textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated the aviation section for the 12z TAFs.

KEY MESSAGES

1. A system moves in today into tonight bringing the threat of both heavy rainfall with an associated flash flood threat along with the threat of severe weather.

2. A HIGH risk of rip currents today for coastal Ocean, Atlantic, and Cape May county.

3. Another cold front is expected to approach the region late this week, bringing one more chance for showers and storms.

DISCUSSION

KEY MESSAGE 1...A system moves in today into tonight bringing the threat of heavy rainfall with an associated flash flood threat along with the threat of severe weather.

Forecast models continue to better align regarding the track of low pressure that will impact the region for today into tonight bringing both severe weather and flash flood threats. Surface low pressure is expected to track eastward across southern PA this afternoon before moving across the heart of our CWA tonight en route to southern New England. This will occur on the downstream side of a fairly broad upper level trough as embedded upper disturbances in the WSW flow aloft move through. Unfortunately this set up in the warm season is quite favorable for both severe weather and flash flooding as it will bring both strong low level and deep layer shear with veering winds profiles along with PWATs maxing out in the 1.8 to 2.4 inch range, close to climatological maxes. ML CAPE values look to max out around 750 to to 1500 j/kg which combined with deep layer shear of 40+ knots and 0-3 km shear of 30-35+ knots along with sufficient forcing will be enough for severe weather. And the high PWAT values along with the potential for there to be multiple rounds of storms with some training has us concerned about flash flooding as well.

Getting into the details, the low will be approaching by this afternoon as it moves from Ohio into southern PA. Ahead of the low, its warm front looks to lift northward through the day as it moves from near Delmarva / southern New Jersey early in the day, ending up somewhere between I-195/east-west PA Turnpike and the I-80 corridor by late day. There will likely be some initial showers and possible storms around this morning into the early afternoon, especially over NE PA into our northern NJ zones north of the warm front but the more significant convection in terms of severe weather and flooding looks to hold off until mainly after 18z. Expect that getting into the mid afternoon and beyond that we'll see convection developing/strengthening both near the warm front in our aformentioned northern/central zones as well as near and ahead of a developing lee surface trough over western MD into SE PA/Delmarva ahead of the low. So timing wise, the window of most concern for our forecast area is roughly 3-11 PM. There may be some discrete cells initially but with time the convective mode should become multi-cell clusters with the potential for embedded supercells as storms move through the area during the late afternoon and evening period. Damaging winds will be the biggest threat in terms of severe weather but with the strongly veering low level winds there will be a tornado threat as well. The current update from the Storm Prediction Center keeps just about all of our area in a SLIGHT risk for severe weather, meaning scattered severe storms will be possible.

Regarding the flash flood threat, cell motion will be fairly fast but there will be the potential for multiple rounds of storms during the late afternoon / evening period along with the potential for storms to train getting into the evening period. The PWAT values progged to be near or above 2 inches indicate the potential for 1-2+ inch per hour rain rates in convection. FFG values are not terribly low given how dry it's been but are as low as 1 hr values of 1.5 to 2.0 inches and 3 hour values of 2.0 to 3.0 inches in the urban corridor. And given the setup just described these values could be reached or exceeded. After collaboration with NWS NYC, we decided to hoist a Flood Watch for flash flooding for our urban corridor zones.

As mentioned, the peak of the convection with its associated severe weather and flash flood threats looks to be through the evening with storms weakening and moving out through the overnight period as the low and its associated cold front move eastward towards the coast.

For Tuesday, a secondary cold front moving through along with some lingering instability could result in some additional scattered showers/storms by the afternoon. But these are not expected to pose a severe weather or flood threat and should be confined mainly to areas mainly near and S/E of the urban corridor.

KEY MESSAGE 2...A HIGH risk of rip currents today for coastal Ocean, Atlantic, and Cape May county.

For today, south-southeasterly winds will increase to around 15-20 mph with gusts upwards of 25-30 mph. South- southeasterly onshore swells will increase to around 3 to 4 feet with a 6 second period. Breaking waves in the surf zone will generally be 2 to 4 feet. Due to the increase in a more onshore wind and wave heights also increasing, a HIGH risk for the development of dangerous rip currents is forecast for coastal Ocean, Atlantic, and Cape May county. A Rip Current Statement has been issued from 8 AM to 8 PM today for these coastal areas. There is a MODERATE risk for the development of dangerous rip currents at coastal Monmouth and the Delaware Beaches. For these coastal zones, the wind is more shore parallel.

For Tuesday, winds are offshore with a north-northwest wind at around 10 mph. South- southeasterly onshore swells will be around 2 to 3 feet with a 5-6 second period. Breaking waves in the surf zone will generally be 1 to 2 feet. Due to these forecast conditions, there is a LOW risk for the development of dangerous rip currents at all the New Jersey and Delaware Beaches.

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

KEY MESSAGE 3...Another cold front is expected to approach the region late this week, bringing one more chance for showers and storms.

Another cold front is expected to approach our region late in the week (likely crossing through our region Friday into Saturday. At this point, the majority of guidance keeps the main low well north of our region (generally crossing across southern Canada). This will mean that the area of most synoptic scale forcing will also stay north of our region. However, even the modest mesoscale forcing ahead of and with the front should be enough for some scattered showers and storms as the front approaches.

Southwesterly flow ahead of the front should allow for plenty of moisture advection ahead of the rain chances. However with the forcing more centered north of the region, that could limit how widespread significant rain is with this event. As for what hazards to expect with this event, that will be highly dependent on the timing of the showers and storms, which the spread in model guidance is as much as 18 hours at this point.

AVIATION /12Z MONDAY THROUGH FRIDAY/

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

Today and tonight...VFR through most of the day with gradually lowering ceilings. Showers and some thunderstorms mostly in the afternoon from west to east. This could be mostly in the form of a line of strong to severe thunderstorms with locally strong winds. MVFR to IFR conditions will occur with the convection. As of now, included a TEMPO for thunderstorms. East to southeasterly winds increasing to 8-12 knots with some gusts to around 20 knots. Low confidence regarding the timing details of the showers/thunderstorms.

Outlook...

Tuesday...Showers and some thunderstorms will result in MVFR to IFR conditions. Coverage of storms will be lower than today/tonight, but may still be significant.

Wednesday...VFR. No significant weather.

Thursday and Friday...Chances for shower/storms return (30 to 50 percent chances) and these could bring brief restrictions.

MARINE

Small Craft Advisory in effect today and tonight with southerly wind gusts increasing to 30 knots with seas building to 4 to 6 feet. Some gusty thunderstorms will be possible this evening.

Outlook...

Tuesday through Friday...Sub SCA conditions.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Flood Watch from 2 PM EDT this afternoon through late tonight for PAZ070-071-101>106. NJ...Flood Watch from 2 PM EDT this afternoon through late tonight for NJZ008>010-012-013-015>019. High Risk for Rip Currents through this evening for NJZ024>026. DE...Flood Watch from 2 PM EDT this afternoon through late tonight for DEZ001. MD...None. MARINE...Small Craft Advisory from 3 PM this afternoon to midnight EDT tonight for ANZ430-431. Small Craft Advisory from noon today to 6 AM EDT Tuesday for ANZ450>455.