Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 170420 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1220 AM EDT Thu May 17 2018 .SYNOPSIS... As a front remains nearly stationary over the Mid-Atlantic through late Friday, several weak low pressure systems will move across the region. As this front dissipates, a cold front then moves through the region later Sunday into Monday before stalling nearby Monday night through Wednesday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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One area of showers has moved offshore, and there is some drier air across the region ahead of another area of showers moving in from the west. Will remove mention of thunderstorms, as storms downstream have ended. Temperatures should remain fairly steady through the night owing to overcast cloud cover and a light E-NE wind. With the cloud cover already in place and widespread rain, do not anticipate the fog from becoming dense except along the ridge lines (which will basically be in the base of the low stratus deck).
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM FRIDAY/... Steady rain is expected to persist through the morning and at least through the first part of the afternoon from roughly south of I-78 and north of Dover, DE. We should see an earlier end to the precip across NE PA and NW NJ around midday with drying conditions gradually expanding S-SE later in the afternoon. From roughly the Delaware Bay, southward, there is a potential for some breaks in the cloud cover to develop by afternoon with the warm front lifting northward. If this happens, there may be enough instability to support isolated thunderstorms by mid to late afternoon. Highs may struggle to reach the 70s except across the far north where breaks in the clouds are more likely and across central Delmarva which could see the front lift slightly farther north enough to be back in the warm sector. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Summary...Flood Watch for much of the area Thursday evening through Friday night. Rounds of showers including heavy rain keeping temperatures down through late this week, then warming over the weekend and early next week. Synoptic Overview...A ridge remains parked over the western Atlantic while some troughing slides from the Plains toward the Northeast. A moisture conduit is in place through late this week with injection of tropical moisture from the south. A little more of a trough may slide across the Northeast late in the weekend and early next week, pushing the western Atlantic ridge southward some. The pattern evolution should result in cooler air transitioning to warmer next week. For Thursday night through Saturday...The potential for times of heavy rain is increasing, especially Friday into Saturday. Our region will continue to be situated between a ridge to the east- southeast and a trough back across the Ohio and Tennessee Valley`s. The trough is forecast to get closer to our area late Friday and Saturday, and ahead of this feature are several focused areas of short wave energy sliding northeastward. At the surface, a front is forecast to be stalled mainly just to our south and this will serve focus for some weak low pressure systems to move along. While the short wave energy will serve focus for ascent at times, mid to upper level flow increases especially Friday into Saturday. This will assist in transporting deeper moisture into our area, with precipitable water values around 2 inches. There will be some initial low-level drier air from the north getting into our northern zones, however this should be replaced as surging moisture overtakes the area by late Friday. This pattern will offer rounds of showers, some of which will produce times of moderate to heavy rain. The more widespread heavier rain looks to occur Friday into Saturday as the system aloft to our west nears and a plume of higher precipitable water values become focused into our area. Given the amount of moisture and rounds of ascent will result in plenty of rainfall. A flood watch has been issued for much of our area starting Thursday evening and going through Friday night for now. More on this in the hydrology section below. The guidance overall does not show much in the way of instability, especially since the airmass is forecast to be or become nearly saturated in the vertical. However, some forecast soundings especially for the southern half of the area Friday afternoon shows enough elevated instability for the potential for some thunder. As a result, we focused more of the thunder mention at this time (slight chance of thunder). Any thunder that gets involved will result in increased rainfall rates. Temperatures will remain on the cooler side especially Friday, then some warming is expected to start on Saturday as low pressure tracks to our west and brings a weakening warm front northward. For Sunday and Monday...As an upper-level trough shifts eastward from the Great Lakes and upper Ohio Valley Sunday, surface low pressure is forecast to track to our north. Its cold front swings into our area later Sunday, however it may stall in our vicinity due to the flow becoming more parallel to it and another surface low develops on its tail end. While this should lower the precipitation chances, a few showers or thunderstorms cannot be ruled out especially in the afternoon and evening both days as much warmer temperatures are expected. For Tuesday and Wednesday...We will have to see how the timing goes as we get closer, however the passage of a short wave trough should drive surface low pressure nearby with an accompanying frontal zone eventually getting pushed a little farther south. The overall flow though may be westerly enough to keep the frontal zone closer to our area. For now went with some mainly slight chance PoPs especially across the southern half of the area. Temperatures should be knocked back some given the nearby presence of a frontal zone and weak high pressure sliding by to the north. && .AVIATION /04Z THURSDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of tonight...CIGs are generally IFR, but should settle in the LIFR range going through the overnight. Steady light rain expected. E-NE winds 4-8 kt. Thursday...Will be starting the day IFR or LIFR depending on the ceilings at day break. However, expect some improvement to occur initially N/W of Phila terminal (e.g., ABE-RDG) and also to the S/E (e.g., ACY-MIV) after 18Z. There is low confidence on timing of improvement in CIGs or with how much conditions will improve. Winds will be light and variable. Outlook... Thursday night and Friday...MVFR/IFR with rounds of showers and perhaps a few thunderstorms. Saturday and Sunday...MVFR/IFR conditions persist with showers, however conditions may improve at least some during Sunday. Monday...Sub-VFR conditions possible especially with any showers and thunderstorms.
