Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 241938 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 338 PM EDT Tue Apr 24 2018 .SYNOPSIS... An area of low pressure located near the Carolinas will continue to move northward across the eastern seaboard, crossing our area Wednesday into Wednesday night, then into New England by Thursday. Very weak high pressure tries to build into the region Thursday, before another low pressure moves northward across the east coast Friday and into New England Saturday as it combines with another low pressure. A cold front is expected Saturday, with additional surface troughs Sunday into Monday. High pressure across the Great Lakes and Ohio Valley Sunday will build to our south Monday into Tuesday. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... The surface high and subsidence are propagating further away from our region. As a result, rain, which has been to our southwest all day should start to propagate into northern Delmarva within the next few hours. Models have backed off a little on how much moisture advection we may see. Latest runs show precipitable water values by late tonight between 1 and 1.2 inches, which remains quite high for this time of year, but less than what the models were depicting with earlier model runs. None the less, there remains a potential for locally heavy rain. For the most part, it looks like the rain amounts will not be enough to cause flooding in our area. However, in the urban corridor from Philadelphia up to Morris County, if the forecasted rain amounts fall in just a few hours, some flooding is possible in poor drainage areas, so will add a mention to the HWO for this. Still see only very limited elevated instability. Thus will not mention any thunderstorms in the forecast for now. Temperatures will stay up higher than what we have seen the last several nights, thanks to the moisture advection and persistent cloud cover. Minimum temperatures are forecast to be in the 40s and 50s. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/... Dry slot should develop in our region tomorrow morning, bringing a quick end to the widespread rain. Depending on how much standing water there is by daybreak, we could have some patchy fog across the region, though it looks like more of a low stratus set up than fog. The lull in rain may not last very long as the trailing mid and upper level trough could result in additional periods of light rain, generally through the second half of the day tomorrow. However, thanks to dry air advection moving in, the threat for heavy rain is very limited tomorrow. Cloud cover may limit heating somewhat, resulting in highs generally in the 50s and 60s across the region. && .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/... After a brief period of possible unsettled weather at the beginning of the period, dry weather takes hold of the forecast for the rest of the long term forecast. Although the low pressure system that affects our area during the day Wednesday lifts to our north Wednesday night, there will remain a chance of showers on the backside of the low into Wednesday evening as there remains a couple of vorticity impulses that will move across the area. By Thursday, very weak high pressure may briefly affect the area, leading to dry conditions. However, on Thursday night, an area of low pressure will be approaching the area from the southwest. The low itself is not expected to make it across, or near the area, until during the day Friday, or as late as Friday evening. There are timing and locations differences with the low between the guidance. Still, they all indicate an increase in moisture while several short wave/vorticity impulses slide across the area as well. This will lead to a chance of showers from late Thursday night through early Friday night, although it will likely not rain everywhere, nor the whole time. Once this low lifts to our northeast, it will combine with another low pressure as it moves out of the Great Lakes region and across eastern Canada and northern New England. This will pull a cold front across the area during the day Saturday. This could bring another chance of showers during the day as the front moves across the area. Behind this front, drier conditions return to the forecast. The low pressure across eastern Canada will near the Canadian Maritimes through Monday, while high pressure across the Great Lakes and Ohio Valley Sunday will build to our south Monday into Tuesday. A couple of surface troughs will cross the area during this time period, but are expected to remain dry. For Sunday and Monday, there will be a pressure gradient in place between the high to the south and low to the north, so a steady breeze with gusts likely in the 20s is expected, with Sunday having the strongest winds. && .AVIATION /20Z TUESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of today...Mostly VFR conditions are expected through the remainder of the day time today. Lower ceilings and light rain will start to approach the TAF sites from the south late today. Winds will stay southeasterly, gradually shifting to easterly, with gusts near 20 kt likely. High confidence. Tonight...Ceilings are expected to lower to IFR through the evening and early morning hours. Localized LIFR conditions are possible. Additionally, visibility in rain, heavy at times, could be reduced (mostly to MVFR, but occasional IFR visibilities are possible). Easterly winds around 10 kt are expected through the overnight hours. Moderate confidence on the flight category, but low confidence on the timing of changes. Tomorrow...though the rain may end at many TAF sites during the morning, low clouds are expected to linger through the morning. Improvement to MVFR or even VFR is possible in the afternoon, but confidence in improving conditions is low. Winds are expected to start easterly, but should shift to northwesterly during the afternoon. OUTLOOK... Wednesday night...MVFR conditions possible early with a chance of showers, before improving overnight. Thursday...VFR conditions expected. Thursday night...Generally VFR, although MVFR conditions may begin late in the night across the southern areas as rain approaches. Friday...MVFR, possible IFR, with a chance of scattered showers. Friday night...MVFR conditions possible early with scattered showers, becoming VFR during the evening and overnight. Saturday...Mostly VFR. A chance of showers which may temporarily lower conditions. Saturday night-Sunday...VFR. Winds becoming gusty out of the northwest Sunday morning and may gust 20-25 knots. && .MARINE... Southeasterly and easterly winds gusting above 25 kt are expected to continue through tonight. By tomorrow morning, winds may diminish, though seas above 5 feet are expected to continue on the coastal waters. The small craft advisory remains in effect until 6 AM on the Delaware Bay, and through the day tomorrow (and into Thursday) on the coastal waters. Rain and patchy fog could reduce visibility especially early Wednesday morning. OUTLOOK... Wednesday night-Thursday...Small Craft Advisory remains as seas expected to remain over 5-7 feet. Thursday night-Saturday night...Conditions expected to remain below advisory levels. Sunday...Winds may approach Small Craft Advisory levels on Sunday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EDT Thursday for ANZ450>455. Small Craft Advisory until 5 AM EDT Wednesday for ANZ430-431. && $$ Synopsis...Robertson Near Term...Johnson Short Term...Johnson Long Term...Robertson Aviation...Johnson/Robertson Marine...Johnson/Robertson

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