textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Aviation section updated for 12z TAFs.

KEY MESSAGES

1. Much less humid today.

2. Showers and thunderstorms return Sunday afternoon into Sunday night with the potential for some of these to be severe. Cooler and less humid for the first half of next week.

3. Widespread minor coastal flooding with the evening high tide cycles today and Sunday along the Atlantic coast and Delaware Bay. A Coastal Flood Advisory is in effect for this evening.

DISCUSSION

KEY MESSAGE 1...Much less humid today.

A cold front has moved through the region and this is bringing in cooler and less humid air. Although highs today will be in the upper 80s to near 90, dew points will be falling into the 50s by this afternoon meaning the heat index will be close to the actual air temperature.

Sunday will feature similar temperatures to today with dew points starting to creep back up. That said, heat indices are not expected to reach advisory levels.

KEY MESSAGE 2...Showers and thunderstorms return Sunday afternoon into Sunday night with the potential for some of these to be severe. Cooler and less humid for the first half of next week.

An expansive upper-level trough across much of eastern Canada Sunday will gradually shift eastward into early next week. Some stronger shortwave energy within the southern portion of the trough will slide across our area later Sunday. A cold front will also be tied to this feature and that will cross our area later Sunday afternoon and Sunday evening. The low to mid level flow will increase some in response to this incoming shortwave and cold front, which will also increase the warm air advection and the low-level moisture advection. Guidance shows a moisture return in the lower levels as the air mass recovers from the substantial drying on Saturday. This will result in it feeling more humid Sunday, however it is not expected to be at the levels recently experienced.

The main forecast challenge is still the timing, coverage and intensity of convection with this incoming system. It appears to be later in the afternoon into the evening regarding the timing (mainly between 4-11pm). There will be a period of stronger synoptic forcing arriving, and with the low to mid level flow increasing and the shear as well. Guidance is generally depicting bulk shear values of 40+ knots with 0-3 km shear around 25-30 knots by late Sunday. ML CAPE values should max out around 1000-1500 j/kg. Given these factors, the Storm Prediction Center has maintained a SLIGHT risk for severe storms across the entirety of our forecast area. This means scattered severe storms will be possible. Low level lapse rates will be steep indicating that damaging winds will be the biggest threat. However some of the guidance is depicting backed low level winds so an isolated tornado can't be ruled out over SE PA into adjacent areas of southern NJ as well as Delmarva. Finally, given that PWATs will be maxing out around 2+ inches, we can't rule out localized flash flooding over the urban corridor. However fast storm motion should tend to limit this threat.

In the wake of the cold front, a less humid and cooler air mass overspreads our area through the first half of next week. An upper- level trough starts to approach mid to late week bringing a return of shower and thunderstorm chances Wednesday through Friday. The highest chances for storms will generally be in the afternoon and early evening period with Thursday currently looking to be the day most favorable for greater coverage of showers/storms.

KEY MESSAGE 3...Widespread minor coastal flooding with the evening high tide cycles today and Sunday along the Atlantic coast and Delaware Bay. A Coastal Flood Advisory is in effect for this evening.

A New Moon this weekend will result in higher than normal astronomical tides. This will result in widespread minor coastal flooding with the evening and nighttime high tide cycles Saturday and Sunday, and potentially Monday as well on the Atlantic coast and Delaware Bay. The tidal Delaware River and its tributaries could experience some spotty minor tidal flooding by Sunday night's high tide but generally these astronomical tide driven flooding events produce more noteworthy impacts for the Atlantic coasts and Delaware Bay. The highest of the New Moon tides will be Sunday night and Monday night. A Coastal Flood Advisory is in effect for this evening for portions of the Delaware Bay from Kent Co. DE and Cumberland Co. NJ southward as well as along our Atlantic coastal zones.

Tidal flooding is not currently expected along the eastern shore of Chesapeake Bay.

AVIATION /10Z SATURDAY THROUGH WEDNESDAY/

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

Today...VFR/SKC. West-northwest winds around 5-10 kt. High confidence.

Tonight...VFR. Winds generally SW 5 knots or less. High confidence.

Outlook...

Sunday...Sub-VFR conditions probable for a time due to showers and some thunderstorms, especially in the afternoon and evening.

Monday...VFR. Northwesterly wind gusts up to 20 knots possible.

Tuesday...Mainly VFR.

Wednesday...Some sub-VFR conditions with some showers and storms possible.

MARINE

No marine headlines are expected through Saturday night as any wind gusts will be below 25 knots with seas 2-3 feet.

Outlook...

Sunday...A brief period of Small Craft Advisory conditions are possible in the afternoon and evening due to wind gusts to 25 knots. There will also be some showers and storms moving over the waters Sunday evening.

Monday through Wednesday...No marine hazards anticipated.

Rip Currents...

For today, a medium period 6-8 second swell, offshore winds, and breaking waves 2 feet or less will result in a LOW risk for development of dangerous rip currents at the Jersey Shore and Delaware Beaches.

For Sunday, winds become southerly and increase to around 15-20 MPH. However, a medium period swell around 6-8 seconds will continue along with breaking waves once again 2 feet or less for most beaches. The exception will be at southern facing beaches in Cape May and Atlantic counties where breaking waves in the surf zone will be up to 3 feet as the winds will be most directly onshore here. Therefore, the risk for the development of dangerous rip currents will be higher for these beaches, and a MODERATE risk is currently forecast for these two counties. For the remainder of the Jersey Shore and the Delaware Beaches, a LOW risk for the development of dangerous rip currents is forecast.

Ocean water temperatures are mostly in the mid to upper 60s.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...Coastal Flood Advisory from 6 PM this evening to midnight EDT tonight for NJZ013-014-020>024-026-027. DE...Coastal Flood Advisory from 6 PM this evening to midnight EDT tonight for DEZ002>004. MD...None. MARINE...None.