textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

A Severe Thunderstorm Watch has been issued for much of the CWA until 8PM.

KEY MESSAGES

1. A significant system begins to impact the region today with flash flooding and severe weather all on the table. The flooding threat will continue into Wednesday. A Flood Watch is in place for most of New Jersey, all of eastern Pennsylvania, and northern Delaware.

DISCUSSION

KEY MESSAGE 1... A significant system begins to impact the region today with flash flooding and severe weather all on the table. The flooding threat will continue into Wednesday. A Flood Watch is in place for most of New Jersey, all of eastern Pennsylvania, and northern Delaware.

Update (120PM). A Severe Thunderstorm Watch has been issued for much of the region until 8PM this evening. The initial batch of convection is moving across Northeast PA and northern NJ attm with intense rains and gusty winds. More showers and storms will fire up to the West/Southwest of the area later this afternoon with more Severe weather possible with that activity. Please report any severe weather to our office by phone or social media.

Only one key message in the discussion this morning but impacts are expected to be felt region-wide and over multiple days. The most significant and impactful day will be today though as a deepening area of low pressure moves toward New York, with a warm front lifting through today and a strong cold front coming in tonight. The front, aided by strong jet dynamics, will provide ample lift for widespread showers and thunderstorms and perhaps multiple rounds today and into tonight.

We will break out the hazards and threats by starting with the flash flooding threat over the next two days. A lifting warm front will only strengthen southerly flow through today, ushering in a very moist airmass. PWATs will approach two inches which is over the 90th percentile even for late July. A warm cloud layer over 10k feet will also aid in producing very efficient rainfall rates. Rates of 1-3" per hour are expected later this afternoon and evening. This, combined with the potential for multiple rounds of precipitation will result in a pretty significant flash flood threat. The WPC Excessive Rainfall Outlook reflects that, having a MODERATE (3/4) Risk for Excessive Rainfall for Northern New Jersey with a SLIGHT (2/4) Risk extending through the rest of the region. The biggest concern will be the more urban areas which begin to see flash flooding generally around 1" per hour and that seems very likely considering today's setup especially in the afternoon and evening. Some areas may also be more flood prone, given yesterday's rainfall where there were some flooding reports.

No significant changes in the Flood Watch, which remains in place for most of New Jersey, all of eastern Pennsylvania, and northern Delaware. Lower rainfall amounts and areas more in need of rainfall over far South Jersey and most of Delmarva will result in a lesser flood threat and those areas staying outside of the watch. Cannot rule out flash flooding entirely though in these areas (as we saw yesterday in localized spots) and most of South Jersey and Delmarva remains in either a Marginal (1/4) or Slight (2/4) Risk for Excessive Rainfall.

The one minor change was extending the Flood Watch north of I-78 through Wednesday. As low pressure meanders, another front will come through on Wednesday, capable of producing some heavy downpours yet again for the northern third of the region. This will move over some more urbanized areas in North Jersey that should get a decent amount of rainfall today, potentially causing more problems. While mainstem river flooding is not currently forecast, some smaller creeks and streams within the Passaic and Raritan basins could see some rises that result in flooding.

When all is said and done, the current QPF forecast through Wednesday has around 2 to 5 inches of rain north of I-78, with 1 to 3 inches elsewhere. As is usual in summer convective setups, locally higher amounts are possible. HREF Max in northern New Jersey has highly localized amounts up to 8" through Wednesday. Have multiple ways to receive any flood alerts.

In terms of the severe threat, no significant changes were made to the forecast. Unlike the hydro threat, the severe threat will be more focused on the southern half of the area. SPC did nudge the Slight Risk (2/5) a touch south, having far southeast PA, South Jersey, and all of Delmarva in. The Marginal Risk (1/5) is in place from Philadelphia on north. Instability will increase quickly once the warm front passes. Delmarva and South Jersey will have more time in the warm sector and have higher amounts of shear to work with. The limiting factor further north towards Philly and points north will be the amount of cloud cover. Main timing window is from 2 PM through about 11 PM and damaging wind gusts the primary threat. Cannot rule out a tornado, though the tornado threat will be more focused over Delmarva. No area is currently in any severe outlook category for Wednesday.

Guidance continues to have the surface low to the north potentially lingering through Thursday, though impacts should mainly be confined through tomorrow. High pressure nudges in towards the end of the week and for the weekend, giving us a break from the active pattern.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

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

This afternoon... Showers and tstms across easter PA and much of northern NJ will continue to the east this afternoon. Looks like most TAF sites will be away from this activity shortly. More Showers/TSTMs develop behind this (first batch) 21Z-00Z and will affect all terminals into the early overnight period. MVFR with IFR at times. Low/medium confid overall.

Tonight...MVFR/IFR conditions expected for at least the first half of the night with rain and thunderstorms moving out, though some guidance has conditions improving for the second half of the night. Low confidence.

Outlook...

Wednesday through Thursday...Primarily VFR though an unsettled pattern will result in some restrictions with showers, low clouds, and isolated thunderstorms, especially from KPHL on north.

Thursday Night through Saturday...VFR. No significant weather.

MARINE

Small Craft Advisory are now up for all marine zones as southerly wind gusts around 25 kt are expected with seas around 5 feet. The SCA expires at 8 PM for Delaware Bay but continues through tonight for all ocean zones as 5 foot seas linger. Showers and thunderstorms expected on the waters through tonight.

Outlook...

Wednesday through Saturday...No marine headlines anticipated.

Rip Currents...

For today, southerly winds around 15 to 20 MPH either shore parallel or onshore and increasing breaking wave heights of 2 to 4 feet will result in a MODERATE risk for rip currents at all beaches. Other factors contributing to the rip current risk are the upcoming full moon, and multiple swell groups within the surf zone. None of the swell groups will have a period longer than 8 seconds though.

For Wednesday, winds turn more offshore around 10 MPH or less. Breaking waves will also decrease to around 2 feet or less. While the Full Moon is on Wednesday, decreasing wave heights, more offshore flow, and no wave period longer than 8 seconds should limit the rip current threat. As a result, elected for 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 through Wednesday afternoon for PAZ060-070-071- 101>106. Flood Watch through Wednesday evening for PAZ054-055-061-062. NJ...Flood Watch through Wednesday afternoon for NJZ013>020-026-027. Flood Watch through Wednesday evening for NJZ001-007>010-012. DE...Flood Watch through Wednesday afternoon for DEZ001. MD...None. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ430- 431. Small Craft Advisory until 6 AM EDT Wednesday for ANZ450>455.