textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Added more detail on the coastal/tidal flooding threat later this week.

KEY MESSAGES

1. A cold front moves through the region late today into tonight, bringing rain at times through the day.

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

3. A closed mid-level low ejecting from the Ohio Valley could help to spin up surface low pressure off our coast by Thursday, potentially bringing another chance for rain.

4. There will be an increasing risk for coastal/tidal flooding through the second half of the week.

DISCUSSION

KEY MESSAGE 1...A cold front moves through the region late today into tonight, bringing rain at times through the day.

The warm front associated with the main surface low will briefly move northward into the region early this afternoon, but will be quickly followed by a cold front late today into this evening.

Low level flow will keep an onshore component through much of the day. As such, low level moisture is expected to increase through mid day, 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 for this time of year.

However, with little instability, there will generally be a lack of embedded convection within the areas of rain. Instead, this will primarily be a more steady, stratiform rain event. As a result, the risk for excessive rainfall leading to flash flooding continues to be low. Given the high PWATs, there may still be some pockets of moderate rainfall at times, particularly for areas northwest of I-95. But the risk for flooding remains low (less then 5 percent) across the entire region.

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

The front will be south of the region on Monday and remain there through mid week. Confidence is not the highest pertaining to all aspects of sensible weather, namely precipitation. How far south that front sags south 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 the mid week period. At least for Tuesday, multiple models trended slightly further north with the front, and as such, rain is likely on Tuesday generally for areas along and south of the PA Turnpike/195 corridors.

Regardless, our region will be on the cool side of the front through this period keeping temperatures below normal. Confidence is higher here.

KEY MESSAGE 3...A closed mid-level low ejecting from the Ohio Valley could help to spin up surface low pressure off our coast by Thursday, potentially bringing another chance for rain.

As a mid level low digs in from the northwest, models are showing surface cyclogensis by Wednesday off the Carolinas. There remain significant differences in the track and strength of the low. The closer the track is to the coast, the more likely our region will see rain and gusty winds on Thursday and Friday. Further complicating the picture, now multiple models depict the low initially progressing northeast before retrograding westward towards the coast at the end of the week. All that to say, the rain and wind forecast from Thursday onward remain very uncertain at this time.

KEY MESSAGE 4...There will be an increasing risk for coastal/tidal flooding through the second half of the week.

Regardless of the exact track of the coastal low, it looks as if we will be in persistent onshore flow from at least Tuesday onward for the Atlantic Coast, Delaware Bay, and Raritan Bay. This long of an onshore flow component increases the risk for tidal flooding, especially for back bays. For the later half of the week, the track, strength of the low, and fetch will all play a role with tidal departures. In addition, the full moon is on Saturday, meaning astronomical high tides are increasing as we get towards the end of the week.

It is still uncertain, but we could see minor tidal flooding as early as the late Wednesday tide cycle.

For the Chesapeake Bay, the expected wind direction tends to limit tidal flooding concerns, but it will need to be watched if persistent easterly flow at the mouth of the Chesapeake.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

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

Today...The prevailing MVFR ceilings that have already developed at most TAF sites are expected to continue for most, if not all of the TAF period. For areas that are seeing more marine layer stratus, especially KACY and KMIV, could see the prevailing VFR conditions continue until very late in the day (after 21Z). Brief IFR conditions possible at KRDG/KABE with more moderate rainfall. Southeast winds around 5-10 kts gradually shifting more SSE. Moderate confidence overall, low confidence in timing of IFR conditions.

Tonight...Sub-VFR conditions likely due to rain, stratus, and fog. Southwest winds abruptly shifting to northwest with a frontal passage from west to east between 21 and 09Z. Low confidence.

Outlook...

Monday...Sub-VFR conditions possible due to precipitation, low stratus, and fog.

Tuesday through Thursday...Sub-VFR conditions will remain possible due to showers/rain. Watching a potential coastal low. Breezy northeast winds could develop, but they'll be dependent on the the track and strength of the potential low.

MARINE

Conditions are expected to remain below SCA criteria through tonight. Winds around 10-15 knots gradually transition from east- southeast tonight to southerly Sunday afternoon and evening. A front will shift winds to the northwest tonight.

Outlook...

Monday...On Monday, winds will gradually shift to northeasterly and begin to increase. An SCA has been issued for the New Jersey coastal waters during the day on Monday, where there is higher confidence of SCA conditions. However, this may need to be expanded later.

From Monday Night onward, there is at least a chance of gale conditions for part of the period. However, differences in model timing and track of a coastal low in this period make it difficult to pinpoint when gale conditions will be the most likely.

Rip Currents...

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.

Monday... We'll continue with the MODERATE risk for dangerous rip currents on Monday as winds will switch around to Northeast and increase to 15 to 20 mph during the day. Breaking waves in the surf zone will be around 2 to 3 feet mostly, but could end up a bit higher as winds remain moderate and gusty.

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 from 11 AM to 6 PM EDT Monday for ANZ450>453.