textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

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

KEY MESSAGES

1. Showers with possible strong to severe thunderstorms may impact the are this evening, followed by another round of showers and possible thunderstorms Thursday morning with the passage of a strong cold front.

2. Turning windy and much colder on Thursday in the wake of a strong cold front.

3. Gusty winds continue on Friday and Saturday, increasing the risk for fire weather conditions.

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

DISCUSSION

KEY MESSAGE 1...Showers with possible strong to severe thunderstorms may impact the are this evening, followed by another round of showers and possible thunderstorms Thursday morning with the passage of a strong cold front.

Bermuda high pressure remains off the western Atlantic. A warm front has lifted north of the area, and a warm and humid airmass lies over the region with temperatures well in the 70s and low 80s and surface dew points in the 50s and low 60s. Though SBCAPE values are generally up to 500 J/kg, MUCAPE values range from 1000 to 2000 J/kg. A strong cold front is moving through the Great Lakes and Ohio Valley, so it will not make it to the local area until late tonight. However, a pre-frontal trough is out ahead of this front, and this trough is triggering scattered showers and thunderstorms across southeast Pennsylvania and New Jersey. There may be enough clouds and showers to stabilize the area ahead of this trough, and that may be enough to inhibit convection this evening. However, 0-6 km Bulk Shear is currently 30 to 40 kt, but will increase to around 50 kt as the evening progresses. The storms currently over western Pennsylvania are moving fairly quickly due to the strong winds aloft, so if they hold together, they should get to southeast Pennsylvania and southwest New Jersey by this evening. If they hold together, and additional storms are capable of developing with the pre-frontal trough, then strong to severe thunderstorms will be capable of damaging winds, small hail, and an isolated tornado. Most of the convection will taper off by midnight tonight, then conditions will be warm and muggy until the early morning hours on Thursday.

The cold front passes through the region Thursday morning, and with it, another round of showers with scattered thunderstorms may impact the area around that time. The severe threat will be minimal, but a few rumbles of thunder are possible.

KEY MESSAGE 2...Turning windy and much colder on Thursday in the wake of a strong cold front.

The cold front will be mostly offshore on Thursday, but it may be close enough for clouds and rain to impact most of New Jersey and Delmarva. Farther inland, with strong cold air advection developing behind the cold front, there may be some snow mixing with the rain before ending. Any snow accumulation will be minimal at best. Winds abruptly shift to the northwest and will range from 15 to 20 mph with 25 to 30 mph gusts. The high temperature will likely occur just after midnight, then will fall into the 40s and 50s by daybreak Thursday, before falling further into the upper 30s to low 40s late Thursday afternoon.

KEY MESSAGE 3...Gusty winds continue on Friday and Saturday, increasing the risk for fire weather conditions.

A strengthening low passing to our north across southern Canada Friday into Friday night will keep a strong gradient across our area to end the week and start the weekend.

As a result, southerly winds on Friday will potentially be around 15-25 mph with gusts of 30-40 mph possible. Robust mixing will also allow for RHs to drop to 30-40% across much of the area. On Saturday, winds will turn to the west after the passage of a front and likely remain elevated around 15-20 mph with gusts of 25-30 mph. The westerly winds will also be advecting in more dry air, with minimum RHs on Saturday of 20-30% across the region.

The combination of gusty winds and low RHs will lead to an increased risk of fire weather conditions. The main limiting factor will be fuel moistures. The entire region is expected to receive a wetting rainfall (0.1 inches of rain or more) Wednesday night through Thursday so the threat will be dependent on how quickly the strong winds and low RHs dry out the fuels. The passing front may bring a few showers to our area Friday night but an additional wetting rainfall is not expected with its passage.

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 late Monday into Monday night. A plume of deeper moisture and stronger forcing for ascent should result in a period of rain ahead of the cold front itself later Sunday night and continuing through 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 the middle of next week, resulting in a much colder air mass across our area. This may result in high temperatures Tuesday and Wednesday struggling to reach 40 degrees across most of our area.

AVIATION /19Z WEDNESDAY THROUGH MONDAY/

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

Rest of this afternoon...Generally VFR. Some TSRA possible, mainly after 20Z. Confidence is low, so will keep PROB30 groups for KRDG/KABE/KTTN/KPNE/KPHL/KILG. S-SW winds around 10 kt with ocnl gusts to 25 kt. Low confidence.

Tonight...VFR initially. Scattered SHRA/TSRA in the evening, but will keep PROB30 groups, as confidence is low that any given storms will pass over a given terminal. Another round of SHRA/TSRA are possible after 08Z as cold front approaches. Will go with MVFR conds in SHRA with VCTS, as convection threat will be lower late tonight than it will be this evening. S-SW winds 10 to 15 kt with 20 to 25 kt gusts, veering to the W, then to the NW at 15 to 20 kt with 25 kt gusts late tonight. Moderate confidence overall.

Thursday...Conds improve to VFR at KRDG/KABE/KTTN and perhaps KPNE. MVFR conds could linger throughout the day at KPHL/KMIV/KACY in low CIGs and RA. NW winds 15 to 20 kt with 25 to 30 kt gusts. Low confidence.

Outlook...

Thursday night...VFR with no significant weather.

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

Saturday and Sunday...VFR overall.

Sunday night and Monday...Sub-VFR conditions likely with rain expected.

MARINE

Small Craft Advisories remain in effect for all waters through Thursday.

South winds 15 to 20 kt with gusts up to 25 kt will veer to the southwest as the night progresses, then will abruptly shift tot he northwest Thursday morning behind the passage of a strong cold front. 25 to 30 gusts then expected on Thursday. Seas 3 to 5 ft this evening will build to 5 to 7 ft on Thursday.

Showers with isolated thunderstorms possible this evening, then another round of showers with isolated thunderstorms Thursday morning. Showers may persist through the day Thursday.

Outlook...

Thursday night...Sub-SCA conditions expected.

Friday...Small Craft Advisory conditions likely beginning in the afternoon and continuing into the overnight hours.

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

Sunday and Monday...Small Craft Advisory conditions probable, especially from Sunday night onwards.

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 until 6 PM EDT Thursday for ANZ430-431- 450>455.