Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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982 FXUS61 KPHI 190207 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1007 PM EDT Fri May 18 2018 .SYNOPSIS... A nearly stationary front just to the south of the region and several weak areas of low pressure will bring generally unsettled conditions to much of the area through the weekend. A cold front will cross the area later Sunday followed by high pressure moving in for early Monday. Another low with its associated fronts will affect the area Tuesday into Wednesday before high pressure returns Thursday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... Multiple updates to the forecast were sent this evening with the latest 930 PM ESTF update focusing on hourly temperatures. Temperatures will generally be slow to fall tonight owing to dense cloud cover and a continued onshore breeze. However, a more precipitous drop in temperatures can be expected in eastern PA and NJ when precip arrives overnight due to evaporative cooling near the surface. Conversely, temperatures may start to rise in the pre-dawn hours across southern DE and eastern MD as the warm front approaches from the south. PoPs, weather and QPF were also tweaked in an earlier update this evening. Utilizing trends from radar/satellite imagery and hi- res models, PoPs were cut way back north of the Mason-Dixon line early in the period as dry air advection/subsidence from high pressure has been effective in keeping most of the shower activity suppressed to the south across Delmarva and VA so far this evening. However, this will change later in the night in response to the surface high retreating offshore and the upper ridge axis progressing downstream of the region. Expect widespread showers to expand northward through eastern PA and NJ overnight. Hi-res CAMs seem to be picking up on the area of moderate to heavy showers that is currently to our south and southwest in central and southern VA. This is expected to move northward through the region late tonight and early Saturday morning. The greatest risk for flooding toward morning will be across southern DE, northeastern MD and southeastern PA. This area has already been primed with moderate to heavy rainfall in recent days (i.e., saturated soils and elevated streams) and this is where an additional 1"+ of rain is most likely to fall based on upstream radar trends. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/... A warm front should finally lift through our region slowly during the day tomorrow. Although we will technically be in the warm sector, temperature-wise it may not feel like it as the front is expected to move through very late in the day, and lingering cloud cover will temper the warm up. The juxtaposition of the southerly low-level jet and several mid-level shortwave disturbances tracking northward will likely keep showers rather widespread along and west of I-95 during the morning while the coastal plain in DE and NJ may see a break in the precip. Showers are likely to continue across inland areas (E PA and N/W NJ) in the afternoon. Forecast soundings do not show much daytime instability developing in these inland zones owing to the cloud cover and the likelihood that the warm front will struggle to make it this far inland. However, showers will be efficient rainfall producers, so we cannot rule out locally moderate to heavy rainfall across our western zones. Additionally, breaks in the cloud cover may develop in lower Delmarva and far SE NJ during the afternoon. If this happens, isolated convection (including a thunderstorm) may develop mid to late afternoon. These would also be heavy rainfall producers but the risk of flooding for the afternoon in the coastal plain would be highly conditional on deeper convection initiating farther away from the better forcing to the west. && .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... Saturday night through Monday... To start the period Saturday evening, upper level trough will be approaching from the west with an associated surface low moving through Ontario into Quebec helping to lift a warm front across the area. As a result, temps will hold fairly steady in the 60s or even rise Saturday night with dewpoints also creeping up. There will be some scattered showers around in response to weak impulses moving through the upper level flow and these could still be capable of producing brief periods of moderate to heavy rain due to the abundant moisture in the atmosphere. There also looks to be enough instability for the chance of some thunder, especially north and west of the I-95 corridor. For Sunday, a cold front will gradually work southward across the area with time...reaching the I-95 corridor in the afternoon and clearing the coast in the evening. There will continue to be abundant deep moisture in the atmosphere until this front passes with PWATS over 1.7 inches. Also, surface Tds will be in the 65 to 70 range in the warm sector so very muggy! The front along with its attendant upper trough and diurnal instability will be the focal point for more showers and storms to develop...especially late morning through about mid to late afternoon. Biggest question will be how much instability there will be and this will depend on if we get any sunny breaks ahead of the front. Currently, still doesn`t look like a lot of sun so this should limit instability to the 500-1000 j/kg range of CAPE. Still, combined with the high PWATs any storms will be capable of producing very heavy rain resulting in continuing flooding concerns exacerbated by the already saturated conditions. Also, some indication from NAM soundings that Corfidi Vectors will result in slow moving and/or backbuilding storms. Urban and small stream type flooding would be the biggest concern but can`t rule out flash flooding as well. Highs Sunday will generally be in the upper 70s to low 80s but again, combined with muggy dew points until the front passes. Sunday night...finally some relief as the cold front clears the area bringing an end to any showers and storms...at least temporarily. In fact, expect a period of mainly clear to partly cloudy skies overnight as high pressure begins to nose in. With the saturated conditions and diminishing winds this will likely result in areas of fog, especially in valleys. Lows generally range from the mid 50s to mid 60s...coolest north, warmest south. For Monday, reprieve continues as most of the day looks to be dry dominated by transient high pressure. However there will be some mid and high clouds around ahead of the next approaching wave and very late in the day some showers could approach eastern PA zones from the west so we mention slight chance POPs here. Highs will generally be in the 70s to near 80 but with much lower and more comfortable dew points in the drier airmass. Monday Night Through Friday... As the next upper level wave and associated surface low approaches, this will lift the warm front back to the north across the region into Tuesday bringing the return of some showers and possible thunderstorms. This will be followed by the system`s cold front Tuesday night into Wednesday bringing continuing chances for showers and storms though it won`t be a steady all day rain. The front should finally clear the area by Wednesday night and at this early vantage point, high pressure with fair weather looks to return by Thursday. It should be stressed though that the models have been struggling in the extended (day 6-7) so this could still change as the moist, rainy tropical airmass plaguing us will never be pushed too far southward away from the area...even right through next week. && .AVIATION /02Z SATURDAY THROUGH WEDNESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...CIGs to start the evening were VFR at RDG and ABE, MVFR for TTN, PNE, PHL and ILG and borderline MVFR-IFR at MIV and ACY. Expect CIGs to gradually lower from S to N in concert with the steadier showers. For the 00Z TAFs, the onset of IFR was forecast to occur from S to N between 07-10Z for the I-95 terminals and from 09-12Z for ABE/RDG, which are 2-4 hours slower than LAMP guidance (which has been too pessimistic). Overall, confidence in timing of IFR is slightly lower than normal. Showers may become heavy temporarily for an hour or two from S to N between 06-10Z period. E winds will continue to be in the 8-14 kt range with occasional gusts to 20 kt. Saturday...Showers and IFR CIGs are expected for most terminals for the morning. Although a break in the showers may occur toward the coast (e.g., MIV-ACY) by mid morning, CIGs aren`t expected to improve much initially. Even though there isn`t much support from the guidance, there seems to be an opportunity for CIGs to improve to MVFR after 18Z in this setup, especially once winds shift from E to S-SE later in the afternoon. However, confidence in timing of this potential improvement is rather low. Wind speeds will be in the 6-12 kt range. Outlook... Saturday night thru Sunday...Mostly sub-VFR conditions expected with showers and psbl thunder at times in addition to low cigs. Sunday night...Areas of fog possible with associated restrictions. Monday...mainly VFR. Monday night through Wednesday...chances for showers/storms with associated restrictions returns as next frontal system moves through.
