textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Aviation section updated for the 06Z TAF issuance.

There is increasing potential for excessive rainfall leading to flash flooding early this week, particularly on Tuesday.

KEY MESSAGES

1. Showers and thunderstorms return and increase along with increasingly more humid for the first half of the week. There is potential for strong to severe thunderstorms as well as areas of heavy rain leading to flash flooding on both Monday and Tuesday.

DISCUSSION

KEY MESSAGE 1...Showers and thunderstorms return and increase along with increasingly more humid for the first half of the week. There is potential for strong to severe thunderstorms as well as areas of heavy rain leading to flash flooding on both Monday and Tuesday.

As a strong upper-level ridge expands into the Plains early this week, northwesterly flow aloft will occur across much of the East. This ridge gets pushed back some as a strong upper- level trough in eastern Canada sends spokes of energy southeastward across the Great Lakes and Northeast. Some guidance maintains a vigorous shortwave riding over the ridge and sliding southeastward across the Great Lakes Monday and then across the Northeast during Tuesday. This results in some model solutions continuing to close off the trough across the Northeast Tuesday into Wednesday. Some other guidance is slower and weaker and thus does not close it off or briefly closes it off a day or two later. The upper-level system will drive surface low pressure and a cold front our way Tuesday into Wednesday. It will become much more humid during the first half of next week, with dew points looking to surge on Tuesday.

This will all result in an increase in convective chances Monday and especially Tuesday into Wednesday. While there remains some differences among the model guidance on the timing and strength of the features diving southeastward early next week, the setup would continue to favor upstream convective development given northwesterly flow aloft.

Monday's setup looks to be a pulsy, single to multi cell cluster environment with low shear and modest instability. Deep layer shear will be weak, only around 10-20 kts. Instability looks to be in the 1000-2000 J/kg range. DCAPE is also modest around 500-1000 J/kg. This setup will favor scattered, hit or miss type showers and thunderstorms developing during the afternoon hours, with a marginal risk of severe thunderstorms. Any stronger storms could produce isolated strong to marginally severe wind gusts and brief heavy downpours. Our entire area is outlooked in a MARGINAL Risk (level 1 out of 5) for severe thunderstorms on Monday. There is also a MARGINAL Risk (level 1 out of 4) for excessive rainfall leading to flash flooding for areas near and northwest of I-95 on Monday. Scattered showers and storms may continue overnight as better forcing arrives aloft, though the severe and flash flooding threat will likely be minimal overnight.

Tuesday looks to feature the most widespread showers and thunderstorms. Height falls and jet dynamics aloft will favor convective development nearly all day as surface low pressure develops just to our west. This will keep a steady stream of warm, moist air pumping into the area and keeping the environment conditionally unstable despite cloud cover. PWats will rise to near 2" by the end of the day. Some models suggest we could have a stalled or slow moving frontal boundary across the central or northern portion of the forecast area, which could act as a focusing mechanism for heavy rainfall. Heavy rain leading to flash flooding could end up being a bigger concern and greater impact than the severe thunderstorm threat.

Long story short, a very active day is expected convection wise on Tuesday. A due southerly surface flow and the dynamics aloft will favor areas along and northwest of the I-95 corridor and/or the frontal boundary to be the focus of the heaviest rain, and areas along and southeast of the I-95 corridor and/or the frontal boundary where convection is less messy and the environment a bit less contaminated being the greater severe thunderstorm threat depending on exactly how the convection evolves. That still remains a little unclear at the moment, and may depend on how storms evolve upstream Monday and Monday night.

WPC has outlooked most of our area outside of central/southern Delmarva in a SLIGHT Risk (level 2 out of 4) for excessive rainfall for Tuesday. SPC has outlooked the southern half of our area from the Philly metro and south in a SLIGHT Risk (level 2 out of 5) for severe thunderstorms on Tuesday.

On Wednesday, low pressure looks to be offshore to our east. Scattered showers and storms will likely develop again by afternoon, at least across our NJ and PA zones, but the severe thunderstorm and heavy rain threat look to be lower and focused across southern New England given the setup. We'll continue to monitor this period though.

As we head through Thursday and Friday, the risk of scattered showers and storms will slowly decline as the Tuesday-Wednesday system slowly pushes east. Details on the late week period are still very uncertain, however. We may briefly dry out completely Saturday before the next system approaches Sunday.

AVIATION /06Z MONDAY THROUGH FRIDAY/

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

Rest of Tonight (through 12z)...Primarily VFR. Some patchy fog over the Lehigh Valley could result in some MVFR VSBYs though only put a TEMPO group in at KRDG. Winds generally light out of the south/southwest around 5 kt or less. Moderate confidence.

Today...Primarily VFR though scattered showers and thunderstorms develop during the early afternoon, mainly focused from the I-95 corridor and points east. Converted the PROB30 to TEMPO groups for the I-95 and South Jersey terminals. The main window will be from 16z-17z to about 22z-23z, with lingering showers after. Lesser chances for showers/thunderstorms for the Lehigh Valley terminals, so kept the PROB30 for now at KRDG/KABE. Winds out of the south/southwest around 5 to 10 kt. Moderate confidence.

Monday Night...Primarily VFR. Cannot rule out some patchy fog at all terminals (10-15%) but confidence was not high enough to include in the TAFs at this time. Light southerly winds 5 kt or less. Low confidence.

Outlook...

Tuesday...Restrictions likely in widespread showers and thunderstorms. Southerly wind gusts up to 20 kts possible.

Wednesday thru Friday...Prevailing VFR, but isolated to scattered showers and storms could cause brief restrictions, with a decreasing probability of this each day.

MARINE

No marine hazards expected through Monday. Winds out of the east this evening turn southerly tonight, remaining around 10 kts or less. Southerly winds increase to 10-15 kts on Monday. Seas of 2-3 feet throughout the period. A few scattered thunderstorms possible near the coast Monday afternoon.

Outlook...

Tuesday...Small Craft Advisory conditions possible as southerly flow increases. Showers and thunderstorms.

Wednesday thru Friday...No prevailing marine hazards expected, however scattered thunderstorms will be possible near the coast, especially Wednesday

Rip Currents...

For Monday, south winds around 5-10 mph. Breaking waves of 1-2 feet along most of the coast, though they could approach 3 feet for southern parts of the Jersey Shore. There will be multiple swell groups, but all of them look to be fairly light and short in period. Given the more shore-parallel flow and light swell, we'll continue with the LOW risk for the development of dangerous rip currents at all area beaches.

On Tuesday, we'll see an overall increase in winds and seas as a storm approaches. In addition, we will also be a day closer to the full moon. These factors point to more enhanced conditions compared to Monday. With this in mind, we will issue a MODERATE risk for Tuesday.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...None.