textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

The Flood Watch from I-78 on north was extended to now go through Wednesday Evening.

Moderate Risk for Excessive Rainfall (Level 3 of 4) was expanded to now include the Philadelphia metro area and I-95 corridor.

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.

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. And the new WPC Excessive Rainfall Outlook reflects that, expanding the MODERATE (3/4) Risk for Excessive Rainfall into the Philly area and extending along the I-95 corridor down into northern Delaware. These more urban areas 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 Jersy 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 /06Z TUESDAY THROUGH SATURDAY/

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

Rest of Tonight...VFR for the next few hours before some MVFR/IFR stratus develops and moves over the region around 09z-10z. Cannot rule out some patchy fog either, mainly at KRDG and KMIV. Winds generally out of the south around 5 kt or less. Low confidence.

Today...Tricky forecast ahead as an active and unsettled day is expected. MVFR/IFR stratus should be in place but could lift to VFR as the morning goes on. Showers and thunderstorms develop off to the west by the late morning, and should impact all terminals, mainly in the afternoon and evening. Have added TEMPO TSRA groups at all terminals, generally in the 18z-22z window give or take an hour or two depending on the site. Gusty and erratic winds are also possible within any convection. Periods of rain and thunderstorms could continue beyond 00z, especially from the I-95 corridor on east, but kept all thunder mention prior to 00z. Confidence is very low in how things evolve. Winds will generally be out of the south around 5-10 kt, with stronger winds of 10-15 kt and gusts near 20 kt at KMIV/KACY. Low confidence.

Tuesday Night...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

Sub-SCA conditions expected through the morning with southerly winds around 10-20 kt and seas 2 to 4 feet.

The Small Craft Advisory remains, beginning at noon 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 this afternoon through tonight.

Outlook...

Wednesday through Saturday...No marine headlines anticipated.

Rip Currents...

For Tuesday, southerly flow will increase as the day progresses causing winds to increase from around 10 to 15 mph to around 15 to 25 mph in the afternoon with gusts up to 30 mph possible. Multiple swell groups generally around 2 to 4 feet are expected. With only 1 day prior to the full moon phase and average swell periods around 5-7 seconds, opted to continue with a MODERATE risk for dangerous rip currents for both the Jersey Shore and Delaware Beaches.

For Wednesday, winds will vary depending on the beaches where the southern Jersey Shore and Delaware Beaches will generally have west winds around 10 to 15 mph where northern Jersey Shore beaches will have south winds around 10 to 20 mph. Breaking wave heights will average between 1 to 3 feet. With the Full Moon Phase occuring on Wednesday, have opted to go with a MODERATE risk for dangerous rip currents at all beaches.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...Flood Watch from 8 AM EDT this morning through Wednesday afternoon for PAZ060-070-071-101>106. Flood Watch from 8 AM EDT this morning through Wednesday evening for PAZ054-055-061-062. NJ...Flood Watch from 8 AM EDT this morning through Wednesday afternoon for NJZ013>020-026-027. Flood Watch from 8 AM EDT this morning through Wednesday evening for NJZ001-007>010-012. DE...Flood Watch from 8 AM EDT this morning through Wednesday afternoon for DEZ001. MD...None. MARINE...Small Craft Advisory from noon today to 8 PM EDT this evening for ANZ430-431. Small Craft Advisory from noon today to 6 AM EDT Wednesday for ANZ450>455.