textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

A Tornado Watch has been issued for the entire region through 9 PM tonight. A Flood Watch also remains in effect for portions of the region through late tonight.

KEY MESSAGES

1. Widespread severe weather as well as flash flooding remains anticipated for the remainder of today into tonight.

2. Lingering smoke from Canadian wildfires will be swept away by late today. Hot temperatures and high dew points will also make conditions uncomfortable.

3. Mainly fair and dry weather is expected for Sunday through Monday before the next chance for showers and thunderstorms returns on Tuesday.

DISCUSSION

KEY MESSAGE 1...Widespread severe weather as well as flash flooding remains anticipated for the remainder of today into tonight.

Bottom line up front: There remains an elevated threat for widespread severe weather and flash flooding for the remainder of today into tonight. A Tornado Watch has been issued for the entire region through 9 PM tonight. A Flood Watch remains in effect for a large portion of the area from the urban corridor northward until 2 AM tonight. The area remains in an Enhanced risk from SPC for severe weather. This means that instances of severe weather are likely to be fairly widespread across the area compared to the more typical scattered coverage that we often see. The primary threats are damaging winds as well as tornadoes, with potential for a stronger tornado, potentially as strong as an EF2.

Getting into the weeds, low pressure will be moving into portions of eastern Ontario and southern Quebec by late today. A warm front pushed through earlier with an initial round of showers and storms and will be followed by a cold front later tonight.

ML CAPE values look to really surge this afternoon as well aided by 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. This all means that the second round of storms this 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, and has highlighted most of our forecast areas with the potential for an EF2 tornado.

In terms of the heavy rainfall and associated flash flood threat, this looks to be highest from northern Delaware to northern parts of 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 this evening just ahead of the actual cold front.

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

KEY MESSAGE 2...Lingering smoke from Canadian wildfires will be swept away by late today. Hot temperatures and high dew points will also make conditions uncomfortable.

Smoke from the Canadian wildfires remains in the region early this afternoon. However, the developing southerly flow should start to push the smoke out by late this afternoon into this evening. Expected convection may also help improve things.

With the southerly flow though, 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. A Heat Advisory has been issued for portions of the eastern shore of Maryland and southern Delaware where 105 or greater heat indices are occurring. Further north, lingering smoke and cloud cover/precip look to hold temps down just enough to prevent advisory criteria from being reached.

KEY MESSAGE 3...Mainly fair and dry weather is expected for Sunday through Monday before the next chance for showers and thunderstorms returns on Tuesday.

Sunday is shaping up to feature sunshine with both lower temperatures and lower humidity levels as a high pressure system builds in. 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 mid to 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 remains in control for Monday with temperature and humidity levels remaining similar to Sunday. It remains mainly sunny on Monday with light winds. Beyond Monday, it turns unsettled. The next frontal system approaches Tuesday with increasing humidity returning ahead of it and showers/storms likely Tuesday 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. Highs on Tuesday are generally in the 80s with heat indices in the upper 80s to mid 90s from around the urban corridor extending southward. It's possible some showers and storms linger into Wednesday but the general trend should be towards improving conditions, especially by late day.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

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

Remainder of Today...Several waves of SHRA/TSRA continuing, with sub-VFR conditions. Stronger TSRA will be capable of brief wind gusts in excess of 50 kt. Outside of TSRA, some lingering FU has kept VSBYs low but is gradually improving. Winds will be out of the S-SW around 10 kts. Low confidence.

Tonight...Sub-VFR conds in lingering SHRA/TSRA through 02-04Z then improving to VFR. Winds turning from west-southwest to northwest, around 5-10 kts. Low confidence.

Sunday...VFR. Northwest winds around 10 kts. High confidence.

Outlook...

Sunday night through Monday...VFR. No significant weather.

Tuesday through Wednesday...Sub-VFR conditions possible in SHRA/TSRA.

Thursday...VFR. No significant weather.

MARINE

Winds shift more south-southwest and increase ahead of a strong cold front today. A Small Craft Advisory is in effect for all near shore ocean zones and continues until midnight tonight 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.

A Tornado Watch is also in effect for all near shore coastal waters as well as the Delware Bay through 9 PM this evening. Waterspouts will be possible with strong to severe thunderstorms in addition to wind gusts potentially over 50 kts and lightning.

Outlook...

Sunday through Monday...Conditions are expected to remain below SCA criteria.

Tuesday through Wednesday...Winds and seas likely approach SCA criteria by late day Tuesday and could linger through Wednesday night.

Thursday...Conditions are expected to remain below SCA criteria.

Rip Currents...

For today, 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 until 2 AM EDT Sunday for PAZ054-055-060>062-070- 071-101>106. NJ...Flood Watch until 2 AM EDT Sunday for NJZ001-007>010-012>020- 026-027. DE...Flood Watch until 2 AM EDT Sunday for DEZ001. Heat Advisory until 8 PM EDT this evening for DEZ003. MD...Heat Advisory until 8 PM EDT this evening for MDZ015-019-020. MARINE...Small Craft Advisory until 4 AM EDT Sunday for ANZ450>453. Small Craft Advisory until midnight EDT tonight for ANZ454-455.