Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 270736 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 336 AM EDT Fri Apr 27 2018 .SYNOPSIS...
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Low pressure will move across our region today, then into New England tonight. A cold front is expected to move through during Saturday, then high pressure across the Great Lakes and Ohio Valley Sunday will build to our south Monday. High pressure shifts off the Mid-Atlantic coast Monday night and remains there through Thursday, while a cold front approaches from the west Thursday night.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Weak surface low pressure and an upper low will approach early this morning and then cross the region this afternoon. Clouds will lower and thicken and showers will arrive from the SW by dawn. The showers and scattered tstms will then affect the area into the early afternoon. While the CAPE values are not that great, we will include the mention of thunder across Delmarva, SE Pennsylvania and southern New Jersey. Temperatures will remain cool today with highs in the mid/upper 50s across the north and low 60s in most other areas. Some mid 60s are possible over srn Delaware, depending if any breaks develop in the clouds later this afternoon. The system doesn`t have a strong pressure gradient associated with it, so winds will be rather light, favoring mostly SE or E this morning then switching to N or NW late today.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/...
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The disturbance will move away this evening, but only gradual improvement in the weather is expected overnight. Clouds will decrease a little before midnight, then begin to break up and move away after midnight. Low temperatures will drop into the low 40s north/west and mid upper 40s elsewhere. Winds will be light and variable overnight.
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&& .LONG TERM /SATURDAY THROUGH THURSDAY/...
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Summary...Some showers Saturday followed by cooler air Sunday, then a significant warm-up during next week. Synoptic Overview...An upper-level trough closes off as it moves across the Great Lakes to the Northeast and northern Mid-Atlantic regions during the weekend. Strong energy then rolling through the western U.S. allows for a ridge to build in the Plains during the weekend which then shifts eastward early next week. The guidance shows a robust closed low which therefore shifts east a little slower early next week. The arrival of the trough/closed low during Saturday along with a surface cold front will produce some showers. The evolving pattern thereafter with an incoming ridge favors significant warming during the course of next week. The transition day of Monday may feature some delay in the warming with the closed low moving east a little slower. For Saturday and Sunday...An upper-level trough closes off as it moves into the Northeast and northern Mid-Atlantic region. Meanwhile, a ridge shifts eastward from the Plains. Since this becomes a closed low, the tendency for them to shift eastward is typically slowed. As it arrives Saturday, forcing for ascent along the leading edge of the more pronounced height falls, positive vorticity advection and a surface cold front results is some showers arriving from the west especially in the afternoon. The more widespread showers may occur across portions of eastern Pennsylvania and into the I-95 corridor given the timing of the cold front and height falls coinciding with peak heating. There may be enough instability Saturday afternoon with the arrival of the cold front to result in some low-topped convective elements. Given the uncertainty regarding the amount of instability (may not be enough for charge separation), therefore we continued with no thunder mention. Despite the incoming closed low, warmer air hangs on Saturday ahead of the showers and cold front then cooling arrives at night and especially for Sunday. Regarding Sunday, it should be dry overall although a shower or two cannot be ruled out mainly in portions of northeastern Pennsylvania and northwest New Jersey given lingering moisture within a northwest cyclonic flow. Cold air advection within a stronger flow and deeper mixing will result a gusty northwesterly wind Sunday. If the winds drop off fast enough and the sky clears, there could be some frost in parts of the region late Sunday night. For Monday through Wednesday...As the pattern shifts to a trough out West, a ridge builds eastward and this places surface high pressure to our south Monday then just offshore on Tuesday and Wednesday. The presence of the building ridge will result in warm air advection and therefore the likely start of a significant warm-up Tuesday, followed by even warmer air on Wednesday. The lingering closed low though that will be exiting the Northeast and northern Mid-Atlantic regions looks a little slower in doing so on Monday. This results in some delay of the main warming. The cyclonic flow is forecast to be gradually waning during Monday as the closed low slides eastward, and enough drying should be in place and therefore carried a dry forecast. Given the presence of nearby surface high pressure and a ridge aloft, dry conditions are forecast for Tuesday and Wednesday. For Thursday...The northern portion of the ridge should start to get pushed southward some as an upper-level trough moves into the Northern Plains and Midwest. Low pressure moves across northern New England and this will have a cold front draped southwestward with the potential for ripples of energy traveling along it. The flow aloft looks to be more parallel to the surface front and with the ridge still in place to our south, the south and eastward movement of the surface front may be delayed. We are still expecting the warmth to continue, and while a lee side trough may be present during Thursday it is unclear this far our in time if some convection will be able to develop from it. As a result, we did not stray far from continuity.
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&& .AVIATION /08Z FRIDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...A slide towards lower CIGS/VSBYS early this morning with the approaching disturbance from the VA/MD region. Conditions will settle to lower MVFR or IFR later this morning and then continue into the afternoon. Occasional rain will taper to showers late. There is a chance for some thunder across KILG-KMIV-KACY, but as of now, the chances look low enough to exclude from the taf. Winds will be mostly light and variable, favoring NE this afternoon. Tonight...The conditions will not improve rapidly tonight with the dry air only gradually arriving. We will likely keep the MVFR conditions into the overnight with perhaps some VFR towards dawn. A low confid fcst for tonight. Winds will become mostly NW late. OUTLOOK... Saturday...Local fog may result in MVFR conditions early, otherwise mostly VFR. Some showers mainly in the afternoon and evening may result in times of sub-VFR conditions. Southwest winds increasing to near 10 knots, becoming west to northwest late in the day and at night. Sunday...VFR. Northwest winds 12-18 knots with gusts to around 25 knots, diminishing some at night. Monday and Tuesday...VFR. Northwesterly winds 10-15 knots with gusts to 20 knots Monday diminishing to 5 knots or less at night, then west to southwest winds around 10 knots Tuesday.
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&& .MARINE...
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We`ll keep the SCA flag for hazardous seas for the ocean today and extend into tonight. The latest guidance indicates that the swells will continue to slowly decrease, and the southern waters will likely lose the SCA flag tonight, if not sooner. Conditions up north will decrease slower so there is a better chance for keeping the SCA flag there possibly into the daytime Saturday. Showers today with possible thunder across the Delaware Bay, Delaware coastal waters and southern NJ coastal waters. Areas of fog will develop later today as well. OUTLOOK... Saturday...The conditions are anticipated to be below Small Craft Advisory criteria. Sunday and Monday...Northwesterly wind may gust to 25 knots at times mainly Sunday afternoon and evening. Otherwise, the seas are anticipated to remain below 5 feet. Tuesday...The winds and seas are anticipated to be below Small Craft Advisory criteria.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory for hazardous seas until 6 AM EDT Saturday for ANZ450>455.
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&& $$ Synopsis...Gorse Near Term...O`Hara Short Term...O`Hara Long Term...Gorse Aviation...Gorse/O`Hara Marine...Gorse/O`Hara

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