textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

The Aviation section was updated to account for the 00Z TAFs.

The severe weather and flash flood threat for Saturday has increased.

Monmouth County, Ocean County, and SE Burlington County were added to the Flood Watch

Updated SRF discussion. Downgraded Sussex County, DE and Monmouth County, NJ to a LOW risk for rip currents Saturday.

KEY MESSAGES

1. Smoke from Canadian wildfires will remain in the area tonight through at least the first part of Saturday.

2. Widespread severe weather as well as flash flooding will be possible for Saturday into Saturday night. Conditions will also be warm and very humid.

3. Mainly fair and dry weather is expected for Sunday through Monday before the next chance for shower and storms returns by late Tuesday.

DISCUSSION

KEY MESSAGE 1...Smoke from Canadian wildfires will remain in the area tonight through at least the first part of Saturday.

Smoke from the Canadian wildfires continues to impact the region this afternoon, though much of the area has seen some improvement in visibility since earlier this morning. The improvement has occurred due to continuing northerly winds pushing the smoke a bit farther south. Unfortunately, as we get into this evening and overnight the flow will be turning more southerly and this will likely result in deteriorating conditions again over portions of eastern PA and New Jersey. Expect that visibility may once again get down to 1 to 2 miles. These conditions likely linger into Saturday morning before the continuing southerly flow should start to push the smoke out Saturday afternoon into Saturday evening. Expected convection (see below) may also help improve things.

KEY MESSAGE 2...Widespread severe weather as well as flash flooding will be possible for Saturday into Saturday night. Conditions will also be warm and very humid.

Bottom line up front: There is an increasing threat for relatively widespread severe weather and flash flooding for Saturday into Saturday night. A Flood Watch is now in effect for a large portion of the area from the urban corridor northward starting at 10 AM and continuing until 2 AM Sunday. Also, the Storm Prediction Center has now placed the region in an Enhanced risk for severe weather. This means that instances of severe weather could be fairly widespread across the area compared to the more typical scattered coverage that we often see. The greatest threat will be for damaging winds but there is also the threat of a few tornadoes. The greatest threat for severe weather looks to be between noon and 10 PM.

Getting into the weeds, low pressure will be advancing through the Great Lakes tonight before moving into portions of eastern Ontario and southern Quebec by late Saturday. This will push a warm front through in the morning followed by a cold front later at night. We are still expecting there will be multiple rounds of showers and storms that occur.

The first looks like it will move through around mid morning to midday Saturday and affect areas mainly near and especially north of the urban corridor. Heavy rainfall with the potential for flash flooding will be possible but the risk of severe weather looks relatively lower compared to later in the day.

Following this first round most areas should see a break of several hours in the afternoon. It is during this time that temperatures and dew points ramp up. Generally expect highs in the 80s with heat indices of over 90 for much of the area with the potential for heat indices to hit near 100 to 105 near and south of around Philly where highs will range from the upper 80s to mid 90s. This will be very close to advisory criteria but given some uncertainty on how warm we get due to cloud cover and the earlier convection, we are holding off on any heat products at this time.

ML CAPE values look to really surge in the afternoon as well aided by the very high dew points. CAPE should max out at least in the 1500-2500 range with the potential for it to even go 3000+ depending on how warm we get. In addition, deep layer shear looks to be 40-50 knots with strong low level shear and turning of the winds as well. The anomalously high dew points expected will also help LCL values get quite low with PWAT values progged to surge to over 2.2 inches. The upshot is that the second round of storms that will likely arrive during the latter part of the afternoon into the evening will be easily capable of producing damaging winds and torrential rainfall leading to flash flooding.

Expect convection to take the form of multi-cell clusters and/or line segments with the potential for embedded supercells. A few tornadoes will also be possible. In fact the SPC has much of the area in a relatively rarely issued for this area 5% risk of tornadoes.

In terms of the heavy rainfall and associated flash flood threat, this looks to be highest from northern Delaware to South Jersey on northward where we have the Flood Watch in effect. Given the very high PWATs described above, storms will be capable of producing rain amounts of 2-3 inches within a period of just 1-2 hours. The flood threat will be greatest for urban, low lying areas, and areas near streams and creeks. Areas that get hit hard repeatedly by the multiple rounds of convection will also be particularly vulnerable. And in fact there will be the potential for a final round of convection to move through later Saturday evening just ahead of the actual cold front.

Showers/storms should finally wind down heading into the overnight period Saturday night as the cold front pushes through.

KEY MESSAGE 3...Mainly fair and dry weather is expected for Sunday through Monday before the next chance for shower and storms returns by late Tuesday.

