textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

The risk for some strong to severe thunderstorms has increased some Wednesday night.

KEY MESSAGES

1. Areas of patchy dense fog possible again tonight, mainly along the coast. Warmth and increasingly more humid conditions Wednesday with record high temperatures possible.

2. A strong cold front crosses our region late Wednesday night and Thursday morning with showers and some potentially strong to severe thunderstorms.

3. Lingering showers will be possible in the wake of the cold front Thursday morning.

4. Another round of widespread precipitation is likely Sunday Night into Monday Night as a strong cold front approaches the region.

DISCUSSION

KEY MESSAGE 1...Areas of patchy dense fog possible again tonight, mainly along the coast. Warmth and increasingly more humid conditions Wednesday with record high temperatures possible.

Another round of fog and mist is possible again this evening into early Wednesday morning, but confidence in occurrence is low. Latest guidance continues to suggest that this slug of marine fog will likely remain mostly over the waters or right along the coastal strip.

Southerly flow increases on Wednesday as a strong cold front approaches from the west and the pressure gradient tightens. South to southwest winds will increase with gusts 20-25 mph. Warm air will remain in place, and a more humid airmass spreads into the region with dew points rising into the upper 50s to low 60s. High temperatures are forecast to be in the mid to upper 70s for many places, although still much cooler along the coast due to a southerly wind off the cold ocean. See the Climate section below for the records. The warmth could be muted some however if thicker cloud cover arrives faster during the day Wednesday. Some showers should start to develop during the course of the day, especially later in the afternoon across our western zones.

KEY MESSAGE 2...A strong cold front crosses our region late Wednesday night and Thursday morning with showers and some potentially strong to severe thunderstorms.

Low pressure will track to our north Wednesday and Wednesday night. This pulls a strong cold front across our region late Wednesday night and Thursday morning. Showers develop with the arrival and passage of the front. A warm and moist environment will be in place ahead of the cold front and some thunder will be possible with the front or with a pre-frontal zone of forcing. Ahead of the upper- level trough, very strong deep- layer southwesterly flow is forecast with around 80 knots at 500 mb and 50-60 knots at 850 mb. This will result in strong shear profiles that would favor line segments with locally strong to damaging wind gusts, with a lower potential for some quarter size hail or an isolated tornado. Overall, the greater risk for severe thunderstorms looks to be just to our west where stronger forcing coincides with greater instability with strong shear earlier in the day. However, the latest trend in guidance potentially shows a slightly earlier arrival time in the late afternoon to early evening. This earlier time would potentially result in a higher risk for severe storms with greater instability remaining across our region. SPC has expanded the SLIGHT risk to include most of the western and southern portions of the area as a result.

KEY MESSAGE 3...Lingering showers will be possible in the wake of the cold front Thursday morning.

The cold front clears our region Thursday morning, then strong cold air advection develops throughout the day. Dry air advection is also expected in the wake of the front, but there should be a small window of opportunity between the cold front passage and the driest air arriving for lingering showers to continue. In fact for some areas, this will be the highest chances. By this point however, a strong low level inversion will be developing, limiting instability. Thus, expect the mode of precipitation through this period to be rain showers as the risk for any thunderstorms appears very low.

Daily high temperatures are likely to occur just after midnight, then temperatures Thursday fall into and through the 50s in the morning to the 40s by late afternoon as strong northwest winds develop behind the front.

KEY MESSAGE 4...Another round of widespread precipitation is likely Sunday Night into Monday Night as a strong cold front approaches the region.

A low pressure system, taking a similar track to the Wedensday system, will bring a strong cold front to our region early next week. Widespread precipitation is likely ahead of and just behind the front. For now, forecast favors a mostly rain event. Any potential for wintry precipitation will be highly dependent on the timing of both the precipitation and the front. At this range, those details are highly uncertain.

Models show very strong cold air advection in the wake of this cold front. Thus, by Tuesday, widespread high temperatures in the 20s and 30s are possible.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

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

Tonight...Prevailing VFR expected for most terminals. The exception is near KACY where another round of marine fog, mist, and stratus is possible, though probability is only around 20%, thus the chance was not high enough to include in the TAF at this time. Winds generally out of the south around 5 kt or less. Moderate confidence.

Wednesday...Mainly VFR with increasing clouds during the day. Chance (30-50%) for showers or a thunderstorm in the afternoon. Highest chances for the Lehigh Valley and I-95 terminals during the daytime, with the threat peaking between 20z-02z. Showers/thunderstorms should hold off around KACY/MIV until after 00z. Scattered showers and isolated thunderstorms could result in brief IFR conditions. South-southwest winds around 10 knots with gusts up to 20 knots. Moderate confidence.

Wednesday Night...Sub-VFR conditions possible (40-60% chance) with areas of showers and scattered thunderstorms moving through up until about 08z or so. Conditions could get as low as IFR in heavier showers. Low-level wind shear possible at all terminals with a 40-50 kt low-level jet overhead during the overnight hours. Winds out of the west/southwest around 10-15 kt, becoming west/northwest around or shortly before daybreak Thursday.

Outlook...

Thursday...Conditions gradually improving to VFR by mid day. NW winds gusting up to 35 kt.

Friday through Sunday...Prevailing VFR. On Friday, winds could gust to 30 kt.

MARINE

South-southwest winds around 10-15 kt are expected through tonight with seas around 2-3 feet. Areas of patchy marine fog through the remainder of the day. Another round of dense marine fog possible tonight into early Wednesday morning.

Winds increase on Wednesday to around 15 knots with gusts up to 25 knots possible. A chance (15-30%) for showers in the afternoon.

Outlook...

Wednesday Night...SCA conditions likely developing with increasing winds and seas. Chance (25-45%) for showers and thunderstorms.

Thursday...SCA conditions are likely to continue through much, if not all of the day time hours. A lull in SCA conditions is likely Thursday Night.

Friday...SCA conditions could redevelop, especially Friday Afternoon into Friday Night.

Saturday...Elevated seas may linger into Saturday morning. Otherwise, winds and seas should be below SCA criteria.

Sunday...SCA conditions may redevelop, primarily late in the day.

CLIMATE

Six record high temperatures were set on Tuesday, March 10th. This included Allentown, Georgetown, Mount Pocono, Philadelphia, Reading, and Wilmington. These were all in the top 5 earliest 80 degree days on record. The earliest 80 degree day on record was tied at the Atlantic City Airport.

Near record high temperatures are forecast for March 11th. Records for our climate sites are listed below:

Record Warmest Low Temperatures: March 11 Site............................Record/Year Allentown (ABE).....................44/2006 AC Airport (ACY)....................48/1955 AC Marina (55N).....................51/2016 Georgetown (GED)....................55/1955 & 1986 Mount Pocono (MPO)..................47/1955 Philadelphia (PHL)..................54/1955 Reading (RDG).......................51/1955 Trenton (TTN).......................51/1955 Wilmington (ILG)....................48/1955

Record High Temperatures: March 11 Site............................Record/Year Allentown (ABE).....................73/2021 AC Airport (ACY)....................79/1967 AC Marina (55N).....................70/1879 Georgetown (GED)....................77/2021 Mount Pocono (MPO)..................66/1977 Philadelphia (PHL)..................74/2021 Reading (RDG).......................77/2021 Trenton (TTN).......................73/2021 Wilmington (ILG)....................74/2021

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...None.