Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 271052 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 652 AM EDT Wed Mar 27 2024 .SYNOPSIS... High pressure will recede away from the area toward the northeast today. A frontal boundary will gradually approach our region from the west tonight, then a wave of low pressure will track northeastward off the coast on Thursday resulting in a period of unsettled weather. A weak clipper system may move through on Saturday, otherwise dry and seasonable conditions will prevail over the weekend. Another rain making system will impact the region early next week. && .NEAR TERM /THROUGH TONIGHT/... The day will start off with high pressure still generally in control, centered over the Canadian Maritimes, with ridging still extending southwestward across the Mid-Atlantic. By the end of the day, low pressure developing along the coast of the Carolinas will begin taking aim at our region, as high pressure gives way. This low will be triggered in part by a deep upper- level trough approaching from the Midwest, with a slowing cold front trailing a surface low lifting north of the Great Lakes. As colder air pushes across the Appalachians tonight with that front, frontogenetic forcing will increase along a tightening baroclinic zone, as the low to the south advects warmer, moist air northward along the coast. That combined with additional forcing mechanisms in the form of vorticity advection ahead of the upper-level trough, and upper-level divergence at jet-stream level, will result in steadier rain developing, especially late tonight from the I-95 corridor southeastward. Prior to that steadier rain developing, today will start off with just a few showers clipping areas north and west of the Lehigh Valley, while onshore flow will continue to bring overcast skies to our entire area. While yesterday turned out surprisingly sunny and mild, there is much more confidence that will not be the case today, as the cloud layer is more well- established with a strong inversion and very light winds in the lower atmosphere to aid in any mixing or erosion of the layer. That said, it is a milder start to the day, with lows mainly in the low 40s this morning, so most places should still see us reach the low 50s this afternoon outside of the immediate shore and the Poconos. Those showers will gradually spread eastward, becoming more numerous across SE PA by this afternoon, with scattered light showers spreading across much of the rest of our area for the second half of the day. Expect only a tenth or two of an inch at most northwest of the I-95 corridor through early evening, with less to the southeast. Later tonight, with the aforementioned steadier rain, amounts of a half-inch to one inch can be expected from I-95 southeastward, with lighter amounts northwest as better forcing and deeper moisture focuses along the coastal plain. Winds today and tonight will be very light, even calm at times. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... Troughing shifting eastward toward the East Coast will amplify by late Thursday. Some subtle jet dynamics will be at play with a developing low pressure system, which will have implications on direct sensible impacts we receive from the system Wednesday through Thursday night. Surface low pressure located near the Carolina coast early Thursday will lift northeastward and eventually offshore later Thursday along a baroclinic zone in place due to a stalled frontal boundary. There remains a notable amount of spread and lack of discernible consensus in the deterministic and ensemble guidance regarding where the heaviest axis of QPF will be located. The synoptic forcing modeled does not really support a far eastward solution as some guidance has suggested. Thus, we`ve held the QPF forecast fairly close to the previous update. The strongest forcing with this system will arrive by early Thursday as the surface low begins making its closest pass to our area. This is when more widespread, steadier/heavier rain is currently anticipated. The system and rain will depart late Thursday and Thursday evening. Currently, QPF ranges from 0.25-0.5" in the far interior areas of eastern PA and northwest NJ, to 1-1.5" along I-95 and the interior coastal plain, to near 2" along the coast. This forecast is still subject to change though, given the aforementioned subtle synoptic features at play. This will have implications on an impacts due to flooding as soils remain very saturated from the weekend rain. This rain may lead to additional rises on creeks, streams, and river across the area, however the likelihood of any significant flooding with this event is low (10-20%). Winds will settle out of the north to northwest on Thursday and increase into Thursday night to near 15-25 mph by Friday with gusts up to 35 mph possible. Temperatures will remain mild in the 40s and 50s for much of this period, although the cold advection beginning Thursday night will drop us back into the mid to upper 30s for low temperatures through Friday night. && .LONG TERM /SATURDAY THROUGH TUESDAY/... Long wave ridging building across the central CONUS should result in seasonable temperatures and relatively benign weather conditions this weekend into Monday of next week. A quick moving shortwave trough may impact the area Saturday with a 20-30% chance of rain and mostly cloudy skies, however it will not be the washout last Saturday was and the system overall does not look very impactful. More widespread rain and a period of unsettled weather looks likely as early as Monday, as most of the deterministic guidance has an upper level low diving down from Canada. A lot of timing and impact discrepancies with this system but it bears watching over the next several days. