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Area Forecast Discussion
Issued by NWS Mt. Holly, NJ
Issued by NWS Mt. Holly, NJ
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945 FXUS61 KPHI 170152 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 952 PM EDT Fri Aug 16 2024 .SYNOPSIS... High pressure over the Northeast slides offshore late tonight. Low pressure over the Great Lakes will slowly track east and drag a cold front towards the region over the weekend. The cold front will finally cross the region on Monday, and then high pressure builds in from the west. A weak cold front may pass through on Tuesday afternoon, and then high pressure gradually builds overhead through the end of the new week. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... 950 PM...No major changes were made to the forecast with the mid evening update. Some echos can be seen on radar not too far away from our western zones but there doesn`t appear to be a lot actually reaching the ground on this leading edge as there`s lot of mid level dry air. In other words, leading edge of what moves in overnight may be virga initially. Previous discussion follows below. Expect clouds to increase through tonight. Guidance maintains a slightly slower progression so most of the area should stay dry tonight and any areas that do see some widely scattered showers, likely won`t see them until well after midnight tonight. PoPs 20-40% for the western third of the region (increasing from east to west) with lower PoPs everywhere else. With increasing clouds and continuing light southerly winds, lows look to be in the upper 60s to near 70 across most of the region. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH MONDAY/... Unsettled weather on tap for the weekend and start of the new week. Hurricane Ernesto will track from south to north along, but well east of, the Delaware and New Jersey coasts. While there will be no direct impacts from Ernesto (any thunderstorm winds or flooding will be unrelated), large swells emanating from Ernesto will result in dangerous rip currents at the Delaware Beaches and Jersey Shore, as well as the potential for minor coastal flooding around the times of high tide. Those impacts will be discussed in the Marine and Tides and Coastal Flooding sections below. Broad low pressure will have a cold front trailing from it, and will be associated with a deep upper trough that will dig down through the Ohio and Tennessee Valleys and into the Gulf Coast states. This system will meander in those areas Saturday through Sunday, though several shortwaves will rotate through the base of the trough, keeping conditions unsettled. The first of these shortwaves will approach on Saturday, followed by several stronger shortwaves Saturday night. Although there are chances for showers in the morning, the bulk of the activity will hold off until the afternoon. By then, southerly flow will usher an increasingly humid airmass into the region with highs in the 80s and surface dew points rising through the 60s and into the low 70s. Although there will be chances for thunderstorms, the severe threat will be low on Saturday. PWATs, however, will rise to 1.8 to 2.0 inches. Locally heavy rain will be possible, mainly north and west of the I-95 corridor, Saturday afternoon and evening. Showers and scattered thunderstorms, mainly producing heavy rain, will really get going Saturday night and into Sunday morning. The Weather Prediction Center has a Marginal Risk (1 out of 4) for Excessive Rain for southeast Pennsylvania and a portion of northern New Jersey during this time. A secondary low looks to form west of Delmarva Saturday night, and then this low will lift to the northeast across New Jersey Sunday. On Sunday, the broad low over the Great Lakes will move into western New York, dragging the cold front closer. The southerly pressure gradient will tighten over the area, ushering tropical moisture into the region. As the front approaches and that secondary low passes through, showers and thunderstorms will develop in the afternoon and into Sunday night. Surface dew points will rise into the low to mid 70s, resulting in PWATs over 2 inches. SB CAPE values will rise to over 1000 J/kg, and even 0-6 km Bulk Shear will rise to 35 to 40 kt. From Sunday afternoon through Sunday evening, there is a risk for severe thunderstorms. The Storm Prediction Center has a Marginal Risk (1 out of 5) for severe storms for much of the forecast area during this time, with damaging wind gusts being the primary threat. Heavy rain producing flash flooding is also expected, and the Weather Prediction Center has the region in a Slight Risk (2 out of 4) for Excessive Rainfall for much of the region. The secondary low should be northeast of the region by Monday morning, and showers and thunderstorms will taper off then. There should be a lull in the showers and thunderstorms for most of the day until the cold front passes through in the afternoon. This will trigger another round of showers and thunderstorms over the area, though behind the departing low in the morning, a somewhat less humid airmass will spread into the region. The threat for flash flooding will be more based on if storms pass over flooded areas than storms being capable of flooding themselves. && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... The base of a deep upper trough will settle over the East Coast for the mid-week period. The cold front working through the region late Monday will move offshore Monday evening. A secondary cold front follows through on Tuesday afternoon, and then