Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 211634 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1234 PM EDT Fri Apr 21 2017 .SYNOPSIS... A front will remain across our area today. Low pressure tracks across southeastern Canada later today and its associated cold front crosses our area late this afternoon into this evening. This front then stalls to our south over the weekend as low pressure tracks along it. This low will remain off the Mid Atlantic coast through early next week, then a cold front slowly moves across the eastern Great Lakes region Wednesday and Thursday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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The low cloud deck will be slow to erode from the west and southwest as warm air overrides it. Today`s temperature forecast remains one of low confidence in parts of our region due to the uncertainty surrounding the cloud cover. We expect the low clouds to linger all of the day in northern and central New Jersey with highs there expected to be in the upper 50s. Much of northeastern Maryland and Delaware will likely continue to break out of the low clouds with maximum readings there anticipated to be in the 75 to 85 degree range. The most uncertain is the temperature forecast for eastern Pennsylvania, northern Delaware and southern New Jersey where highs should range anywhere from the lower 60s to the lower 70s depending upon how much clearing takes place. Weak low pressure is forecast to develop in the Chesapeake Bay region late this afternoon as a cold front approaches our region from the northwest. Conditions will destabilize in northeastern Maryland and Delaware due to the increasing sunshine and surface heating. As a result, we will keep the mention of a chance of showers and thunderstorms in that area. The wind should remain onshore in northern and central New Jersey. Where the clouds break, the wind direction is forecast to become southwest. Some fog will linger in the elevated terrain of eastern Pennsylvania and northern New Jersey, and along the coast into the afternoon.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/... Patchy fog, otherwise considerable cloudiness. Leftover early evening showers/iso tstms vcnty s NJ and the Delmarva. At least partial clearing late. This part of the fcst was a 50 50 blend of the 00z/21 GFS/NAM MOS. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Summary...Some wet weather for at least parts of the area over the weekend; watching an offshore storm Monday through Tuesday; much warmer air should start to arrive at the end of the week. Synoptic Setup...A weakening upper-level trough is forecast to be sliding across the Northeast Saturday. A trailing strong short wave in the Central Plains shifts eastward Saturday with a surface low developing just ahead of it. This feature looks to strengthen and close off as it moves into the Southeastern States Sunday. This system will then slowly move eastward to off of the coasts of the Carolinas Monday. This closed low should then gradually open up and shift northeastward through Wednesday. The track of this feature will be key in the amount of rain our area may get Sunday and especially early next week. The overall trend is for a farther south track along a stalled baroclinic zone. As we go into the end of next week, significant short wave energy is forecast to sharpen a trough as it ejects into the Plains. This will start to allow for downstream ridging. Despite this, during this time of the year the potential for backdoor fronts is present. A frontal boundary is forecast to be to our northwest while high pressure builds east of James Bay Canada. While currently this front is forecast to remain out of our area, this evolution will be monitored in the event stronger high pressure allows for the boundary to slip farther south. We used a model/continuity blend for Saturday through Sunday night, then blended in the 00z WPC Guidance. Several adjustments were then made following additional collaboration with our neighboring offices. For Saturday and Sunday...What appeared to be initially a wet period of time for the region especially Sunday has lessened overall. The trend in the guidance is for much of the rain to be farther south, focused mainly in Delmarva for our area. This depends on an upper- level trough that starts to close off Sunday as it gets to the Southeastern States. Meanwhile, an upper-level trough sliding across the Northeast Saturday is forecast to gradually weaken. There will be a frontal boundary stalled to our south and a lead short wave Saturday may result in showers overspreading our southern areas. There is uncertainty with this, given some timing differences. Additional energy with some more organized lift may then slide across the southern half of the area Saturday night with more showers or a period of rain. Thereafter, surface low pressure associated with the closing off low in the Southeast States tracks rather far south. This appears to place much of the convergence and associated lift south of nearly our entire area Sunday. Some overrunning may occur though with showers for portions of Maryland, Delaware and far southern New Jersey, however farther north Sunday may end up being dry as high pressure builds into southeastern New New England or just offshore. We note that some guidance lowers the surface dew points over the weekend from north to south, indicating a stronger push from northern high pressure. As a result of the above including collaboration with WPC and our neighboring offices, we made significant changes to the PoPs for the weekend. There is a tighter gradient from south to north with any