textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Aviation section updated for the 00Z TAF issuance.

KEY MESSAGES

1. Tranquil weather continues today, though a few very isolated instability/sea breeze showers will occur this afternoon.

2. Showers and thunderstorms increase in coverage Sunday night into Monday with the potential for areas of heavy rain leading to a risk of flash flooding. There is also some risk for strong to severe thunderstorms.

DISCUSSION

KEY MESSAGE 1...Tranquil weather continues today, though a few very isolated instability/sea breeze showers will occur this afternoon.

Surface high pressure ridges across the area begins slowly moving offshore tonight. The high is relatively weak and with the warm and relatively humid airmass, there could be a few pop- up showers across the sea- breeze fronts thru the rest of the afternoon. Coverage on Friday was rather sparse, and it appears it will be that way again today. Clouds and southerly flow tonight will promote a milder night, with lows mainly in the upper 60s to lower 70s. Sunday will be notably more humid, with most areas seeing dew points creep back into the low-mid 70s and actual temps again in the mid-upper 80s. This will result in a day that feels warmer despite relatively minimal sunshine under thickening high and mid clouds. The next system approaching may bring some showers and storms into the area late, but most likely hold off until evening, so have generally held POPs below likely.

KEY MESSAGE 2...Showers and thunderstorms increase in coverage Sunday night into Monday with the potential for areas of heavy rain leading to a risk of flash flooding. There is also some risk for strong to severe thunderstorms.

An upper-level trough will shift across eastern Canada into early next week. While this occurs, strong energy crests a ridge in the West and this carves the trough out more into the Tennessee and Ohio valleys by later this weekend. There continues to be signals in model guidance showing this trough potentially becoming closed off into early next week as it gradually approaches the East. The main trough may detach and depart to the northeast leaving behind a lingering closed low. This scenario would lead to the system slowing its eastward progression through the first half of next week. The model trends continue to show a slower arrival of this system, with the bulk of the showers and thunderstorms occurring later Sunday night and especially on Monday.

Ahead of the strengthening upper-level trough, deep south to southwest flow will increase and this will ramp up the low-level moisture and temperature advection. It will therefore be a humid time frame as the precipitable waters surge to 2+ inches. There looks to be multiple rounds of showers and thunderstorms, starting with an initial round late Sunday night into Monday with the increased warm air advection regime. Instability will increase, however the timing at night should limit the thunderstorm intensity especially since it may be more of elevated convection initially. Areas of heavy rain however will be probable given the increasing forcing for ascent and high water content air. When looking at model soundings, the high precipitable water values, warm cloud layer processes, and tall skinny CAPE profiles show the heavy rain potential. For Sunday night into Monday, the Weather Prediction Center has us in a Marginal Risk (level 1 out of 4) for excessive rainfall.

On Monday, ripples of energy along the incoming cold front during the day may enhance the convective potential and with deep southerly flow, some training convection will be possible. Excessive rainfall rates would enhance the flash flooding risk, especially from the I-95 corridor to the north and west where the elevation rise could boost the ascent some given a southeast to south low- level flow. There is a Slight Risk (level 2 out of 4) for Excessive rainfall for Monday into Monday night for mainly northern portions of New Jersey. There is a Marginal Risk (level 1 out of 4) for the rest of our area.

In terms of the severe weather risk, uncertainty remains around the exact extent of instability present due to cloud cover and the round of showers/thunderstorms Monday morning. If enough heating develops during the course of Monday, then this combined with some increase in shear (some initially backed low- level flow) could lead to at least some severe thunderstorm risk. The Storm Prediction Center has highlighted areas near and north of Philadelphia in a Marginal risk (level 1 out of 5) for isolated severe thunderstorms with the primary risk being damaging wind gusts.

Beyond Monday night, the slow eastward progression of this system looks to keep the chance for convection going for multiple days next week as the cold front may stall in our vicinity. The current forecast does have shower and thunderstorm chances each day next week, mainly in the afternoon and evening. The most active day, as of now, however looks to be Monday given the overlap of favorable parameters for more widespread convective development.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

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

Tonight...VFR with gradually lowering ceilings. Southeast winds 5-10 kts. High confidence.

Sunday...VFR to start. Ceilings will continue to gradually lower, with MVFR ceilings expected first at RDG and ABE after 19-20Z and then spreading east towards 00Z. Scattered showers and storms are also possible after 18-19Z, with restrictions possible in any heavier shower or storm. More widespread activity is expected to hold off until after 00Z. Winds generally southeast, increasing to 10-15 kt with gusts near 20 kt in the afternoon. Moderate confidence overall.

Outlook...

Sunday night and Monday...Periods of sub-VFR conditions along with showers and thunderstorms.

Tuesday through Thursday...Times of sub-VFR conditions are probable as some showers and thunderstorms continue, especially each afternoon and evening.

MARINE

No marine headlines expected through Sunday. Southerly winds around 10-15 kt with a few gusts up to 20 kts. Seas 2 to 3 feet.

Outlook...

Sunday night and Monday...A Small Craft Advisory has been issued for all of the nearshore Atlantic coastal waters from midnight Sunday night to midnight Monday night. A south to south-southwest flow increases Sunday night into Monday with wind gusts up to 25 kt. Seas also increase to 4-6 feet. Some gusty thunderstorms possible on Monday.

Tuesday through Thursday...No marine headlines are anticipated.

Rip Currents...

For Sunday, winds turn more southeasterly, which is more directly onshore, and increasing to around 15 mph. While there will be a short 6-7 second period swell, given the strong winds, wave heights in the surf zone may be up to 3 feet, particularly at southeasterly facing beaches. As a result, there is a MODERATE risk for the development of dangerous rip currents for the beaches of Cape May, Atlantic, and Ocean counties of NJ. At the beaches of Monmouth County, NJ as well as the Delaware Beaches, the risk for Sunday remains LOW given that winds at these beaches will be more shore parallel.

For Monday, the risk for the development of rip currents increases to MODERATE for all beaches. Winds will be more shore parallel out of the south-southwest but increasing to around 15-20 mph. Additionally, a strengthening swell will result in offshore seas of 5-6 feet. In the surf zone, this translates to breaking waves of 2-3 feet with a medium 8-9 second period.

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 midnight Sunday night to midnight EDT Monday night for ANZ450>455.