textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Models have trended faster with the system Sunday into Monday. Consequently, rain amounts have trended down slightly, though there remains an isolated threat for flash flooding.

Increasing confidence of a coastal low late in the week, though exactly how close it will get to the coast remains to be seen.

KEY MESSAGES

1. Low and cold front approach the region Sunday into Monday, with rain showers likely across the region.

2. A stationary front south of the region could keep rain chances around, especially for Delmarva through mid week.

3. A closed low could develop off our coast by Thursday, potentially bringing another chance for rain, and increasing the risk for coastal flooding.

DISCUSSION

KEY MESSAGE 1...Low and cold front approach the region Sunday into Monday, with rain showers likely across the region.

High pressure shifts away from the region late tonight as a low organizes and develops over the Midwest. The warm front associated with this low may briefly lift into the region on Sunday, but will be quickly followed by a cold front Sunday Night into Monday Morning.

Regardless of the exact evolution of the warm front, should start to see rain showers move into the region by late this evening in response to modest mid and upper level synoptic scale lift associate with the cyclogensis.

As far as hazards, there remains a concern for localized flash flooding, particularly in urban areas, however that threat has decreased slightly. At the surface, a persistent easterly flow will be in place through tomorrow, gradually becoming southeasterly. As such, low level moisture will be on the increase, with surface dew points rising well into the 60s and precipitable water values increasing to near 2 inches. Precipitable water values that high are well above the 90th percentile.

Most models have trended faster with the low and then front sweeping through the region. This is the main factor why forecasted rain amounts now range from 0.1 to 1.5 inches from Tonight through Monday. Additionally, the mid and upper level synoptic scale lift never really line up with low level frontogenesis, thus limiting lift.

The risk for excessive rainfall is a marginal, or 1 out of 4, through this period. However, with abundant moisture, locally heavy rain will still be possible, and if it occurs over urban and other poor drainage areas, localized flash flooding is possible. Additionally, if the warm front is able to lift into our region, areas along and south of the warm front could have

The risk for other thunderstorm hazards is low due to very limited instability.

KEY MESSAGE 2...A stationary front south of the region could keep rain chances around, especially for Delmarva through mid week.

Once the cold front sweeps through on Monday, it is expected to stall south of our region. How far south that front stalls will be a big factor in how much rain we see through the week. The further south it stalls, the less likely our region will see continued rain chances through mid week. Regardless, our region should generally be on the cool side of the front through this period keeping temperatures below normal and low level moisture lower than what it is expected to be on Sunday.

An upper level low digging in from the northwest could spark another round of cyclogensis by Wednesday, which could result in the stationary front sliding north as a warm front briefly, though it will still likely stay south of our region.

KEY MESSAGE 3...A closed low could develop off our coast by Thursday, potentially bringing another chance for rain, and increasing the risk for coastal flooding.

As early as Tuesday, there looks to be an inverted surface trough off the coast of the Carolinas. However, most models don't show this low closing off until Wednesday or Thursday. That is not the only way the models differ; there are significant differences in the track of the low, ranging from the GFS showing the center of the low as close as 120 miles from our coast, to the ICON, with the center of the low getting no closer than 500 miles from our coast!

The closer the track of this low is to the coast, the more likely our region is to see rain, and gusty winds in the Thursday and Friday time period.

It also bears watching as it could increase the risk for coastal flooding. The full moon is on Saturday, Sept 26, so persistent onshore flow Thursday and Friday will only increase the risk as astronomical tides increase ahead of the full moon.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

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

Prevailing VFR conditions are expected through at least 00Z. After 00Z, MVFR ceilings are likely to start building in from the west as areas of rain approach. Another area of clouds may build in from the east due to onshore flow/marine stratus moving in. Consequently the I95 TAF sites (KPHL, KPNE, KTTN, and KILG) may be the last to see MVFR ceilings (possibly as late as 09Z), while the other TAF sites (KRDG, KABE, KACY, and KMIV) may have prevailing MVFR conditions by 03Z. That being said, models could be struggling with how marine stratus will evolve.

Winds are expected to gradually veer from NNE to E to eventually SE through the TAF period. Wind speeds should be mostly at or below 10kt, except at KACY where wind gusts up to 20 kt will be possible.

Outlook...

Sunday through Monday...Periods of MVFR conditions with SHRA, isolated TSRA (mainly Sunday afternoon and Sunday night).

Tuesday through Wednesday...Sub-VFR conditions possible with showers around with breezy northeast winds.

MARINE

A Small Craft Advisory continues for the NJ Atlantic coastal waters north of Great Egg Inlet through 2 PM This Afternoon. Northeast winds will increase to 15-20 kt with gusts near 25 kt through the morning, with seas increasing to near 5 feet. Winds will shift to east- northeast gradually this afternoon and slowly diminish, with diminishing seas as well. Outside of the advisory area, winds are expected to remain below 25 kt with seas remaining below 5 feet.

Once winds and seas subside, conditions are expected to remain below SCA criteria through Sunday Night.

Outlook...

Monday through Wednesday...SCA headlines may be needed starting on Monday for east to northeast winds 15 to 20 kt with gusts up to 25 kt and 4 to 6 ft. Gale force winds are possible as early as Tuesday with wind gusts exceeding 35 kts. Seas may build as high as 10 to 15 ft from Tuesday through Thursday.

Looking ahead, there is at least a chance for gale conditions through Friday. However, it will be highly dependent on the track of a coastal low.

Rip Currents...

Saturday...East winds around 15 mph with gusts up to 20 mph. Easterly swells with a 10 to 12 second period. As a result, there is a MODERATE risk for the development of dangerous and life threatening rip currents for Monmouth county and Ocean county shores and a LOW risk for Atlantic county and coastal Cape May county shores and Delaware Beaches.

Sunday...The winds shift to south/southeast but remain around 15 to 20 mph. Breaking wave heights in the surf zone look to be around 3 to 4 feet for the Jersey shore and 2 to 4 feet for the Delaware Beaches. There is a forecast of a MODERATE risk of dangerous and life threatening rip currents along the NJ and DE coast.

For specific beach forecasts, visit weather.gov/beach/phi

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 2 PM EDT this afternoon for ANZ450>452.