textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Forecast for tonight still on track. Warm front slowly progressing northward through the region at this hour.

KEY MESSAGES

1. Low pressure lifting through the region through the morning will produce showers and thunderstorms with the potential for flash flooding across portions of the forecast area.

2. A cold front Thursday into Friday may bring some showers and thunderstorms.

DISCUSSION

KEY MESSAGE 1...Low pressure lifting through the region through the morning will produce showers and thunderstorms with the potential for flash flooding across portions of the forecast area.

The low and associated warm front is slowly lifting through Delmarva overnight. As it does so, expect low level moisture advection to continue. For areas south of the warm front already, dewpoints were quick to increase into the lower to mid 70s. Precipitable water values are still expected to increase above 2 inches. Given the high precipitable water values and relatively deep warm cloud layer, rain rates up to 2 inches per hour will be possible.

While the heavy rain won't last long, this could be enough to lead to flash flooding particularly in urban areas. The primary area of concern is the I-95 corridor and surrounding urban areas and small streams where flash flooding is more possible given any rapid runoff. The Flood Watch for flash flooding in these areas continues through Noon.

This system will be fairly progressive, lifting to Long Island by around midday Sunday. Precipitation tapers off late this morning the low departs. A weak cold front swings through Sunday afternoon behind the low followed by a secondary cold front late tonight/early Monday Morning. Some light rain showers will be possible with the secondary cold front, but with dry air already filtering in to the area by then, do not expect much more than light rain showers.

KEY MESSAGE 2...A cold front Thursday into Friday may bring some showers and thunderstorms.

An upper-level trough across central into eastern Canada is forecast to push a cold front into our area at the end of next week. While the guidance, including ensembles, are in decent agreement showing this trough, there are significant differences with the amplification and therefore speed of the trough. This will have impacts to the cold front itself. If the main energy slides far enough north of our area, the front may tend to be slower moving. This scenario would open the door for a greater chance for a round of convection. A faster movement to the front would lessen the length of convection over our area. The front may then stall to our south into next weekend, with one or more possible waves of low pressure tracking along it with time. A plume of deeper moisture looks to get drawn northward ahead of this front, therefore some heavy rain potential may exist with it and this also depends on whether it stalls or not. Confidence remains low during this time frame.

AVIATION /04Z SUNDAY THROUGH THURSDAY/

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

Tonight...Clouds lowering and thickening with showers and some embedded storms moving in mainly for the overnight period. MVFR cigs beginning as early as 02-03z with IFR expected overnight. Moderate confidence.

Sunday...Sub-VFR in SHRA in the morning, then becoming VFR by 20/21z or shortly after that as clouds scatter out. W-NW winds generally around 5-10 knots. Moderate confidence.

Outlook...

Sunday night...Mainly VFR. A few showers possible.

Monday...VFR. North-northwest winds gusts 20-25 knots possible during the day.

Tuesday and Wednesday...VFR.

Thursday...Mostly VFR. A few showers possible.

MARINE

Winds and seas should stay below Small Craft Advisory conditions through tonight. Some visibility restrictions are possible in showers and fog through the morning. Expect a wind shift from southeasterly to northwesterly during the day on Sunday.

Outlook...

Monday...There is a slight chance (20%) of Small Craft Advisory northwesterly wind gusts (25 knots). At this point, the chance is too low to issue an SCA, but will be monitoring trends over the next 24 hours.

Tuesday through Thursday...The conditions are anticipated to be below Small Craft Advisory criteria.

Rip Currents...

Sunday...Southwest winds around 10-15 mph in the morning diminish to around 5-10 mph from the west in the afternoon. Breaking wave heights will range between 1 to 3 feet. An underlying 1 foot southeast swell around 8 to 10 seconds will continue. Given these conditions, have opted to go with a LOW risk for rip currents for all beaches.

Monday...West-northwest winds will be around 15-20 mph with seas will remaining at 1-2 feet. The underlying 1 foot 8 to 10 second southeasterly swell remains. Given these conditions, currently forecasting a LOW risk for rip currents for all beaches.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...Flood Watch until noon EDT today for PAZ070-071-103>106. NJ...Flood Watch until noon EDT today for NJZ008>010-012>020-026- 027. DE...Flood Watch until noon EDT today for DEZ001. MD...None. MARINE...None.