textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated the Aviation section with the 00Z TAFs.

KEY MESSAGES

1. The remnants of Isaias Sunday and Sunday night brings rain, some potentially heavy at times, gusty winds and a low risk for an isolated severe thunderstorm.

2. Minor tidal flooding possible with Sunday morning's high tide, mainly for coastal areas within Delaware Bay. A Coastal Flood Advisory is in effect for Kent County, DE.

DISCUSSION

KEY MESSAGE 1...The remnants of Isaias Sunday and Sunday night brings rain, some potentially heavy at times, gusty winds and a low risk for an isolated severe thunderstorm.

A closed mid level low and surface low, the remnants of former Hurricane Isaias, is forecast to slide across the Mid-Atlantic region Sunday night into early Monday morning. As this occurs, it is forecast to weaken further with the closed mid level low opening up more into a trough. The main impacts for our area will be widespread rainfall and some gusty winds.

Large scale ascent will start to increase from the south late tonight as the system approaches. An east to southeast robust low- level jet (around 925 mb) on the order of 40-60 knots will overspread our area Sunday morning and this will rapidly increase the warm air advection initially above a marine stable layer. This will enhance the lift (strong 700-850 mb frontogenesis) as the low to mid level flow veers with height. In addition, deeper moisture will be advected northward into our area with this peaking Sunday afternoon into the evening as a warm sector gradually advects northward. The precipitable water values are forecast to quickly increase from south to north late tonight through Sunday to 1.75- 2.25 inches. As this coincides with strong forcing for ascent Sunday, some periods of heavier rain should occur. Total rainfall amounts still look to be 1-2 inches across the entire area, however up to 3 inches will be possible where some enhancement and/or embedded convective elements occur. This event should initially be in the form of stratiform rain before transitioning to more convective/showery Sunday night into early Monday morning. WPC has maintained most of the region in a Marginal Risk (level 1 out of 4) for excessive rainfall for Sunday into Sunday night, which means isolated instances of flash flooding will be possible. There are currently no river gauges forecast to reach minor flood stage, however if even heavier rain occurs into northern NJ (especially 2.5+ inches), this could cause some gauges in the Raritan basin (the Millstone River) to rise to minor flood stage. If rainfall rates become high enough for a time, then some urban/roadway flooding will also be possible.

As the low-level jet moves across the area and the warm sector gradually spreads northward on Sunday, an easterly wind will increase and become gusty. Where the better warm sector gets, the surface winds will veer more out of the southeast to south with time. The surface wind speeds will be driven by how much mixing occurs within the boundary layer, and this looks to be greatest along the coast given the onshore wind and warmer ocean allowing for a bit more mixing onto the adjacent land. There is a robust wind field forecast to be in place not to far above the surface, however a moist/stable layer should keep this from fully mixing down. Wind gusts up to 30 mph are anticipated inland with gusts up to 40 mph along the coast. The winds should diminish Sunday night as the remnant surface low slides across our area and the low-level jet shifts away from our region.

While this is the remnants of a tropical system, it has had plenty of time being a post-tropical system prior to reach our area. Nevertheless, low-level shear profiles increase quite a bit for a time Sunday into Sunday evening. For awhile there will be a marine influence given the onshore flow and with a period of stratiform rain this will keep the instability limited to near zero. However, as the main rain shield starts to shift north and east of our area either later Sunday afternoon and/or Sunday evening, some instability may try to advect northward across Delmarva into southern New Jersey especially. Model forecast soundings show the flow veering with time and this would weaken the low-level shear/helicity (spin). There appears to be a very small window of opportunity though in Delmarva where lingering stronger deep-layer shear overlaps with some increase in instability for perhaps a few transient low-topped supercells with an isolated damaging wind gust possible. The severe scenario however is highly contingent on how much higher dew points advect northward with enough low-level instability, and this looks to be greater to our south and southwest. The risk for a weak/brief tornado in our area is therefore very low.

Some showers linger early Monday morning, especially from about the I-95 corridor on south and east, then drying should quickly occur during Monday as a north to northwest wind increases for a time and the entire weakening system exits to our east and northeast.

KEY MESSAGE 2...Minor tidal flooding possible with Sunday morning's high tide, mainly for coastal areas within Delaware Bay. A Coastal Flood Advisory is in effect for Kent County, DE.

With a period of east/southeasterly flow expected and the upcoming New Moon, increased tidal levels are expected later this weekend. This will result in spotty to widespread minor tidal flooding, mainly for coastal communities along Delaware Bay. The only forecast point currently forecast to reach widespread minor is Bowers Beach. As a result, went ahead an issued a Coastal Flood Advisory from 7 AM to 1 PM for Kent County, DE.

This won't be a significant event for the Atlantic Ocean coastline, where only spotty minor tidal flooding is expected from Barnegat Bay on south. This includes the Delaware Beaches and back bay communities in Sussex County, DE.

The eastern side of Delaware Bay in NJ and northern part of the Bay near Reedy Point are also only expected to reach spotty minor. Overall though, this event will be nothing like late September.

The current forecast has no tidal flooding for areas along the Atlantic coastline north of Barnegat Bay. The forecast for the tidal Delaware River and Chesapeake Bay also has tide levels staying below flood stage at this time.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

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

Tonight...VFR. Ceilings lower overnight. Winds remain out of the east at 5-10 kts. Gusts of 15-20 kts develop at KMIV and KACY after 08Z. High confidence.

Sunday...MVFR conditions develop around daybreak and quickly drop to IFR towards mid-morning due to lowering ceilings. Widespread rain moves in around daybreak and continues through the day with visibility restrictions. Pockets of moderate to heavy rain are possible at times. Winds also remain out of the east but increase to 10-15 kt with gusts to 20-25 kt. A few gusts up to 30 kts possible at KMIV and KACY. Moderate confidence.

Outlook...

Monday...Sub-VFR possible in the morning due to low clouds. Conditions gradually trend towards VFR throughout the day.

Monday night through Tuesday...Primarily VFR. No significant weather.

Wednesday...VFR. Gusty WNW winds possible.

Thursday...VFR. No significant weather.

MARINE

Winds continue to increase with gusts up to 20 kt by this evening. Wind gusts begin to reach 25 kts by late tonight into early Sunday morning. Seas 1-3 feet, increasing to 3-5 feet overnight. Fair weather.

On Sunday, winds increase further to 30 kts with occasional gusts upwards of 35 kts, particularly Sunday morning. Seas continue to build to 7-10 feet by Sunday afternoon. A Small Craft Advisory is in effect for the Delaware Bay and all of the Atlantic coastal waters from early Sunday morning into Sunday night. Due to the occasional nature of gale force gusts, the Gale Watch has been cancelled for the offshore waters. Visibility restrictions are likely in rain which could be moderate to heavy at times.

Outlook...

Monday...SCA conditions probable due to lingering seas of 6-8 feet and wind gusts upwards of 25 kts that gradually diminish into Monday night.

Tuesday through Thursday...No marine hazards expected.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...Coastal Flood Advisory from 7 AM to 1 PM EDT Sunday for DEZ002. MD...None. MARINE...Small Craft Advisory from 5 AM to 10 PM EDT Sunday for ANZ430- 431. Small Craft Advisory from noon Sunday to 6 AM EDT Monday for ANZ450. Small Craft Advisory from 8 AM Sunday to 6 AM EDT Monday for ANZ451. Small Craft Advisory from 5 AM Sunday to 6 AM EDT Monday for ANZ452>455.