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&& .MARINE...
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Winds and seas are expected to stay below SCA conditions tonight through Thursday. Some visibility restrictions are possible in rain showers and mist through this period. Outlook... Thursday night...The conditions are anticipated to be below Small Craft Advisory criteria. Friday and Saturday...Increasing onshore winds and building seas are expected. A Small Craft Advisory is looking likely for at least the Atlantic coast water zones. Sunday and Monday...The winds are anticipated to be below advisory criteria, however seas may take much of Sunday to drop below 5 feet.
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&& .HYDROLOGY... Numerous rounds of showers and thunderstorms will continue across the region through the end of the week and into the weekend. Heavy rainfall is possible during this period, with the heaviest rain falling Thursday night through at least Friday night. As of this afternoon, the heaviest rain is forecast along and south of the I-78 corridor. The current forecast calls for 2-4 inches of rain between this afternoon and Saturday. This does not include the rain that fell the last few days. Up to three inches have already fallen across portions of southern New Jersey and the Delmarva. A variety of flooding types are all possible within the Flood Watch we issued. First, low-lying and poor drainage flooding is the most likely type of flooding to develop. Second, flash flooding will be possible under the stronger thunderstorms. Flash flooding occurs quicker than other types flooding due to the torrential rain thunderstorms can produce in a short amount of time. Flash flooding is most likely to occur across urban areas where rainfall runoff is maximized and where small creek and stream respond quickly. The rain has to go somewhere and it eventually leads to larger streams and rivers. This is the third type of flooding possible across the Flood Watch area. If you live across the Passaic, Raritan, and Rancocas basins in New Jersey or the Neshaminy, Schuylkill, and Brandywine basins across Pennsylvania and Delaware, keep an eye on later forecasts. These forecasts, as well as all of our hydrologic forecasts, can be accessed via our Advanced Hydrologic Prediction Service website. Tides are also expected to increase through the end of the week as well. This could exacerbate flooding across those areas where fresh water and tides meet. && .TIDES/COASTAL FLOODING... Observed tidal levels as we approach high tide along the NJ and DE oceanfront this evening have increased to around 1/2 to 3/4 ft above astronomical prediction in response to the onshore flow (albeit light winds). Many of the tidal forecast points along the coast should reach their minor flood stage but Coastal Flood Advisories were not issued with the expectation that levels fall short of advisory criteria (0.3 ft above minor). The latest total water level forecast with "ensemble" support from various tidal models indicate that each high tide cycle through Friday morning should remain below minor criteria. However, with onshore flow forecast to strengthen during the day Friday, there is a potential for a more widespread occurrence of minor coastal flooding with the Friday evening/night high tide cycle. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Flood Watch from this evening through late Friday night for PAZ060>062-070-071-101>106. NJ...Flood Watch from this evening through late Friday night for NJZ007-009-010-012>027. DE...Flood Watch from this evening through late Friday night for DEZ001>004. MD...Flood Watch from this evening through late Friday night for MDZ008-012-015-019-020. MARINE...None.
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&& $$ Synopsis...Gorse Near Term...Johnson/Klein/MPS Short Term...Johnson/Klein Long Term...Gorse Aviation...Gorse/Johnson/Klein/MPS Marine...Gorse/Johnson/MPS Hydrology... Tides/Coastal Flooding...

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