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Ceilings are leveling off between 007-012 this morning for most of the area; locally lower along the immediate coast. Some modest improvement can be expected later this morning through early afternoon, but would not expect higher than 020 for the vast majority of our region. VSBY is mainly unrestricted aside from patchy MVFR BR near the coast and that should continue to the case into the early evening, even as some light showers spread eastward toward I-95 by this afternoon. Very light winds, tending easterly but variable or calm at times. Moderate confidence. Tonight...CIGs will lower into the IFR category this evening, with potential for occasional LIFR toward midnight. VSBY will initially be VFR but should also lower to IFR in most locations by dawn as steadier rain develops after midnight, especially from I-95 southeastward. Winds remain very light. Moderate confidence. Outlook... Thursday...MVFR and IFR conditions likely with rain. North to northwest winds 10-15 kts. Moderate confidence. Thursday night through Sunday...Mainly VFR. Northwest to west winds during this period, strongest on Friday with gusts near 30-35 kts possible. Light rain possible Saturday or Saturday night.
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&& .MARINE... Onshore flow will persist today, but winds are settling down to around 10 kt from the NE this morning, and will only tend to ease further later today, tending easterly at times. Winds remaining NE 4-8 kt tonight. Seas are still 5-7 ft with a lingering easterly swell, resulting in an ongoing Small Craft Advisory for all ocean zones. That may drop below 5 ft and end the advisory toward the predawn hours on Thursday. Outlook... Thursday...No marine headlines anticipated. Rain will restrict visibility to 1-3 NM. Thursday night through Friday...North to northwest winds increasing with gale force wind gusts possible. A Gale Watch is in effect for this period. Saturday and Sunday...Some gusts near 25 kts possible early Saturday, otherwise no marine headlines anticipated. && .TIDES/COASTAL FLOODING... The low pressure system that is lingering well off the coast resulting in an extended duration of long period swells, rough surf, and widespread minor tidal flooding impacting the Atlantic coast is finally losing its influence on our coastal areas. Coastal Flood Advisories remain in effect for the tidal Delaware River as widespread minor flooding is expected with the high tide early this morning. A Coastal Flood Advisory goes into effect for the Chesapeake Bay communities within the Eastern Shore of Maryland for this morning`s high tide as widespread minor tidal flooding is expected there as well. Tidal surge anomalies have started to recede due to the lessening influence of the offshore storm and moving further away from the full moon. Following the early morning high tide cycle, coastal flood impacts will cease for the most areas. The exception will be for the back bays in Ocean and Sussex (DE) Counties, where it will take some time for the water to drain. The advisory was extended into this afternoon for these areas. Additionally, there is some potential for lingering minor tidal flooding near the times of high tide through the week along the tidal Delaware River. An additional advisory may be needed for the early Thursday morning high tide. But otherwise, no further impacts/threats are expected. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Coastal Flood Advisory until 7 AM EDT this morning for PAZ070- 071-106. NJ...Coastal Flood Advisory until 4 PM EDT this afternoon for NJZ020-026. Coastal Flood Advisory until 7 AM EDT this morning for NJZ017>019. DE...Coastal Flood Advisory until 4 PM EDT this afternoon for DEZ003-004. MD...Coastal Flood Advisory until noon EDT today for MDZ012. Coastal Flood Advisory until 10 AM EDT this morning for MDZ015- 019-020. MARINE...Gale Watch from Thursday evening through Friday afternoon for ANZ430-431-450>455. Small Craft Advisory until 6 AM EDT Thursday for ANZ450>455.
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&& $$ SYNOPSIS...Staarmann NEAR TERM...Dodd SHORT TERM...Staarmann LONG TERM...Hoeflich/Staarmann AVIATION...Dodd/Staarmann MARINE...Dodd/Staarmann TIDES/COASTAL FLOODING...

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