textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Aviation section updated for the 00Z TAF issuance.

The probability of precipitation increase is slower during the day Sunday, then these values have increased Sunday night and Monday.

KEY MESSAGES

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

Surface high pressure settles in later today with drier air moving in. Radar is bearing out earlier guidance which hinted at a few rather isolated showers this afternoon, mainly focused along the sea and bay breezes. So, kept some mainly slight chance POPs along the coastal strip into this evening. A large majority will stay dry however given the closed low/trough continuing to move away. Any showers dissipate early this evening with a quiet night unfolding. Temperatures warm some more to start the weekend with high temperatures on Saturday right near average along with dry conditions. Guidance again hints that a few isolated pop-up showers could develop but signal is weak and so is forcing, so kept forecast mostly dry for now except perhaps an isolated convergence shower in the Delmarva.

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. Some ripples of energy along an incoming cold front during Monday 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 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 /00Z SATURDAY THROUGH WEDNESDAY/

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

Tonight...Primarily VFR. A low chance of a shower through around 02Z at ACY. There is also a 15-20% chance of patchy fog development, but widespread restrictions are not anticipated. Southwest winds this evening 5-10 kts generally becoming light and variable with calm periods likely after 03-05Z. Moderate confidence overall.

Saturday...VFR. South winds increasing to around 10 kts, with a few gusts 15-20 kts possible, especially at ILG, MIV, and ACY. High confidence.

Outlook...

Saturday night...VFR.

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. Southerly winds around 10-15 kt through Saturday. Seas 2 to 3 feet.

Outlook...

Saturday night...No marine headlines are anticipated.

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...

For Saturday, south/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.

For Sunday, winds turn more southeasterly, becoming 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 the development of dangerous rip currents at all beaches as well.

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

PHI WATCHES/WARNINGS/ADVISORIES

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