Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 210414 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1214 AM EDT Fri Apr 21 2017 .SYNOPSIS...
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A cool front is expected to stall across the Mid Atlantic states through Friday. Low pressure moving from the Great Lakes region into southeast Canada, will pull a cold front across the area Friday night. This front will stall to our south over the weekend. An area of low pressure is forecast to strengthen as it moves off the southeast coast late Sunday, then move north offshore of the east coast through early next week. A cold front is expected either next Wednesday or Thursday as an area of low pressure moves out of the Great Lakes region into southeast Canada.
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&& .NEAR TERM /THROUGH FRIDAY/...
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Midnight estf: two updates for cooler overnight temps and more expansive showery weather have already posted 11P-1145P. Lead bands of showers e PA southward through DE with isolated embedded thunder should exit6 or weaken off the NJ coast by 3 am. The primary band of showers and sct tstms across central PA should weaken as it arrives overnight. We are not anticipating any severe weather with tonight`s system. However, there could be localized brief downpours and perhaps some small hail. Otrw...light east wind. The daylight hours Friday...Probably running cooler temps, especially PHL north and northeastward. light wind and too much cloud cover, residual leftover from the overnight showers, most of which should have ended by 9 or 10 am as the primary band of showers in central PA at midnight dwindles-weakens in its passage across the Garden state around 8 or 9 AM Friday. Can there renewed showers and thunderstorms today...possibly but probably reserved more for where the instability is greater, over the probable warm sector of the Delmarva. The 330 am fcst will probably be a 50 50 blend of the 00z/21 GFS/NAM MOS and then that blended with mesoscale models hrrr, rgem to handle the temperature differencing along the coasts and in the cooler boundary layer airmass across NNJ.
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&& .SHORT TERM /FRIDAY NIGHT/...
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No change below from the 330 PM post, previously in our long term section of this AFD. Friday night, the frontal boundary that affects our area during the day pushes south of the area. With the boundary to our south, there is also the possibility for some vorticity impulses to move across the area withing the southwest flow aloft. The NAM is the most aggressive bringing in precipitation overnight, while the GFS, ECMWF, and Canadian aren`t as aggressive. We will keep a slight chance of showers overnight due to the proximity of the frontal boundary to the south and potential short wave interaction.
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&& .LONG TERM /SATURDAY THROUGH THURSDAY/...
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There is the potential for some unsettled periods during the long term, but not the entire time expected to be active. Some uncertainty with some model differences at times during the forecast. As we move into Saturday and continuing into Sunday, the front is expected to stall to the south, while an area of low pressure moves along the boundary well to our south. With stronger short wave/vorticity impulses expected to move across the area this weekend north of the frontal boundary and low pressure system, there will be a better chance of showers across the area Saturday through Sunday. The most likely areas for showers is expected to be from around the Philadelphia area southward. There will remain a chance of showers Sunday night across southern New Jersey and southern Delmarva, but chances decrease through the night as the short wave/vorticity impulses begin to move past the area. By Sunday night into Monday the low pressure to our south is expected to move offshore of the southeast coast, then begin lifting northward offshore of the east coast into the middle of the week. There is some timing differences in how fast the model guidance lifts this low offshore of our area, but there is a general consensus for a enhanced chance of showers around Tuesday for much of the area. Depending on how fast this system lifts north of the area, there could also be a small chance of showers into Wednesday. However, if it`s far enough offshore, most places should be precipitation free. There is also the possibility of a cold front approaching the area during the middle of next week as well. The ECMWF has it approaching Wednesday into Wednesday night, while the GFS has it approaching Thursday into Thursday night. This frontal boundary could bring another chance of showers to the area Wednesday or Thursday.
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&& .AVIATION /04Z FRIDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Through 12z... VFR cigs lower to IFR conds in showers/isolated Tstms and possibly stratus/fog. light mostly east wind. After 12z Friday...IFR or MVFR conds in st/fog to start the day with showers near KTTN ending by 14z. Conditions improving to cigs 2000-4000 ft in the afternoon. light wind. Friday night...VFR conditions are currently forecast for Friday night, but there is a chance for lower conditions to develop with a chance of showers. OUTLOOK... Saturday-Sunday...Better chance for showers and lower conditions over the weekend. The highest chances for showers and lower conditions are expected from the Philadelphia area southward. Sunday night...A chance of showers remains for coastal New Jersey and central/southern Delmarva with possible lower conditions there. Elsewhere, improving conditions overnight. Monday-Monday night...Mostly VFR, lowering clouds overnight. Tuesday...MVFR conditions possible with showers, especially eastern New Jersey.
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&& .MARINE...
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The wind should favor the east and southeast tonight and the south and southwest on Friday at speeds of 15 knots or less. Gusts near or above 20 kt may be possible in the vicinity of any thunderstorms late tonight. 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 3 feet. Winds may gust around 20 knots at times Friday night into Saturday, but overall should remain below advisory levels. OUTLOOK... Saturday...Winds may gust around 20 knots at times Friday night into Saturday, but overall should remain below advisory levels. Saturday night-Sunday...Conditions expected to remain below advisory levels. Winds shift from a west-northwest direction to a northeast direction overnight Saturday. Sunday night...Winds and waves begin to increase to advisory levels late Sunday night into early Monday morning. Monday-Tuesday...Small Craft Advisory conditions likely.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Robertson 1214 Near Term...Drag/Iovino/Johnson 1214 Short Term...Drag/Iovino 1214 Long Term...Robertson Aviation...Drag/Iovino/Johnson/Robertson 1214 Marine...Drag/Iovino/Robertson 1214

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