textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Small Craft Advisories reissued beginning Saturday evening through Sunday.

KEY MESSAGES

1. A frontal boundary will sag south through the region on Saturday as a back door cold front.

2. Showers and isolated thunderstorms are expected on Sunday as a cold front tracks through the area.

3. An extended period of dry weather is expected for the upcoming week with the return of warm temperatures later in the week.

DISCUSSION

KEY MESSAGE 1...A frontal boundary will sag south through the region on Saturday as a backdoor cold front.

A warm front that lifted north of the region Friday evening will sag back south as a backdoor cold front Saturday. The timing of when the backdoor front will move south and how far south it will go is, admittedly, rather uncertain with various guidance in broad disagreement, quite typical in these backdoor front setups. At any rate, a difficult temperature forecast is in store for Saturday with a non-diurnal temperature curve today for many locations. Current thinking is this backdoor cold front will begin advancing south/southwest in some fashion by late morning/early afternoon. Areas north of the I-195 corridor in NJ could see temperatures touch the mid 60s to low 70s in the morning before seeing temperatures fall in the afternoon. Further inland across eastern PA and south across southern NJ and Delmarva will see this backdoor front arrive later in the afternoon, therefore seeing a better chance for warmer temperatures. Particularly for areas around the Philly metro and south/west, we could see record-challenging afternoon highs in the upper 70s to low 80s before the backdoor front arrives with cooler temperatures. Depending on how far south the front moves through the afternoon and into the late night period, areas across the Delmarva could also see nighttime temperatures challenge some record warmest lows. See the climate section below for information on record temperatures for April 4th.

As the backdoor front advances south and west across the region, some shortwave energy will ride along that boundary and will trigger some late day showers and possible thunderstorms due to elevated instability. But depending on how far south that back door cold front gets, that may inhibit instability needed for convection. Behind the front, ENE flow will support a strong low-level inversion, causing the development of some stratus and fog Saturday night.

KEY MESSAGE 2...Showers and isolated thunderstorms are expected on Sunday as a cold front tracks through the area.

A sequence of fronts will cross through the area on Sunday as low pressure tracks through eastern Canada. First, a warm front will lift back north across the Mid-Atlantic during the early morning hours on Sunday. A few showers can be associated with this front. A lull in activity is then expected before the cold front approaches. The cold front is then forecast to arrive after daybreak and push offshore during the afternoon. Showers are likely with some isolated thunderstorms possible as a narrow corridor of instability may creep up into the Delmarva and southern New Jersey region. At this point, any severe potential appears to be quite limited as most of the instability looks to be elevated in nature. However, a few rumbles of thunder cannot be ruled out. This is in line with the latest outlook by SPC, which only clips the southeastern-most portions of Sussex County, DE in a Marginal Risk. Any shower and thunderstorm activity will end by mid afternoon.

High temperatures on Sunday will push into the mid 60s to low 70s during the morning hours, before temperatures take a tumble in the afternoon in wake of the cold front. Winds will also be quite gusty, especially in the morning, as southerly flow will be enhanced ahead of the approaching cold front. Gusts may push upwards of 30-35 mph before diminishing in the afternoon.

KEY MESSAGE 3...An extended period of dry weather is expected for the upcoming week with the return of warm temperatures later in the week.

For Monday into Tuesday, the Mid-Atlantic region will be situated within the base of an upper level trough which will depart the region Tuesday night. By Wednesday, a strengthening mid-level ridge will take shape over the central and eastern CONUS through the end of the week. At the surface, high pressure will largely be in control aside for a meager cold front which will pass through the area Monday night into Tuesday. Suppose a stray rain (or snow) shower is possible across far northwestern areas during this timeframe, but largely no precipitation is expected for the upcoming week. As a result, temperatures will be seasonably cool through Wednesday, before beginning to moderate and trend back toward seasonably warm levels by Thursday and Friday. This is supported by the Climate Prediction Center which outlooks the entire forecast area in a 60-70% chance of above-normal temperatures for the late week period.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

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

Saturday...A few sites linger at MVFR VSBY early this morning, but will quickly improve to prevailing VFR through the rest of the morning. CIGS should trend lower in the afternoon as a backdoor cold front pushes south with all sites likely MVFR again by late day. E winds 5 to 10 kt. Cannot rule out late day SHRA and perhaps a TSRA, mainly at KRDG-KABE- KTTN- KPNE- KPHL. Low confidence.

Saturday night...Widespread IFR with possible LIFR restrictions will develop from low CIGS north of the backdoor cold front. VIS restrictions will likely be MVFR with patchy fog and showers. E winds around 10 kts. Low confidence.

Outlook...

Sunday...Sub-VFR conditions probable with rain showers likely and possibly an isolated thunderstorm. Gusty southwest winds up to 25-30 kt possible for all terminals except KRDG/KABE.

Sunday night Wednesday...VFR. Aside for a stray rain shower at KRDG or KABE on Monday night, no significant weather expected.

MARINE

Marine conditions below SCA criteria for a short window Saturday before redeveloping Saturday evening into the overnight hours. SW winds around 10-15 kts early will shift E behind a backdoor front. SCA winds possible Saturday evening across the northern ocean zones with E winds gusting to around 20-25 kts. Overnight, the frontal boundary is expected to lift northward again with winds turning SW and ramping up to around 20-25 kts with seas building to 4-6 feet.

Outlook...

Sunday through Sunday night...SCA conditions likely. Wind gusts up to 25-28 kt with seas around 5-8 feet. Rain showers likely with a slight chance of a thunderstorm.

Monday through Wednesday...Lingering SCA conditions possible on Monday morning, otherwise, no marine headlines are expected through Wednesday. Fair weather.

CLIMATE

A very warm airmass Saturday may challenge record high temperatures as well as record warmest low temperatures for April 4th.

Record High Temperatures for April 4th:

SITE...................RECORD...YEAR(S) AC Airport (ACY).......78.......2017 Philadelphia (PHL).....80.......1892 Wilmington (ILG).......80.......1950 Allentown (ABE)........79.......1928 Trenton (TTN)..........82.......1892 Reading (RDG)..........81.......1921 Georgetown (GED).......81.......2023/2011/1963 Mount Pocono (MPO).....78.......1928 AC Marina (55N)........77.......1999

Record Warmest Low Temperatures for April 4th:

SITE...................RECORD...YEAR(S) AC Airport (ACY).......56.......1974 Philadelphia (PHL).....60.......2025 Wilmington (ILG).......60.......2025 Allentown (ABE)........60.......1974 Trenton (TTN)..........62.......1981 Reading (RDG)..........58.......2025 Georgetown (GED).......60.......2025/1950 Mount Pocono (MPO).....58.......1981 AC Marina (55N)........56.......2025

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 6 AM to 2 PM EDT Sunday for ANZ430- 431. Small Craft Advisory from 6 PM this evening to 6 PM EDT Sunday for ANZ450-451. Small Craft Advisory from 2 AM to 6 PM EDT Sunday for ANZ452>455.