Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 281221 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 821 AM EDT Wed Sep 28 2016 .SYNOPSIS... An area of low pressure will move across the Ohio River Valley today and combine with another low across the central and southern Appalachians Thursday into Friday. This low will lift back across the Ohio River Valley and into the eastern Great Lakes region Saturday into Sunday and bring a frontal system across our area. High pressure is then expected to build across the northeast late Monday into Tuesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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820 am: fog is done. Ceilings 800 to 1800 ft spread southwest onto the NJ coast around 15z and eventually reach PHL around 20z. Cold air damming minimal today since the big push of cool moist boundary layer air doesn`t arrive most of our fcst area until after max heating today. Cold air damming does become a much bigger player tomorrow. Gusty e-ne wind 20-30 mph develops this afternoon. showers and isolated tstms overspread md e shore this morning and then a problematic timing issue on a further n and e spread this afternoon. A strengthening 30-35 kt jet streak at 925 mb will advect moisture off the Atlantic. There should be sufficient isentropic lift to produce showers across southern/western zones this afternoon. So, we expect showers to increase in coverage and intensity later this afternoon. Forecast soundings support a deep enough moist layer for at least patchy drizzle to develop late this afternoon, toward the NJ coast. High temperatures range from the mid/ upper 60s along and north of the I-80 corridor to mid/upper 70s across southeastern PA, southern NJ and Delmarva.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/... The upper low is forecast to slow down over the Midwest states tonight as it becomes cutoff. High pressure will continue to remain anchored to our north while the frontal boundary remains stalled to our south. The easterly low-level jet between these systems over the mid Atlantic will strengthen further to around 40 kt. Expect showers to expand northeastward across the entire area tonight. Expect for the NAM-based guidance, models continue to show a band of heavy rainfall developing very close to the western most zones of our CWA. The trend has been to shift the axis of higher QPF farther south and west, likely as models are more accurately capturing the magnitude of the cold-air damming pattern. Additionally, with the greatest low-level convergence situated at nose of the low-level jet forecast to be positioned to our west, the greatest risk for excessive rainfall may remain on the western side of the Chesapeake Bay. QPF amounts of 1-2 inches are forecast for eastern MD and out toward Chester/Berks Co. in eastern PA. We opted to not issue a Flood Watch at this juncture. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Unsettled weather continues to be forecast through the first half of the extended period, with generally fair weather for the second half of the period. For Thursday into Friday, an area of low pressure at the surface will be located to our southwest, while high pressure remains to our northwest. In between, a stationary boundary will be located just to our south. This will keep an onshore flow across our area. Meanwhile, a closed low aloft will be spinning to our west as well. This will keep a persistent southerly flow aloft across the area at the same time. With the southerly flow aloft, overrunning precipitation is expected to continue area within the onshore flow Thursday and Friday. It will not likely rain the entire time, but several periods of rain will be associated with several short waves/vorticity impulses rounding the closed low aloft to our west. By Saturday, and especially Sunday, the high to our north begins to move eastward, and the low to our west will begin lifting back toward the Great Lakes region. This will eventually pull the frontal boundary to our south across our area on Sunday. There will continue to be a chance of showers Saturday through Sunday as several short wave/vorticity impulses move across the area as the closed low aloft begins to break down and lift to the northeast. By Monday into Tuesday, the surface low and low aloft pass to our east, and drier air begins to move into the area as high pressure tries to move in from the north. The high may remain to our north, Tuesday which could produce an onshore flow which may end up leading to more cloud cover and cooler conditions for Tuesday. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Basically VFR to start with any MVFR Fog dissipating by 1330z. Cigs 800-1800 feet advect-develop southwestward onto the NJ coast in the 15z-18z time frame and spread west to PHl by 20 or 21z. drizzle may develop KACY late in the day. light n-ne wind early this morning becomes gusty northeast 20-28 kt this afternoon. Tonight...widespread cigs 500-1500 ft with vsby occasionally aob 3 mi in stratus, mist and fog with showers developing. ne wind with scattered gusts 20 kt. OUTLOOK... Thursday-Saturday...A prolonged period of MVFR or IFR conditions with periods of rain is expected to affect the TAF sites through the period. East to northeast winds are expected to gust 20-25 knots for much of the area. Saturday night-Sunday...Some improvement, possibly to VFR by Sunday. Showers remain possible.
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&& .MARINE... SCA goes into effect this afternoon for ANZ450/451 and the remainder of the waters this evening as NEly winds surge down the coast. A Gale Watch was previously in effect for ANZ450/451 for tonight but we do not anticipate gales to develop before Thursday. Winds of 15- 25 kt are expected with gusts to 30 kt possible tonight off the NJ coast. Seas in the coastal Atlantic waters will subsequently build to 5-7 ft tonight. OUTLOOK... Thursday...Gale Watch in effect for the northern half of the Atlantic coastal waters. Small Craft Advisory in effect elsewhere. Thursday night...Gale Watch in effect for ALL areas. Friday...Gale force winds may remain across the northern half of the Atlantic coastal waters. Small Craft Advisory level winds may remain elsewhere. Friday night-Saturday...Winds expected to drop below Small Craft Advisory levels, however, seas may remain above advisory levels through the period. Saturday night-Sunday...Winds and seas expected to drop below Small Craft Advisory levels. Rip Currents... There is a moderate risk for the formation of dangerous rip currents today. This is due to the combination of an increasing onshore wind and building waves in the surf zone. Conditions may warrant a high risk late in the day if winds and seas increase quicker than currently expected. The probably risk for the formation of dangerous rip currents is at least moderate to perhaps high on Thursday. && .TIDES/COASTAL FLOODING...
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A persistent onshore flow is expected for the rest of the week into the weekend. The strongest winds are expected Thursday through Friday. Tides levels are forecast to increase through this period, and we could reach minor coastal flood levels.
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&& .CLIMATE...
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Will add extensive climate information late this morning as time permits.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Watch from Thursday evening through late Thursday night for ANZ430-431-452>455. Small Craft Advisory from 6 PM this evening to 6 PM EDT Thursday for ANZ430-431-452>455. Gale Watch from Thursday morning through late Thursday night for ANZ450-451. Small Craft Advisory from 1 PM this afternoon to 6 AM EDT Thursday for ANZ450-451. && $$ Synopsis...Robertson Near Term...Drag/Klein 820A Short Term...Klein Long Term...Robertson Aviation...Drag/Klein/Robertson 820A Marine...Drag/Klein/Robertson Tides/Coastal Flooding... Climate...820A

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