textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No significant changes. Confidence remains high regarding rain through tonight. Confidence in the forecast/details late next week remains very low.

KEY MESSAGES

1) Rainy through tonight.

2) Isolated rain or snow showers possible Sunday afternoon with gusty winds developing following a cold frontal passage Sunday afternoon into Sunday night.

3) Potential for a coastal storm Thursday and/or Friday. Impacts/details are very uncertain as these will depend on how close to the coast the low tracks.

DISCUSSION

KEY MESSAGE 1...Rainy through tonight.

Low pressure continues to organize and develop over the Gulf Coast states on an old frontal boundary. Meanwhile, high pressure remains over northern New England. Low pressure lifts to the north and east, pulling that front north as a warm front, and will move into the Mid- Atlantic early this evening before tracking over New Jersey and lifting into the Gulf of Maine after midnight tonight and into daybreak Sunday. As this low departs, a cold front passes through the region Sunday morning.

Rain will continue to spread over the area tonight. Where the heaviest of the rain will fall will be determined by how far north that frontal boundary gets. Based on latest model trends that front should be through Delmarva and southern New Jersey, where rainfall totals will be from 1.25 to 1.5 inches. Most other areas should pick up around an inch of rain with the southern Poconos, far northwest New Jersey, and into the Lehigh Valley should pick up 1/2 to 3/4 inch. There may also be just enough elevated instability for a rumble of thunder or two over southern Delaware and extreme southeast New Jersey.

Once the rain ends, it should remain cloudy and foggy until the cold front passes through the region late tonight and into daybreak Sunday. West to northwest winds will pick up at 5 to 10 mph, and this should begin to help dissipate the fog. Cold air advection will be underway and temperatures will drop into the upper 30s to low 40s.

KEY MESSAGE 2...Isolated rain or snow showers possible Sunday afternoon with gusty winds developing following a cold frontal passage Sunday afternoon into Sunday night.

While the initial cold front will be through the region Sunday morning and cold air advection will be underway, it will not be until a secondary cold front crosses the region late Sunday afternoon that the colder air arrives. This front will be accompanied with a strong H5 trough with strong shortwave energy. As a result, can expect a period of rain and/or snow showers with its passage. Not expecting much in the way of accumulating snow, though some spots well north and west of Philadelphia could pick up a quick coating. Will go ahead and bump up PoPs to slight chance in the afternoon for the region, and to low-end chance for portions of southeast Pennsylvania and southern New Jersey in and around Philadelphia, as this is where the base of the trough is expected to cross. A tight pressure gradient develops, and northwest winds will increase to 15 to 20 mph with 25 to 35 mph gusts by sunset. The gusty winds continue Sunday night, however some gradual diminishing should occur overnight.

KEY MESSAGE 3...Potential for a coastal storm Thursday and/or Friday. Impacts/details are very uncertain as these will depend on how close to the coast the low tracks.

For Thursday and Friday, the upper air pattern is forecast to become more amplified across the East. The sharpening of an upper-level trough will develop a storm, however the details are much less certain at this time. The model consensus is pointing toward a storm, however whether this will become a coastal storm or one that simply goes out to sea is the forecast challenge. Some guidance suggests a significant winter storm while others are a miss. If the southern energy can remain out ahead of the northern stream energy along with a trough axis more neutral or becoming negatively tilted, then the surface low would tend to be closer to the coast. The guidance that has more of a positive tilted upper-level trough shear the system northeastward and keeps it out to sea. The closer to the coast scenario would favor more precipitation westward across our region, with the timing either Thursday or Friday. Given the wealth of uncertainty in the details, did not alter the forecast away from the NBM guidance. This continues to show low chances for now of rain and/or snow Thursday and Thursday night, however it currently is dry for Friday. If some of the guidance is correct, then PoPs would need to be added/increased for Friday. A colder air mass is forecast to be in place at the of next week and to start next weekend.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

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

Rest of today...Conds lower top MVFR, then IFR in RA/BR/low stratus by 20Z or so. E winds 5 to 10 kt. High confidence.

Tonight...IFR/LIFR in RA/BR/stratus. RA ends from 06Z to 09Z, but IFR conds linger due to BR/stratus. Conds should lift to MVFR prior to 12Z. LGT/VRB winds in the evening, becoming W-NW 5 to 10 kt after 06Z. A period of LLWS with 30-40 kt winds at 2000 feet, mainly from I-95 corridor terminals and KMIV/KACY most of tonight. Cannot rule out a brief TSRA, mainly at KMIV/KACY. Moderate confidence.

Sunday...Conds improve to VFR from 12Z to 15Z. NW winds increase to around 10 kt with gusts up to 20 kt in the morning, then increase further to 10 to 15 kt with 20 to 25 kt gusts in the afternoon. A brief SHRA and/or SHSN possible from around 19Z to 23Z. Moderate confidence overall.

Outlook...

Sunday night...Mainly VFR. Isolated snow showers possible early. West-northwest wind gusts 25-35 knots, gradually diminishing overnight.

Monday...VFR. West-southwest wind gusts up to 25 knots possible through about early afternoon.

Tuesday...VFR.

Wednesday and Thursday...Periods of sub-VFR conditions possible. A 20-30 percent chance of rain and/or snow.

MARINE

Winds and seas will remain sub-SCA Saturday; SCA conditions arrive Saturday night. Rain arrives Saturday moving across the southern waters first (morning) and then for the northern NJ waters during the early afternoon. Behind a cold front passage late Saturday night, winds and seas will increase. While winds should remain below 25 kt, seas will build to 4-6 feet. Sub-SCA conditions expected on Delaware Bay.

Outlook...

Sunday night and Monday...A Gale Warning now in effect for Sunday night for all marine zones due to a period of west to northwest wind gusts up to 40 knots (seas 4-7 feet). Small Craft Advisory conditions then probable on Monday, however the conditions will be improving.

Tuesday...A short duration of Small Craft Advisory conditions possible at night, mainly on the ocean zones.

Wednesday...No marine headlines anticipated.

Thursday...Small Craft Advisory conditions possible depending on the development of a coastal storm.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 6 AM to 6 PM EST Sunday for ANZ430- 431. Gale Warning from 6 PM Sunday to 6 AM EST Monday for ANZ430- 431-450>455. Small Craft Advisory until 6 PM EST Sunday for ANZ450>455.