textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

1. A well offshore nor'easter bringing coastal impacts and hazardous marine conditions through Thursday.

2. A prolonged period of onshore flow will lead to multiple elevated high tide cycles this week resulting in rounds of coastal flooding.

3. Frost impacts possible Friday night and Saturday night where the growing season continues.

DISCUSSION

KEY MESSAGE 1...A well offshore nor'easter bringing coastal impacts and hazardous marine conditions through Thursday.

A closed low off the Mid-Atlantic coast will gradually shift north and eastward through later this week. This will continue to support surface low pressure, which is forecast to strengthen as it slowly moves north and northeastward (even meanders some for a time). Meanwhile, strong surface high pressure will become more centered across northern New England through Wednesday. These systems will result in a tightening pressure gradient into our area through late Wednesday.

While this this nor'easter remains hundreds of miles off of our coast (an ocean storm), some impacts are still being felt for our coastal and marine areas. The wind field has strengthened some today with stronger surface winds being observed across most of the area. Some of this wind will decouple tonight, especially farther inland, but still remain breeze to windy at the coast. This continued north to north-northeast wind will build the offshore seas and these will be breaking into the beaches resulting in beach erosion in some areas especially during times of high tide. The pressure gradient should tend to tighten some more tonight and for portions of Wednesday as the storm is forecast to shift westward some or even make a small offshore loop on Wednesday. This should result in the winds peaking tonight and Wednesday. For areas along the coast, peak wind gusts in the 30-40 mph range are forecast and then up to 30 mph inland. No impacts are expected inland given how far offshore this storm is. Some showers may try and approach the immediate coast though during Wednesday, but overall the drier air on the west side of this system along with the offshore distance should keep these areas mainly dry. Some cloudiness should spread westward for a time Wednesday, however this may be mainly the mid to high level variety.

For Thursday and Friday, the storm will gradually head farther out to sea as an incoming system from the northwest kicks it out. This will quickly lessen the marine and coastal impacts, while keeping our weather dry and providing a fresh shot of chilly air.

KEY MESSAGE 2...A prolonged period of onshore flow will lead to multiple elevated high tide cycles this week resulting in rounds of coastal flooding.

As the nor'easter meanders offshore through the middle of the week, there will be a persistent and strong north-northeast wind in place. This will lead to a prolonged period of onshore flow resulting in multiple elevated high tide cycles. Recent forecast trends have been fluctuating on the crests of the high tide cycles for this event.

The first high tide cycle of concern will be this evening as spotty minor to widespread minor tidal flooding is forecast. A Coastal Flood Advisory remains in effect for the evening high tide cycle for Ocean, southeastern Burlington, Atlantic, Cape May, Kent, and Sussex County Delaware. This Advisory goes from 3 PM to 9 PM today and is for widespread minor coastal flooding.

The high tide cycle late tonight into early Wednesday morning, looks to bring another round of spotty minor to widespread minor tidal flooding. Due to the continued fluctuation of forecast trends with the high tide cycles, the current Coastal Flood Advisory was not extended to cover late tonight into early Wednesday morning's high tide cycle. However, an additional Coastal Flood Advisory may be needed for at least a portion of our area.

Additional coastal flood hazards will likely be needed for high tide cycles Wednesday, and possibly Thursday, as the high tide cycles remain elevated. From the current forecast, Wednesday evening's high tide cycle continues to be the one of most concern with widespread minor coastal flooding. In some of the back bays, moderate coastal flooding is possible with Wednesday evening's high tide cycle. Given fluctuating forecasts, did not quite have the confidence to issue a coastal flood watch just yet, but this may be necessary soon. Beyond Wednesday evening, the nor'easter will gradually be moving away from the region for the end of the week. Another round of coastal flooding is possible Thursday evening with spotty minor to widespread minor coastal flooding. This high tide cycle is forecast to be lower than Wednesday evening's high tide cycle.

KEY MESSAGE 3...Frost impacts possible Friday night and Saturday night where the growing season continues.

As an upper-level trough pivots across the Northeast on Friday, a potentially strong mid level ridge in the Plains to Midwest shifts eastward some into the weekend. This will support strong surface high pressure initially centered near the Great Lakes, which then shifts eastward into the Northeast and Mid-Atlantic Friday and over the weekend.

A chilly and dry air mass will accompany this strong surface high, and as the high itself settles more overhead the winds will decouple. This will set the stage for efficient radiational cooling both Friday night and Saturday night. The forecast lows, as of now, are in the 30s for much of the area (away from the coasts and outside of the urban centers) with a frost potential. It is possible that if the radiational cooling ends up being stronger, then some areas could get to freezing or a little below. Overall though, frost impacts are possible where the growing season remains active.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

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

Rest of this afternoon...VFR. North-northeast winds 10-15 knots with gusts to 25 knots. Moderate confidence.

Tonight...VFR. North-northeast winds diminish to 10 knots or less for a time, then should increase with some gusts to 20 knots especially at KACY. Moderate confidence.

Wednesday...VFR. North-northeast winds 10-15 knots with gusts 20-30 knots, with the strongest winds mainly at KACY and KMIV. Gusts could be limited at KABE and KRDG. Moderate confidence.

Outlook...

Wednesday night and Thursday...VFR. North-northwest wind gusts to 20 knots possible on Thursday.

Friday through Sunday...VFR.

MARINE

The pressure gradient between strengthening low pressure well off the Mid-Atlantic coast and strong high pressure into New England will tighten into Wednesday. The wind increase has continued to be slower than anticipated so far today. Opted to maintain the existing Gale Warning for nearly all of the Atlantic coastal waters and adjacent offshore zones as guidance shows gale force wind gusts developing tonight and for a time Wednesday. Small Craft Advisory conditions develop on Delaware Bay through tonight. Seas will continue to be hazardous. Peak winds across our waters are forecast to be between 30 knots and 45 knots.

Outlook...

Wednesday night and Thursday...Gales for much of the ocean water zones, with Small Craft Advisory conditions on Delaware Bay. The winds are expected to diminish Wednesday night and especially Thursday. Hazardous seas will continue but gradually subside.

Friday...Small Craft Advisory conditions probable.

Saturday...Winds are forecast to be below Small Craft Advisory criteria, however seas 5-7 feet may linger for a while longer for the Atlantic coastal waters.

Sunday...The conditions should be below Small Craft Advisory criteria.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...Coastal Flood Advisory until 9 PM EDT this evening for NJZ020- 022>027. DE...Coastal Flood Advisory until 9 PM EDT this evening for DEZ002>004. MD...None. MARINE...Small Craft Advisory until noon EDT Wednesday for ANZ430-431. Small Craft Advisory until 6 AM EDT Thursday for ANZ450. Gale Warning until 6 PM EDT Wednesday for ANZ451>455-480. Gale Warning until 6 AM EDT Thursday for ANZ481>483-485.