period of high PoPs now across parts of the southern zones. However, did keep a swath of chance PoPs for a time Saturday as a lead impulse may produce a corridor of showers. Kept these PoPs no higher than chance given low model agreement. We also changed the weather grids to showers from rain. There may be enough subsidence Sunday afternoon for some breaks of sunshine especially up north, then we await to see how the surface low well to our south tracks for Sunday night and thereafter. The onshore flow influence looks to be delayed until mostly later Sunday however the airmass looks to be cooler overall. For Monday and Tuesday...The closed low in the Southeast States is forecast to gradually lift east and even northeastward. This will drive surface low pressure across the far western Atlantic, however some guidance now brings this closer to the coast. The trend in the model guidance is for wetter conditions now for much of this time frame. Additionally, a strengthening onshore flow may lead to minor coastal flooding concerns during mainly Tuesday`s high tide (new moon occurs on Tuesday). Given the uncertainty, will leave a mention out of the hazardous weather outlook. This is a dynamic and fluid setup especially when dealing with closed lows, and for now did increase PoPs for much of this time frame (especially Monday) with a chance PoPs everywhere and even a period of low likely closer to the coast. This more unsettled weather with an onshore flow will result in cooler conditions. For Wednesday and Thursday...An upper-level trough is forecast to amplify as it ejects into the Plains. This will start to build downstream ridging. Meanwhile, the closed low near the East Coast should be opening up some as it moves away Wednesday. This results in a loose pressure gradient along with a warming airmass. While it should be turning much warmer, it still looks to be cooler along the coast as there should be a sea/bay breeze influence especially Wednesday. A frontal boundary is forecast to be stretched from the Plains to the eastern Great Lakes then just north of New England. This front should remain there as a ridge builds, however given this time of year one must watch surface high pressure building across eastern Canada. This could push the surface front farther south if strong enough opening the door for a cooling affect from the ocean. As of now, looks like a warmer airmass will dominate overall. After some lingering slight chance PoPs Wednesday, we went dry thereafter. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...IFR and MVFR conds in st/fog. Conditions improving only slightly this afternoon. Right now for this afternoon we think cigs 1000-2000 ft but there will be some cigs below 1000 ft. Mostly east wind 5 to 15 kt. Tonight...MVFR or IFR cigs for a time during the early night should improve late at night. Patchy IFR fog possible. Wind becoming light north. OUTLOOK... Saturday and Sunday...Low confidence with timing and details. VFR may dominate much of this time frame, however an area of showers Saturday probably from about RDG to PNE to MJX southward could bring times of MVFR/IFR conditions. An area of additional showers Saturday night into Sunday mostly south of a ILG to ACY line could also result in periods of lower conditions. Monday and Tuesday...Potential for widespread MVFR/IFR conditions with showers or periods of rain as a storm system parallels the East Coast. Confidence on the low side given uncertainty with the track of the storm system.
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&& .MARINE...
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No headlines at this time. Winds easterly with a more southerly component for a time Cape May southward during early afternoon before winds everywhere turn to northeast late today or this evening and then northerly tonight Gusts near or above 25 kt may be possible in the vicinity of any thunderstorms late this afternoon in the De Waters. Wave heights on our ocean waters are forecast to favor the 3 to 4 foot range. Waves on Delaware Bay are expected to be 1 to 2 feet. OUTLOOK... Saturday and Sunday...The conditions are anticipated to be below Small Craft Advisory criteria. However, an onshore flow should start to increase mainly later Sunday night as low pressure moves off the coast of the Southeast States. Monday and Tuesday...Confidence is on the low side with the timing and details. Low pressure may parallel the East Coast producing rounds of showers or periods of rain. A strengthening northeasterly flow, which may peak on Tuesday, looks to gust to 30 knots at times. This will also significantly build the seas. Small Craft Advisory conditions look to occur, and as of now there is lower confidence if wind gusts can reach gale force sometime during this time frame.
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&& .CLIMATE... Top 10 warmest April is virtually assured for parts of our area. This despite some chilly rains early this coming week and associated potentially significantly colder than normal maximum temps. The reason is the eventual bounce back with very warm temps, probably developing at the end of next week. The numbers this morning have Philly #2, 0.3"F from a record and Allentown at record (0.1F) above. Lets for now say that we`re too optimistic about overall warmth and lose a degree on the average... that will shave the positive departure from 5 and 6 degrees to 4 or 5 above normal. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gorse Near Term...Iovino Short Term...Drag Long Term...Gorse Aviation...Drag/Gorse Marine...Drag/Gorse/Iovino Climate...Drag

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