textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated the Aviation section for the 06Z TAFs.

KEY MESSAGES

1. A frontal system passes through the region later today and tonight producing widespread showers with localized flash flooding.

2. A trailing mid-level short wave trough and weak cold front lead to the potential for additional rounds of showers and thunderstorms on Monday.

3. A cold front is expected to cross through the region Wednesday night into Thursday resulting in more shower and thunderstorm chances.

DISCUSSION

KEY MESSAGE 1...A frontal system passes through the region later today and tonight producing widespread showers with localized flash flooding.

High pressure over the area moves offshore during the early morning hours. Low pressure over the Midwest tracks east. A warm front ahead of that low lifts north through the region this afternoon, and then this will be followed by low pressure passing through the region late tonight.

Several waves of showers and scattered thunderstorms will impact the area this afternoon and tonight. The first wave of showers will develop as the warm front lifts north through the region. Most rainfall associated with this front will average 1/4 to 1/2 inch.

Behind the warm front, moisture increases over the area, resulting in surface dew points rising into the low 70s and PWATs of 2 to 2.25 inches. With low pressure passing through the region tonight, showers and scattered thunderstorms develop with locally heavy rain. While localized flash flooding is possible due to localized amounts of 1 to 2 inches of rain, most showers will be fairly progressive, so will not issue a Flash Flood Watch at this time. The Weather Prediction Center has a Marginal Risk for Excessive Rainfall, which is a 1 out of 4. The Slight risk has been cut back to portions to southeast Pennsylvania west of Philadelphia.

The severe threat is fairly minimal, generally focused on Delmarva where the Storm Prediction Center has a Marginal Risk (1 out of 5) for Delmarva and extreme southern New Jersey.

KEY MESSAGE 2...A trailing mid-level short wave trough and weak cold front lead to the potential for additional rounds of showers and thunderstorms on Monday.

The cold front associated with the low that approaches the region on Sunday will finally progress through the region and move offshore on Monday. Additionally, in the the mid-levels, a trailing short wave trough will cross overhead. The net result is that isolated to scattered showers and thunderstorms will be possible during the day on Monday.

As for hazards, this round appears to have very limited threats for both flash flooding and severe weather. Convection on Monday will be much more weakly forced (as compared to Sunday/Sunday night), and bulk shear values are depicted in the 20 to 25 kt range, which is rather modest. Although there will be instability present, the modest shear will limit storm organization. Model soundings still depict relatively high precipitable water values of upwards of 2 inches, and warm cloud layers above 10K ft, which results in some heavy rain threat. However, with limited shear and coverage of convection, the threat for any one area to get heavy rain for long enough to result in flash flooding remains low.

For those hoping for a significant cool down in the wake of this cold front, unfortunately the temperature gradient is very small. As such highs on Tuesday will be at or just a degree or two lower than Monday. That being said, the front should usher in drier air, so regardless it should feel more comfortable on Tuesday.

KEY MESSAGE 3...A cold front is expected to cross through the region Wednesday night into Thursday resulting in more shower and thunderstorm chances.

The next main focus is another cold front arriving in the region during the Wednesday night into Thursday time period. There are some big timing differences between models on when exactly the front will arrive and when the mid-level trough will cross offshore. As such, there is considerable uncertainty with the potential hazards as the severe and hydro threats will be dependent on the prime time for any showers and thunderstorms.

In terms of temperatures, so far this appears to be a stronger cold front compared to the one earlier in the week. As such, temperatures by the end of the week and into the weekend are currently forecast to be around 5 degrees lower compared to the midweek time period.

AVIATION /06Z SUNDAY THROUGH THURSDAY/

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

Early this morning...VFR. LGT/VRB to nearly calm winds. High confidence.

Today...VFR initially, then MVFR CIGs develop at KRDG/KABE shortly after 13Z, and then MVFR CIGs develop by 15Z at the I-95 corridor terminals, and then by 21Z at KMIV/KACY. SHRA develop by midday with MVFR/IFR VSBYs. A few TSRA possible after 21Z, but will not include in the TAF due to low confidence in a given TSRA passing over a given terminal at a given time. SE wind less than 10 kt, becoming S around 10 kt with gusts up to 20 kt. Moderate confidence overall.

Tonight...IFR conds in SHRA/BR/low CIGs. After SHRA end, CIGs may lower to LIFR late. S winds less than 10 kt. Low confidence.

Outlook...

Monday...Isolated showers and thunderstorms may result in temporary visibility and ceiling restrictions.

Tuesday and Wednesday...VFR conditions expected.

Wednesday night through Friday...Sub-VFR possible with the chance for showers and thunderstorms.

MARINE

Today...Southeast winds less than 10 kt will turn south and increase to 15 to 20 kt with gusts up to 25 kt this afternoon. Seas on the ocean will build to 3 to 5 feet late. A Small Craft Advisory remains in effect for the waters starting this afternoon. VSBY restrictions in showers developing this afternoon.

Tonight...S winds 15 to 20 kt with gusts up to 25 kt and seas around 5 feet. The SCA remains in effect through tonight. VSBY restrictions in showers and fog. A few thunderstorms possible as well.

Outlook...

Monday...Once seas subside below 5 ft Monday morning, winds and seas are expected to then remain below SCA criteria.

Monday night through Friday...Conditions are expected to remain below SCA criteria.

Rip Currents...

Today, southeast winds less than 10 mph will turn south and increase to 15 to 20 mph with gusts up to 25 mph. Breaking waves will average 2 to 3 feet. There will also be a light easterly swell with a period of 8 to 10 seconds. As a result, there is a LOW risk for the development of dangerous and life threatening rip currents for the Jersey Shore in coastal Monmouth county and for Delaware Beaches, and a MODERATE risk for the Jersey Shore for coastal Ocean, coastal Atlantic, and coastal Cape May counties.

On Monday, southwest winds 15 to 20 mph with gusts up to 25 mph in the morning will turn west at 10 to 15 mph in the afternoon. Breaking waves will average 1 to 2 feet. There will be an increasing southerly swell with a 5 to 7 second period. For now, there is a LOW risk for the development of dangerous and life threatening rip currents for the Jersey Shore and Delaware Beaches.

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 2 PM this afternoon to 4 AM EDT Monday for ANZ430-431. Small Craft Advisory from 4 PM this afternoon to 11 AM EDT Monday for ANZ450>455.