textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Isolated to scattered severe thunderstorms remain possible later today, however the threat over coastal NJ and DE has lowered some.

KEY MESSAGES

1. Warm and humid conditions today with record high temperatures possible.

2. A pre-frontal trough may bring strong to severe thunderstorms to portions of the area this afternoon, before a strong cold front crosses through early Thursday morning.

3. Gusty winds and turning much colder Thursday in the wake of a strong cold front.

4. A strong cold front moves through during Monday bringing precipitation followed by a surge of colder air.

DISCUSSION

KEY MESSAGE 1...Warm and humid conditions today with record high temperatures possible.

Southerly flow increases today as low pressure deepens over the eastern Great Lakes and high pressure remains parked over the western Atlantic. This will result in a tightening pressure gradient over the region with gusts up to 20-30 mph this afternoon. Warm air will continue to flow northward where temps today are again expected to top out in the mid 70s to low 80s, with 60s in the Poconos and 50s/60s along the coast. Highs may end up being a few degrees cooler than yesterday though, mostly due to more clouds expected this afternoon. See the Climate Section below for more information.

KEY MESSAGE 2...A pre-frontal trough may bring strong to severe thunderstorms to portions of the area this afternoon, before a strong cold front crosses through early Thursday morning.

Surface low pressure will strengthen as it tracks across the eastern Great Lakes this afternoon and into southern Quebec tonight. This will drag a strong cold front across the Mid-Atlantic region late tonight into Thursday morning.

A warm and moist environment will be in place today with dew points rising into the upper 50s to low 60s with mild temperatures. This environmental set-up along with a pre-frontal trough developing over central PA may allow for some strong to severe thunderstorms to occur this afternoon. While storms may initiate to our west where forcing is stronger near the pre-frontal trough, these storms are expected to progress east by mid/late afternoon into the early evening hours across our region. Strong southwesterly flow aloft in addition to bulk shear around 40-50 kt will support strong shear profiles favoring line segments with locally damaging wind gusts. Cannot rule out some half-inch to quarter size hail along with the potential for an isolated tornado. Overall, the greatest risk for severe weather today favors areas further south and west where stronger forcing overlaps the greater instability axis. The biggest caveat today will be how much sunshine occurs and how unstable the area can get. Latest HREF guidance depicts more clouds than sun to the north with some breaks of sun to the south, mainly south and west of Philadelphia. As a result, the Storm Prediction Center has maintained a SLIGHT risk (Level 2/5) for severe weather this afternoon for areas south and west of Philadelphia. There has been a slight reduction to a MARGINAL risk mainly across coastal NJ and DE where temperatures will be much cooler.

There may be a lull in activity overnight aside for some occasional showers before the actual cold front arrives early Thursday morning. Showers associated with the cold front itself look to mainly be post- frontal, so the threat for any convective threat is minimal. Temps will be very warm tonight until the front arrives and temperatures tumble toward day break on Thursday.

KEY MESSAGE 3...Gusty winds and turning much colder Thursday in the wake of a strong cold front.

As a strong cold front clears our area early Thursday morning, robust cold air advection quickly develops from west to east. This will result in mild temperatures to start the day, with temperatures falling through the 50s and into the 40s for most areas through the afternoon. A ripple of energy looks to ride along the front as it clears the coast, and that should result in some additional forcing for ascent behind the front. Some guidance shows rain changing to wet snow for some areas before ending, however this tends to be overdone as the dry air advection takes hold quicker. This drier air may undercut the lingering midlevel forcing. Therefore, kept precipitation as showery at this point with the PoPs lowering from west to east during the day Thursday. Given this and plus with temperatures above freezing, no snow accumulation expected. Strong flow in the wake of the front along with robust cold air advection will result in a period of deeper mixing occuring. The end result is a gusty northwest wind through about mid afternoon before the wind starts to diminish into Thursday evening. Peak gusts look to be between 30-40 mph, and these winds along with falling temperatures will enhance the chill through the afternoon. It turns much colder Thursday night with most areas dropping into the upper 20s to low 30s before daybreak Friday.

KEY MESSAGE 4...A strong cold front moves through during Monday bringing precipitation followed by a surge of colder air.

As low pressure tracks just to our north Sunday night into Monday, a strong cold front will cross our area during Monday. A plume of deeper moisture and stronger forcing for ascent should result in a period of rain ahead of the cold front itself during Sunday night and continuing for a time on Monday. A milder air mass should be in place, therefore rain is expected. As some cold air begins to pour in later Monday a transition to some snow before ending is possible if enough moisture remains. The amount of precipitation and any snow will be highly dependent on the timing of the system, how much moisture is with it and how quickly the colder air moves in.

A robust period of cold air advection may follow into Tuesday, resulting in a much colder air mass across our area. This may result in high temperatures Tuesday struggling to reach 40 degrees across most of our area.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

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

Through 12Z...VFR. The exception is near KACY where another round of fog/mist is possible, though probability is only around 20%, thus have continued to leave out of the TAF at this time. South winds around 5 kt or less. Moderate-high confidence.

Wednesday...Mainly VFR with increasing clouds during the day. Showers and thunderstorms possible in the afternoon, with the greatest chance for the Lehigh Valley and I-95 terminals. Have opted to include PROB30 groups mainly between 20-01Z. Shower and thunderstorm activity is much less of a concern at MIV and ACY. If any thunderstorm occurs, a brief period of IFR conditions may be possible. South-southwest winds around 10-15 kt with gusts up to 25 kt. Moderate confidence.

Wednesday night...Mainly VFR conditions expected early, with MVFR and IFR conditions developing late with showers likely. Southwest LLWS possible at all terminals around 40-50 kt. Southwest winds around 10-15 kt will gradually veer and become west-northwest by day break on Thursday.

Outlook...

Thursday...A period of MVFR ceilings possible, otherwise VFR ceilings. Some showers mostly during the morning which could mix with wet snow at some locations before ending could result in MVFR visibilities at times. Northwest wind gusts 25-35 knots, diminishing later in the afternoon and at night.

Friday...VFR. South to southwest wind gusts 25-35 knots, diminishing some at night.

Saturday and Sunday...VFR overall.

MARINE

A Small Craft Advisory has been issued for the Atlantic Coastal waters beginning at 4 PM today. A Small Craft Advisory has also been issued for the Delaware Bay beginning at 9 PM tonight.

South winds around 10-15 kt this morning are expected to increase to around 20-25 kt later this afternoon. Southwest winds will then further increase tonight with gusts up to 30 kt possible. Seas around 2-4 feet will build to 4-6 feet.

Outside of SCA conditions, fair weather expected. Showers likely with a slight chance of thunderstorms tonight.

Outlook...

Thursday...Small Craft Advisory issued, with conditions then diminishing at night.

Friday...Small Craft Advisory conditions possible in the afternoon and at night.

Saturday...The conditions should be lowering before Small Craft Advisory criteria.

Sunday...Small Craft Advisory conditions probable, especially at night.

CLIMATE

Near record high temperatures are forecast for today, 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...Small Craft Advisory from 9 PM this evening to 6 PM EDT Thursday for ANZ430-431. Small Craft Advisory from 4 PM this afternoon to 6 PM EDT Thursday for ANZ450>455.