Following the passage of a cold front later Saturday night into early Sunday, the rest of the day Sunday is shaping up to feature sunshine with both lower temperatures and lower humidity levels. Generally expect highs ranging from the 70s to near 80 over our northern zones over NE PA into NW NJ with low to mid 80s near the urban corridor and upper 80s over portions of our eastern MD shore counties and southern Delaware. Dew points will be much lower compared to Saturday so the heat indices will be close to the actual air temperatures.

High pressure builds in to start the week Monday with temperature and humidity levels even a bit lower than Sunday under mainly sunny skies and light winds. However looking beyond this time, the next frontal system approaches Tuesday with heat and humidity returning ahead of it and showers/storms likely by late day into Tuesday night as the system moves through. This is still several days out but it's worth noting that WPC and SPC are already indicating threats for heavy rainfall and flash flooding as well as severe storms. Expect highs Tuesday generally in the 80s to near 90 with heat indices as high as the mid to high 90s from around the urban corridor extending southward. It's possible some showers storms linger into Wednesday but the general trend should be towards improving conditions...especially by late day.

AVIATION /01Z SATURDAY THROUGH WEDNESDAY/

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

Tonight...MVFR/IFR VSBYs in FU. Light S-SW winds. Low confidence.

Saturday...Low VSBYs in FU improve in the morning. Several waves of SHRA/TSRA throughout the day, starting in the late morning, with sub-VFR conds. Stronger TSRA will be capable of brief wind gusts in excess of 50 kt. Outside of TSRA, winds will be out of the S-SW 5 to 10 kt. Low confidence.

Outlook...

Saturday night...Sub-VFR conds in lingering SHRA/TSRA.

Sunday through Sunday night...Lingering sub-VFR conds at KACY Sunday morning. Otherwise, VFR.

Monday...VFR.

Tuesday through Wednesday...Sub-VFR in SHRA/TSRA from time to time.

MARINE

No marine headlines are in effect through early Saturday. Generally expect Southeast winds becoming southerly tonight and increasing to around 10 knots. Seas around 2 feet through early Saturday.

Wildfire smoke will continue to filter into the region through early Saturday. This may cause localized areas of visibility restrictions. Will have to monitor observations closely as denser smoke passes through the region, where marine dense smoke advisories may become warranted if visibilities drop to 1 mile or less, but at this time it appears more unlikely.

By Saturday winds shift more south-southwest and increase ahead of a strong cold front. A Small Craft Advisory is in effect for all ocean zones starting at Noon Saturday and running until midnight for our southern two zones off the coast of Delaware and adjacent to the Delaware Bay. Farther north, the SCA runs until 4 AM Sunday. Expect winds gusting to around 30 knots with seas getting up to 5-6 foot range. Showers and thunderstorms will become increasingly likely as well.

Outlook...

Sunday...Seas may linger near SCA levels early in the day but otherwise the conditions should be sub SCA.

Monday...Fair weather expected with winds under 25 knots and seas around 2-3 feet.

Tuesday - Wednesday...Winds and seas likely approach SCA levels by late day Tuesday and could linger into Wednesday.

Rip Currents...

For Saturday, south winds strengthen to around 15-20 MPH (with the exception of Monmouth County where winds will be a bit lighter). Winds will have an onshore component for Cape May County, Atlantic County, and Ocean County in NJ and be more shore parallel for the Delaware Beaches and Monmouth County beaches. Ended up going a MODERATE risk for rip currents where the winds are strongest and where there is an onshore component (Cape May, Atlantic, Ocean), with a LOW risk elsewhere (Monmouth and Sussex). Breaking waves start around 1 to 2 feet, but gradually increase through the day to around 2 to 3 feet. The wave period will be around 8 seconds. Given lower wave heights and a short to medium period, thinking a high risk is not necessary at this time.

For Sunday, winds turn offshore, becoming northwesterly up to 10 MPH. Still a medium period swell with wave heights around 2 feet as well. All of this will result in a LOW risk for rip currents at all beaches.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...Flood Watch from Saturday morning through late Saturday night for PAZ054-055-060>062-070-071-101>106. Air Quality Alert until midnight EDT tonight for PAZ054-055- 060>062-070-071-101>106. NJ...Flood Watch from Saturday morning through late Saturday night for NJZ001-007>010-012>020-026-027. DE...Flood Watch from Saturday morning through late Saturday night for DEZ001. MD...Air Quality Alert until midnight EDT tonight for MDZ012-015. MARINE...Small Craft Advisory from noon Saturday to 4 AM EDT Sunday for ANZ450>453. Small Craft Advisory from noon Saturday to midnight EDT Saturday night for ANZ454-455.