textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Aviation section updated for the 00Z TAF issuance.

KEY MESSAGES

1. A warm front will move north through the region tonight, followed by a strong cold front that will pose the threat of severe weather on Friday afternoon and evening.

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

DISCUSSION

KEY MESSAGE 1...A warm front will move north through the region tonight, followed by a strong cold front that will pose the threat of severe weather on Friday afternoon and evening.

Low pressure developing over the Midwest this afternoon will lift northeast into the Great Lakes tonight and into southern Quebec on Friday. As the system lifts north, a warm front will lift north through the region overnight. As forcing for ascent increases along with increasing moisture advection, occasional showers and periods of rain are anticipated to develop and spread into the area after midnight. Cannot rule out some embedded thunder overnight as well, as there is some elevated instability present in forecast soundings. Showers and some patchy fog will continue through about daybreak, before the warm front lifts clear of the area by late morning. There should then be a lull in activity through early afternoon.

Once the region is in the warm sector, expect a few hours of mostly sunny conditions on Friday under warm and moist southwest flow. Forecast highs are expected to be in the mid 70s to low 80s across much of eastern PA and NJ with highs possibly topping out in the mid 80s across the Delmarva. Dew points will also rise into the mid to upper 60s in the afternoon.

By mid to late afternoon, focus will be to the west as a strong cold front trails to the west. Convection by this time should already be ongoing across portions of NY and western PA tracking east toward the area. Forecast guidance currently depicts a narrow corridor of instability where MLCAPE is around 500 J/kg, perhaps up to 1000 J/kg over western areas. While instability is meager at best, strong background wind fields aloft will be coincided with bulk shear values in excess of 50 kt along the immediate front. As a result, the Storm Prediction Center has increased wind probabilities across the entire forecast area, now being placed into a Slight risk (Level 2/5) for severe weather on Friday. The primary hazard is damaging wind gusts given the overall set-up, however cannot entirely rule out a brief spin-up as well. Timing appears to around 4-6 PM for western areas, 6-8 PM around the I-95 corridor, and 8-10 PM for coastal areas and across the Delmarva. With diurnal heating ceasing this time of year much earlier than in peak summer, it is expected that the squall line will be losing steam in intensity as it progresses further east. Showers and storms will clear the area from west to east later Friday evening.

KEY MESSAGE 2...Widespread frost and freeze conditions are increasingly likely Saturday night.

The trough axis shifts east away from the region and high pressure builds in at the surface for Saturday Night. This will result in some of the coldest air of the season to date settling in over the entire area. Skies will clear out by sunset on Saturday, with winds gradually diminishing through the night. The elevated winds, especially south and east of the I-95 corridor, will prevent temperatures from bottoming out completely though. So, while we will have decent radiational cooling conditions, it won't be perfectly ideal. Ended up lowering temperatures a few degrees under the NBM though.

As a result, low temperatures are expected to fall below freezing across the Poconos and northwestern NJ. Freezing temperatures could extend down through the I-78 corridor though, with the current forecast for Reading/Allentown having lows around 33 to 34. Rural areas even across southeastern PA and southern NJ cannot entirely rule out seeing their first freeze of the year either. For the rest of the area, lows in the mid 30 to upper 30s are expected, with 40s along the coast.

Elevated winds and a dry airmass may actually preclude frost development, especially along the I-95 corridor and points south and east. There is a scenario where we see temperatures in the mid 30s but no widespread frost given how dry the airmass will be. However, the first widespread hard freeze looks increasingly likely for the Poconos and northwestern NJ. Headlines are going to follow in subsequent forecast updates.

Temperatures moderate from Sunday onward. Temperatures on Sunday Night north of Philadelphia will drop into the mid to upper 30s, but some areas may no longer need frost/freeze headlines beyond Saturday Night. After that, lows each night for most of next week should be above 40 degrees where the growing season is still active. Dry and tranquil conditions are expected with no significant weather hazards anticipated at this time for next week.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

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

Tonight...VFR through evening, with ceilings lowering to MVFR by 04-06Z and IFR by 09-11Z. Showers are likely to develop after 04Z with perhaps an embedded thunderstorm, however confidence in occurrence is not high enough to include in TAFs at this time. Winds generally southeast for much of the night at 5-10 kt, veering to southerly late. Low-moderate confidence.

Friday...IFR conditions are expected to persist through the morning hours with some lingering showers and low ceilings. Conditions will gradually improve from south to north after 16Z, giving way to VFR in the afternoon. South-southwest winds around 10-15 kt gusting up to 20-25 kt. In the late afternoon, a squall line is expected to move in from the west after 20Z. This may cause another round of sub-VFR conditions and gusty winds. Moderate confidence overall.

Outlook...

Friday Night...Sub-VFR conditions expected early, especially from the I-95 corridor on east as scattered showers and thunderstorms move through. Conditions should lift back to VFR later in the night though.

Saturday...VFR. North/northwesterly winds around 15-20 kt with gusts 20 to 30 kt.

Saturday Night through Tuesday...VFR. No significant weather.

MARINE

Small Craft Advisories from Little Egg Inlet to Fenwick Island have been allowed to expire as of 3 PM.

East winds around 10-20 kt this afternoon will become south- southeast overnight tonight with seas around 3-4 feet. For Friday, south-southwest winds will increase to around 15-20 kt with gusts up to 25-30 kt late in the day. Seas around 3-5 feet. As a result, a Small Craft Advisory has been issued for all Atlantic coastal waters beginning at 3 PM on Friday.

Fair weather expected through tonight and into Friday morning. A line of showers and thunderstorms is expected late Friday afternoon into the evening. Locally gusty and erratic winds are possible at times.

Outlook...

Friday Night...The Gale Watch was replaced by a Small Craft Advisory for all marine zones as wind gusts should be around 20 to 30 kt. Wave heights of 4 to 5 feet expected.

Saturday...The Gale Watch remains in place for Saturday for all marine zones. Wind gusts out of the north/northwest will be right around 34 kt. Seas of 4 to 7 feet.

Saturday Night...The Gale Watch was extended to now include Saturday Night for the outer marine zones as lingering gusts near 34 kt are possible (50% chance). Winds on the inner zones fall below 30 kts and an SCA will be needed when Gale headlines end.

Sunday...Winds fall below 25 kt but seas linger near 5 feet, which could result in Small Craft Advisory conditions.

Sunday Night through Tuesday...No marine headlines expected.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 2 AM to 6 AM EDT Saturday for ANZ430- 431. Gale Watch from Saturday morning through Saturday afternoon for ANZ430-431-450>455. Small Craft Advisory from 3 PM Friday to 6 AM EDT Saturday for ANZ450>455. Gale Watch from Saturday morning through Sunday morning for ANZ480>483-485.