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&& .MARINE...
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All Gale Warnings were dropped and converted to a SCA as E winds have gradually subsided toward sunset in response to a weakening pressure gradient. The SCA for the DE Bay was extended through 4 AM with gusts to 25 kt still possible into a portion of the overnight. For the coastal waters, winds will continue to weaken through the night, falling below 25 kt late. Seas, which were 7-8 ft in our offshore buoys, will take longer into Saturday to subside below 5 ft. Accordingly, a SCA will cover these zones through Saturday afternoon. Outlook... Sat night thru Sun...Lingering SCA across the ocean possible. Showers. Sunday night thru Wednesday...Mostly sub-SCA. Sct Showers.
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&& .HYDROLOGY... Surface high pressure pushed south today across northern New Jersey and northeast Pennsylvania. This kept the rain across the southern portion of the HSA all day. While this southern rain will slowly track to the north tonight back across much of the area, rainfall totals are no longer forecast to be as high as they once were across northern NJ and east-central PA. For these reasons, the Flood Watch has been cancelled north of the PA Turnpike and I-195 corridor in NJ. Further to the south, over three inches of rain have fallen in spots today alone across the Delmarva. Lesser amount in southern NJ. This is on top of the rainfall that occurred the last few days. Periods of rain remain in the forecast tonight through Saturday. Going forward, another inch can be expected just about anywhere. With the heaviest rain falling across southern Delaware and the adjacent Maryland Eastern Shore the last few days, the threat of mainstem river flooding across New Jersey and southern Pennsylvania has diminished. Low lying and poor drainage can still be expected to develop through Saturday. Flash flooding also remains possible as thunderstorms are expected on Saturday. Continue to keep an eye on later forecasts. All of our hydrologic forecasts can be accessed via our Advanced Hydrologic Prediction Service website. While the threat of mainstem river flooding has indeed diminished, it has not been completely eliminated. && .TIDES/COASTAL FLOODING...
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At 10 PM, the total water level forecast sent Friday afternoon looks to be on track as we approach high tide in the next hour or two along the coast, overnight for the Delaware Bay, and early morning for the tidal portion of the Delaware River. An east to northeast wind along the coasts of New Jersey and Delaware will continue into tonight. The onshore flow is expected to result in a 1.0 to 1.5 foot surge along the coast. As a result, we are anticipating minor flooding around the time of tonight`s high tide. The Coastal Flood Advisory remains in effect for the coastal counties of New Jersey and Delaware and for the counties along Delaware Bay. The Coastal Flood Advisory has been expanded to include the tidal Delaware River, based on the latest guidance. We should lose the onshore flow during the course of the day on Saturday. Therefore, the potential for coastal flooding will diminish over the weekend and no additional advisories are anticipated. No tidal flooding is forecast for the upper eastern shore of Chesapeake Bay.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Flood Watch through Saturday afternoon for PAZ060-070-071- 101>104. Coastal Flood Advisory from 3 AM to 7 AM EDT Saturday for PAZ070-071-106. NJ...Flood Watch through Saturday afternoon for NJZ016>027. Coastal Flood Advisory until 3 AM EDT Saturday for NJZ012>014- 020>027. Coastal Flood Advisory until 4 AM EDT Saturday for NJZ016. Coastal Flood Advisory from 3 AM to 7 AM EDT Saturday for NJZ015-017>019. DE...Flood Watch through Saturday afternoon for DEZ001>004. Coastal Flood Advisory until 3 AM EDT Saturday for DEZ002>004. Coastal Flood Advisory until 4 AM EDT Saturday for DEZ001. MD...Flood Watch through Saturday afternoon for MDZ008-012-015-019- 020. MARINE...Small Craft Advisory until 6 PM EDT Saturday for ANZ450>455. Small Craft Advisory until 4 AM EDT Saturday for ANZ430-431.
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&& $$ Synopsis...Fitzsimmons Near Term...Klein Short Term...Klein Long Term...Fitzsimmons Aviation...Fitzsimmons/Klein Marine...Fitzsimmons/Klein Hydrology...Kruzdlo Tides/Coastal Flooding...Iovino/Klein

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