ, PA, and northern DE areas. Delmarva could also get in on the frost threat.

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 /18Z WEDNESDAY THROUGH MONDAY/

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

Rest of Today...VFR. Showers possible for the next few hours, diminishing after 21Z from north to south. West winds generally around 10 kt, gusting up to 15-18 kt during the afternoon. Moderate-high confidence overall.

Tonight...VFR. A slight chance of showers at MIV and ACY in the early evening, waning after midnight. Northwest winds around 5-10 kt will settle out of the north-northeast overnight. High confidence.

Thursday...VFR. Mostly clear skies for much of the day, with increasing clouds late. Northeast winds around 5-10 kt in the morning, settle out of the east-southeast in the afternoon. High confidence.

Outlook...

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

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

MARINE

Small Craft Advisories have been issued for the Atlantic coastal waters from Cape May to Fenwick Island beginning at 5 AM Thursday.

West-southwest winds around 10-15 kt this afternoon will become northwest this evening. After midnight, a surge in north- northeast winds is expected around 15-20 kt with gusts up to 25 kt possible, mainly across southern waters through Thursday morning. Northeast winds will then diminish to around 10-20 kt for the rest of Thursday. Seas of 3-4 feet.

Aside for a slight chance of showers this evening, fair weather is expected outside of SCA areas.

Outlook...

Friday and Saturday...Small Craft Advisory conditions developing during the daytime Friday, then gale force winds possible Friday night into Saturday as a strong cold front pushes offshore Friday evening. Seas building over 5 feet. Heavy showers and a few thunderstorms will accompany the front as it pushes offshore Friday night, along with locally stronger winds and higher seas. A Gale Watch was issued for all Atlantic waters and Delaware Bay for Friday night through Saturday.

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

CLIMATE

Near record breaking high temperatures are forecast for today. Records for our climate stations are listed below:

Record High Temperatures October 14 Site Record/Year Allentown (ABE) 86/1975 AC Airport (ACY) 83/1990 AC Marina (55N) 84/1975 & 1990 Georgetown (GED) 87/1954 Mount Pocono (MPO) 79/1995 Philadelphia (PHL) 88/1975 Reading (RDG) 82/1920 Trenton (TTN) 86/1975 Wilmington (ILG) 87/1975

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 from 5 AM to 11 AM EDT Thursday for ANZ454-455.