textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Increased confidence in two distinct rounds of showers and thunderstorms tomorrow into tomorrow night. Increasing confidence in a cold front crossing through the region Thursday into Friday.

Updated rip current section.

KEY MESSAGES

1. High pressure builds offshore of New England tonight ahead of an approaching frontal system that will move across the area Sunday and Sunday night.

2. Rainy conditions Sunday into Sunday night with heavy rainfall possible.

3. A trailing mid level short wave trough may result in another round of showers and thunderstorms Monday Afternoon and Evening.

4. A cold front is expected to cross through the region Thursday into Friday.

DISCUSSION

KEY MESSAGE 1 ...High pressure builds offshore of New England tonight ahead of an approaching frontal system that will move across the area Sunday and Sunday night.

One more night of fair weather tonight as high pressure builds offshore of New England tonight ahead of an approaching frontal system. After dewpoints drop during the day today, they will creep up again tonight into Sunday morning. Afternoon cumulus clouds will continue to develop today and a stray shower might occur for southern New Jersey or central/southern portions of Delmarva. But most areas will be dry.

KEY MESSAGE 2...Rainy conditions Sunday into Sunday night with heavy rainfall possible.

A warm front will lift into the region during the day Sunday, but likely not make it past central/southern portions of Delmarva and southern New Jersey, before a cold front and triple point low approach the area late in the overnight. We expect a couple of rounds of precipitation as these systems move into the area. The first will begin late morning into the afternoon hours as a short wave/vorticity impulse approaches from the west. This round will likely be mostly showers with no thunder activity as we will be north of the warm front and instability will be minimal to non existent. Rainfall with this first batch will likely be mostly 0.25- 0.50 inches or less area-wide.

The next batch is expected to move into the area during the evening and overnight hours as the second short wave/vorticity impulse moves into the area ahead of the approaching cold front and possible triple point low. PWATS continue to increase to 2-2.25 inches, so there will remain the threat for heavy rainfall as this rainfall moves across the area. It looks like the heavy rainfall will be fairly progressive, so we are not planning on a Flash Flood Watch at this time. However, there will may still be some localized flash flooding if any training does occur, especially in urbanized and poor drainage areas as 1-2 inches of rain, with locally higher amounts, will be possible overnight

There is still not a lot of instability since we'll still be north of the warm front, so thunderstorms are not expected to be widespread. They still can't be ruled out however, and any severe threat looks to be across portions of southern New Jersey and central/southern portions of Delmarva which will be closer to the warm front.

KEY MESSAGE 3...A trailing mid level short wave trough and weak cold front may result in another round of showers and thunderstorms Monday Afternoon and Evening.

The cold front associated with the low that approaches the region on Sunday will finally progress through the region and move off shore on Monday. Additionally, in the the mid levels, a trailing short wave trough will cross overhead. The net result is that isolated to scattered showers and thunderstorms will be possible during the day on Monday.

As for hazards, this round appears to have very limited threats for both flash flooding and severe hazards. Convection on Monday will be much more weakly forced (as compared to Sunday/Sunday Night), and bulk shear values are depicted in the 20 to 25 kt range, which is rather modest. This limits the severe threat. Model soundings depict still relatively high precipitable water values, and warm cloud layers above 10K ft, which results in some heavy rain threat. However, with limited shear and coverage of convection, the threat for any one area to get heavy rain for long enough to result in flash flooding.

For those hoping for a significant cool down in the wake of this cold front, unfortunately the temperature gradient is very small. As such highs on Tuesday will be at or just a degree or two lower than Monday. That being said, the front should usher in drier air, so regardless it should feel more comfortable on Tuesday.

KEY MESSAGE 4...A cold front is expected to cross through the region Thursday into Friday.

The next main focus is another cold front arriving in the region during the Thursday into Friday time period. There are some big timing differences between models on when exactly the front will arrive and when the mid level trough will cross off shore. As such, there is considerable uncertainty with the potential hazards as severe and hydro threat will be dependent on the prime time for any showers and thunderstorms.

In terms of temperatures, so far this appears to be a stronger cold front than the early week cold front. As such, temperatures going into the weekend should be around 5 degrees lower as compared to the mid week time period.

AVIATION /20Z SATURDAY THROUGH THURSDAY/

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

Rest of today...VFR conditions continue. Winds may get a little tricky for a few hours this afternoon as they shift from an east or northeast direction, to more of a north or northwest direction for a few hours, then back to northeast to southeast later this afternoon and evening. Wind speeds will be 5-10 knots or less. The timing of these wind shifts will be between 19z-23z.

Tonight...VFR conditions expected. Winds become northeast, then east to southeast this evening and overnight. Wind speeds will be 5 knots or less.

Sunday...VFR conditions begin the day Sunday, before MVFR or lower conditions and showers move in later in the morning into the afternoon. Winds settle into a southeast to direction 5-10 knots.

Sunday night...MVFR or IFR conditions expected with a period of moderate to heavy showers during the evening and overnight hours. Isolated thunderstorms are possible. South to southeast winds 5-10 knots.

Outlook...

Monday...Isolated showers and thunderstorms may result in temporary visibility and ceiling restrictions.

Tuesday and Wednesday...VFR conditions expected.

Thursday...Sub-VFR possible with the chance for showers and thunderstorms.

MARINE

Rest of today-Tonight...Conditions will remain below advisory levels through tonight.

Sunday-Sunday night...Winds and seas are expected to increase to advisory levels Sunday afternoon and continue through Sunday night. Isolated thunderstorms are possible as well, mainly for Delaware Bay and the Atlantic Ocean waters adjacent to Delaware and southern New Jersey.

Outlook...

Monday through Wednesday...once seas subside below 5 kt Monday morning, winds and seas should stay below SCA criteria.

Wednesday night into Thursday...A brief period of SCA conditions are possible Wednesday night into Thursday morning mainly due to elevated seas. However, the latest models have trended down on wave heights.

Rip Currents...

Sunday, southeast winds in the morning will turn south at 10 to 20 mph with gusts up to 25 mph during the afternoon. Breaking waves will increase to 2 to 3 feet. There will be multiple swell groups, but the dominant swell is expected to be onshore with a period of 8- 9 seconds. With that said, all swell groups look to be around a foot or less, so winds and breaking wave heights will be the primary driver of rip current risk. With these factors considered, have opted for a MODERATE risk for the development of dangerous rip currents for the Jersey Shore from Ocean County southward, and a LOW risk for Monmouth County portions of the Jersey Shore and all Delaware beaches where winds will be more shore-parallel.

Monday, southwest winds in the morning at 10-20 mph with gusts near 25 mph will turn west in the afternoon and decrease to 5-10 mph. Breaking waves will decrease to 1 to 3 feet. Dominant southeasterly swell around 3 feet with a period of 6-7 seconds. For now have opted for a LOW risk for the development of dangerous rip currents for all area beaches. If the timing of the wind shift or decrease in wave heights is delayed from the current forecast, certain areas could need upgrades in future updates.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 2 PM Sunday to 4 AM EDT Monday for ANZ430-431. Small Craft Advisory from 4 PM Sunday to 11 AM EDT Monday for ANZ450>455.