surface high pressure builds over the Great Lakes and Ohio Valley. The upper trough slides offshore by Friday. Meanwhile, the surface high builds east and will be overhead by the end of the work week. In terms of sensible weather, any showers and thunderstorms will taper off in the evening with the loss of diurnal heating. A much more autumnal airmass will then spread into the Northeast and Mid- Atlantic for the mid-week period. High temperatures will be several degrees below normal with highs in the 70s both Tuesday and Wednesday. The passage of that secondary cold front Tuesday afternoon may spark off some afternoon showers and thunderstorms, but based on the NBM, PoPs will be capped at slight chance for most of the region, with some low end chance PoPs in northern New Jersey as some shortwave energy passes through. Sunny, dry, and much less humid conditions on tap from Wednesday through Friday. Radiational cooling conditions likely at least Tuesday night and Wednesday night, so went with a blend of NBM:NBM10 for low temperatures those nights. Using this blend, lows will be mainly in the 50s both nights, but around 60 near the coasts due to the proximity of the warmer ocean temperatures, and in the 40s in the southern Poconos! The temperatures on Tuesday and Wednesday will be more similar to late September than late August. Get your pumpkin spice lattes ready! Temperatures moderate a bit on Thursday and Friday with highs in the upper 70s to low 80s. Humidity levels creep up a bit as dew points rise to around 60 by Friday. && .AVIATION /02Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR conditions expected with increasing mid to high level clouds, particularly after 06Z. An isolated shower may approach KRDG/KABE after 09Z. SSE winds around 5 knots or less. High confidence on prevailing VFR conditions. Saturday...Times of sub-VFR conditions possible as showers and storms arrive from the west in the morning and become more numerous through the afternoon into the early evening hours. VCSH as early as 15-18Z for RDG/ABE with chances for TSRA increasing after 18Z for all terminals. Some locally heavy rainfall possible. Outlook... Saturday night through Sunday night...Sub-VFR in SHRA, TSRA, fog, and stratus. Locally heavy rain Saturday evening, then again Sunday afternoon and Sunday night. Monday through Monday night...Lingering SHRA/TSRA Monday morning, then another round of SHRA/TSRA Monday afternoon and evening. Tuesday...Overall, VFR. Isolated SHRA/TSRA in the afternoon. && .MARINE... For tonight, a Small Craft Advisory remains in effect for Atlantic Ocean coastal waters beginning at 10 PM EDT. Seas initially 1-2 feet this morning building to 2-4 feet later this afternoon. Seas begin to exceed 5 feet late this evening and continue to build towards 5-7 feet by late tonight. Winds of 5-10 kts with gusts around 15 knots expected throughout the period. Small Craft Advisory in effect for the ocean for Saturday as seas build to 5 to 7 feet due to larger swells from distant Hurricane Ernesto. Sub-SCA conditions expected for Delaware Bay. VSBY restrictions in showers and thunderstorms in the afternoon. Outlook... Saturday night through Sunday night...SCA is in effect for seas of 5 to 7 feet due to larger swells from distant Hurricane Ernesto. The conditions on Delaware Bay will stay below Small Craft Advisory criteria. VSBY restrictions in showers and thunderstorms, as well as patchy fog. Monday...Lingering SCA conditions as seas will be slow to subside on the ocean. Sub-SCA conditions on Delaware Bay. Tuesday through Wednesday...Sub-SCA conditions. Rip Currents... For Saturday and Sunday, Hurricane Ernesto will track well east of our region. However, larger swells arriving from this storm and breaking into the surf zone will result in very dangerous rip currents at the Jersey Shore and at the Delaware Beaches. As a result, a HIGH risk for the development of dangerous rip currents is in effect for the Jersey Shore and Delaware Beaches through Sunday evening. Breaking waves could reach as high as 4 to 7 feet. Some beach erosion is possible. The high risk of rip currents could linger into early next week as well. For specific beach forecasts, visit weather.gov/beach/phi && .TIDES/COASTAL FLOODING... With long period swells and rough seas/surf expected to arrive this weekend from distant Hurricane Ernesto coinciding with an approaching full moon on Monday, at least some spotty minor coastal flooding is anticipated to develop along the Atlantic coasts of New Jersey and Delaware, Delaware Bay, and the tidal Delaware River. The primary high tide cycles of impact will be evening to nighttime high tides, perhaps beginning as early as Saturday in the most vulnerable areas. As long period swells persist and astronomical high tides increase Sunday and Monday, more widespread minor coastal flooding is forecast with the evening to nighttime high tides those days. Anyone with interests in these areas that are susceptible to minor coastal flooding should monitor the latest forecasts and remain alert for any advisories that may be issued in future updates as confidence in the forecast increases. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...High Risk for Rip Currents through Sunday evening for NJZ014- 024>026. DE...High Risk for Rip Currents through Sunday evening for DEZ004. MD...None. MARINE...Small Craft Advisory until 6 AM EDT Monday for ANZ450>455. && $$ SYNOPSIS...MPS NEAR TERM...Fitzsimmons/MJL SHORT TERM...MPS LONG TERM...MPS AVIATION...Fitzsimmons/MJL/MPS MARINE...Fitzsimmons/MJL/MPS/Staarmann TIDES/COASTAL FLOODING...