textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Small Craft Advisory extended through tonight.

Rainfall totals have decreased some more for Wednesday afternoon and Wednesday night.

KEY MESSAGES

1. A round of showers later Wednesday into early Thursday. Low risk of a strong thunderstorm for mainly Delmarva later Wednesday afternoon and early Wednesday evening.

2. A storm system could track close enough to our area from the south Saturday to bring some rain.

DISCUSSION

KEY MESSAGE 1...A round of showers later Wednesday into early Thursday. Low risk of a strong thunderstorm for mainly Delmarva later Wednesday afternoon and early Wednesday evening.

A strong upper-level trough/closed low centered over or just north of the upper Great Lakes Wednesday night and Thursday will gradually pivot eastward. This will drive a strong cold front across our area early Thursday as low pressure develops along the cold front. As of now, this surface low looks to track mostly across the southern half of our our region early Thursday morning before exiting to the east and northeast.

The precipitable water values are forecast to surge across our area ahead of the cold front to 1.00-1.50 inches. A low-level jet around 850 mb of around 40 knots is forecast to arrive Wednesday evening. This will increase the low-level warm air advection and subsequently augment the forcing for ascent ahead of and with an incoming cold front. A weak area of low pressure is also forecast to develop along the front across mainly the Delmarva region. This will all result in an expanding area of showers to arrive from the west by later Wednesday afternoon with the bulk of the rainfall occurring Wednesday night into early Thursday morning. Some embedded downpours are possible given the higher precipitable water values and stronger forcing, however the overall instability continues to look to be rather limited. While some thunder (low-topped) cannot be ruled out, the risk for a few strong/severe thunderstorms looks rather low/isolated at this time with the anticipation of cloud cover limiting the instability. If more instability however can become realized, then given the shear magnitude there could be some stronger convection. Isolated severe thunderstorm potential (isolated or level 1 out of 5) is currently outlined across mainly our Delmarva zones toward later Wednesday afternoon and early Wednesday evening.

Overall, storm total rainfall looks to be between 0.25-0.75 inches across our entire region. These totals are lower compared to the previous forecast. If an embedded stronger convective contribution materializes though, then locally higher rainfall totals will be probable. This rain is much needed given the ongoing longer term drought conditions in place. The showers rapidly end early Thursday morning as the cold front and surface low clears our area, with a gusty breeze occurring during Thursday.

KEY MESSAGE 2...A storm system could track close enough to our area from the south Saturday to bring some rain.

As a closed low remains across southeastern Canada over the weekend, a stronger shortwave is forecast to rotate through the base of it from the Ohio Valley to the Mid-Atlantic then offshore. This feature should support surface low development, however the model guidance varies on the placement of this low. Some models and ensembles have this low stronger and farther to the north, which would bring a shield of rain across most of our area. Some other guidance, including the GFS, are weaker and farther south with the surface low. Despite the uncertainty with the location/track of the surface low, given the synoptic setup suggests that whatever system occurs would tend to be a quick mover. Also, given the presence of the closed low, temperatures will be chilly over the weekend (well below average) with Saturday being the coolest. This potential system will continue to be monitored as a more developed and farther north storm track would offer widespread rainfall across the area.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

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

This afternoon...VFR ceilings. South to southwest winds mainly 10 knots or less, becoming southeast. A few sprinkles or a brief light shower possible. Moderate confidence.

Tonight...VFR ceilings. Southeast winds around 5 knots, becoming light and variable. Moderate confidence.

Wednesday...VFR ceilings, which may lower to MVFR late in the afternoon at KRDG and KABE. Showers develop mainly late in the afternoon west of KPHL. An isolated late afternoon thunderstorm possible. Low confidence regarding thunderstorm occurrence.

Wednesday night...MVFR/IFR conditions as showers increase for a time before ending from west to east late. An evening thunderstorm possible. Southeast winds 5-10 knots, becoming northwest late.

Outlook...

Thursday...Sub-VFR conditions possible with some lingering showers early, then VFR. Northwest wind gusts up to 25 knots.

Friday...Mainly VFR. West-northwest wind gusts to 20 knots during the day.

Saturday...A period of sub-VFR and showers possible.

Sunday...Mainly VFR.

MARINE

Seas are still hovering around 5 feet on the Atlantic coastal waters this afternoon, and this looks to be remain through tonight. Seas, especially across our southern New Jersey to Delaware Atlantic coastal waters, may build some during Wednesday however it looks marginal at this time. For now, the Small Craft Advisory has been extended through tonight. The conditions are anticipated to remain below Small Craft Advisory criteria on Delaware Bay through Wednesday.

Outlook...

Wednesday night and Thursday...A period of Small Craft Advisory conditions possible.

Friday...Small Craft Advisory conditions, mainly seas to 5 feet, are possible in the morning for the Atlantic coastal waters.

Saturday and Sunday...The conditions are anticipated to be below Small Craft Advisory criteria.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ450>455.