textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

The warm front that will lift north through the region today may return south as a back door cold front on Saturday afternoon, potentially resulting in conditions that would be cloudier and cooler than previously forecast.

The Marine Dense Fog Advisory is no longer in effect.

KEY MESSAGES

1. The warm front continues to lift north through the region this evening, then will sag south through the region on Saturday as a back door cold front.

2. Low pressure and a series of fronts will continue to bring chances for showers and thunderstorms for the last half of the weekend.

DISCUSSION

KEY MESSAGE 1...The warm front continues to lift north through the region this evening, then will sag south through the region on Saturday as a back door cold front.

The warm front continues to lift to the north, and is now through most of Delmarva and southern New Jersey. Skies are clearing from south to north, and temperatures are rising into the 70s in these areas. Clouds are otherwise thinning out, and as these clouds thin out, this should help temperatures to begin to rise, and the warm front should be able to lift at least as far north as the southern Poconos and far northwest New Jersey. So temperatures should at least get into the lower 70s into interior New Jersey and southeast Pennsylvania, and in the upper 70s in Delmarva.

That front will be north of the area, and will extend back through Pennsylvania and into the Ohio Valley. Some shortwave energy will approach from the west and may trigger some showers, mostly north and west of the I-95 corridor, with the best chances in the southern Poconos. Showers taper off by midnight.

Another inversion sets up to around 925 mb. With moisture trapped under the inversion, this sets the stage for another night of fog and low stratus, though visibilities do not look like they will be as low as they were Thursday night into Friday morning.

What makes the Saturday forecast questionable is that high pressure currently centered over Hudson Bay will sag to the south and east, and will be over eastern Canada and near the Canadian Maritimes Saturday afternoon. As it does, it will push the warm front north of the area back to the south as a back door cold front. Winds will take on more of an easterly flow, and this will also make it hard for temperatures to warm up as much as previous forecast indicated, at least for northern areas.

Basically, went with a blend of the 12Z HRRR/NBM/ConsShort guidance for areas north of I-195 in New Jersey and I-295 in southeast Pennsylvania, generally for the northern half of New Jersey and areas north of Philadelphia, as the back door cold front should get at least that far south, and those northern areas should be on the colder side. South of that line, the warmer NBM looks good. Though the fly in the ointment is that once that back door cold front gets going, it may go quickly, and could get through at least southern New Jersey and southeast Pennsylvania in the afternoon.

Will go with highs in the upper 60s to low 70s for the northern half and in the upper 70s to low 80s for the southern half. Temperatures will be colder at the immediate coast regardless.

Meanwhile, low pressure will develop over the Midwest tonight and will track towards the Great Lakes on Saturday. A frontal boundary will extend out from that low through the Ohio Valley and Pennsylvania. 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.

KEY MESSAGE 2...Low pressure and a series of fronts will continue to bring chances for showers and thunderstorms for the last half of the weekend.

One front will lift north of the area Saturday night while a cold front will move through Sunday. We'll continue with chances for showers and a few tstms during these periods. Clouds will predominate through these periods, but some sunshine could break through at times Sunday. Notable impacts Sat night and Sunday will be locally heavy rains and a small possibility of severe weather Sunday. At this time, we are not in an Outlook for severe weather, but we'll continue to monitor this since some of the ingredients are in place. Winds will be a bit gusty on Sunday, but peak speeds will only be 25 to 30 mph, so not too impactful.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

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

Rest of today...Conds improve to VFR from south to north from 19Z through 20Z. S to SW winds increase to 10 to 15 kt with gusts up to and around 20 kt. Moderate confidence.

Tonight...VFR initially, then CIG and VSBY restrictions develop as the evening progresses in fog and stratus. SW winds 5 to 10 kt, becoming LGT/VRB late.Low confidence on extent and how low conds get.

Saturday...Uncertain. Conds should be VFR in the morning, then CIGs lower from north to south as the day progresses. Low confidence on how low CIGs get and timing of when CIGs lower. 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.

Outlook...

Saturday night/Sunday...Widespread IFR or LIFR expected with low CIGS and occasional showers. A few tstms possible. Gusty winds Sunday of around 25 kts at times.

Sunday night...Improving conditions early with VFR most of the period.

Monday thru Wednesday...VFR expected.

MARINE

The warm front continues to lift north through the waters, and fog on the waters continues to lift and dissipate. Therefore, the Marine Dense Fog Advisory is no longer in effect. VSBY restrictions are possible again tonight as patchy fog redevelops.

Southwest winds will increase to 15 to 20 kt with gusts up to 25 kt and seas will build to 5 to 6 feet. The Small Craft Advisory for the ocean waters remains in effect until 6 am Saturday. Conditions will be below SCA criteria on Delaware Bay.

Saturday will feature winds diminishing in the morning, then turning east to northeast at 10 to 15 kt with gusts up to 20 kt as the afternoon progresses.

Outlook...

Small Craft Advisory conditions likely for Saturday night and into Sunday with showers and a chance for a few tstms.

Beyond that, mostly sub-SCA expected with fair weather which should continue into the middle of next week.

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 until 6 AM EDT Saturday for ANZ450>455.