textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Aviation section updated for the 12Z 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 Sunday night and Monday with the potential for areas of heavy rain with some flash flooding risk.

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 today before moving offshore tonight. The high is relatively weak and with the warm and humid airmass, there could be a few pop-up showers across the sea-breeze fronts today. Coverage on Friday was rather sparse, and it appears it will be that way again today. We'll keep the 10%-20% POP that the NBM offered across Delaware and up across North NJ. Highs temps will again reach the mid/upper 80s most spots and few low 90s are expected too. Cooler readings for the shore and southern Poconos.

KEY MESSAGE 2...Showers and thunderstorms increase Sunday night and Monday with the potential for areas of heavy rain with some flash flooding risk.

An upper-level trough will shift across eastern Canada this weekend and 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. This may result in the trough 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 slow the 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 may be multiple rounds of showers and thunderstorms, starting with an initial one later 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. The WPC has included much of our region in a Marginal Risk for Excessive Rainfall for Sunday night into Monday. On Monday, ripples of energy along an incoming cold front during 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 exists for Monday and Monday night for most of our eastern Pennsylvania counties, most of New Jersey and into northern Delaware. There is a Marginal Risk (level 1 out of 4) for the rest of our area. If enough heating develops during the course of Monday, that combined with some increase in shear (some initially backed low- level flow) could lead to at least some severe thunderstorm risk. This potential is less certain though given the chance for multiple rounds of convection/cloud cover, and a moist atmospheric profile which tends to limit the overall instability.

Given the slow eastward progression of this system, the chances for convection look to continue 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 /10Z SATURDAY THROUGH WEDNESDAY/

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

Today...A little patchy fog across KMIV/KACY until 12Z/13Z, but other than that...VFR. South winds increasing to around 10 kts, with a few gusts 15-20 kts possible, especially at ACY. Medium to High confidence.

Saturday night... VFR with high clouds. Light South winds. High confid.

Outlook...

Sunday and Monday...Periods of sub-VFR conditions along with showers and thunderstorms, especially from Sunday night through Monday. Southerly winds may gust to 20 knots on Sunday.

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

MARINE

No marine headlines expected through Saturday night. Southerly winds around 10-15 kt through Saturday. Seas 2 to 3 feet.

Outlook...

Sunday and Monday...Small Craft Advisory conditions probable as southerly flow increases later Sunday. Seas are forecast to increase to 4-6 feet. Some gusty thunderstorms possible on Monday.

Tuesday and Wednesday...No marine headlines are anticipated.

Tuesday...No marine headlines expected.

Rip Currents...

Today... South to southeasterly flow will strengthen a bit with either shore parallel or slightly onshore winds 10-15 mph. However, wave heights still should be 2 feet or less, with an 8 second dominant wave period. This will mitigate the rip current risk and result in a LOW risk for the development of dangerous rip currents at all beaches.

On Sunday, winds turn more southeasterly, which is more directly onshore, remaining around 10-15 mph. However, there will be a short 6-7 second period swell, and wave heights in the surf zone will likely remain around 2 feet or less. As a result, Sunday looks to have a LOW risk for most spots, but areas of MODERATE conditions are possible for the beaches across Cape May and Atlantic counties of NJ.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...None.