Area Forecast Discussion

WHAT HAS CHANGED

Updated Key Message 1 sections with latest guidance. The risk for excessive rainfall on Sunday has decreased.

KEY MESSAGES

1. Low pressure and a cold front move through the region tonight into Sunday with rain showers likely.

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, and increasing the risk for coastal flooding.

DISCUSSION

KEY MESSAGE 1...Low pressure and a cold front move through the region tonight into Sunday with rain showers likely.

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 will move northward into the region early Sunday afternoon, but will be quickly followed by a cold front Sunday evening.

Regardless of the exact evolution of the warm front, the leading edge of a large area of rain will move into the region by late this evening in response to modest mid and upper level synoptic scale lift associate with the cyclogensis. Locations north of Philadelphia will have the greatest chance for a prolonged period of rainfall tonight into Sunday.

An easterly flow will be in place this evening through tonight, becoming southeasterly by Sunday morning. 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.

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 has decreased. 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 is now 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.

A cold front will track through the forecast area on Sunday night. 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. 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, and increasing the risk for coastal flooding.

As a mid level low digs in from the northwest, models are showing surface cyclogensis by Wednesday off the Carolinas. At this point, there are 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. An eye will also need to be kept on tides. Track, strength of the low, duration of an onshore wind component, and fetch will all play a role with tidal departures. In addition, the full moon is on Saturday. Long story short, some sort of tidal flooding may effect the region next week.

AVIATION /21Z SATURDAY THROUGH THURSDAY/

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

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 low clouds may build in from the east due to onshore flow/marine stratus moving in.

Winds are expected to gradually veer from E to SE to S through the TAF period. Wind speeds will be 10 to 15 knots this afternoon, and at or below 10kt thereafter.

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

Easterly winds will diminish to 10 to 15 knots by late this afternoon, with seas less than 5 feet.

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

Outlook...

Monday through Thursday...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 the gales will persist through Friday. However, chances will be highly dependent on the track of a potential coastal low.

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...None.