Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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309 FXUS61 KPHI 281937 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 337 PM EDT Wed Sep 28 2016 .SYNOPSIS... An area of low pressure will move across the Ohio Valley and combine with another low across the central and southern Appalachians Thursday into Friday. A cold front will cross the region around Sunday. High pressure is then expected to build to our north into early next week. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... 1040 am: Ceilings 800 to 1800 ft spreading southwest onto the NJ coast now will eventually reach PHL around 19 or 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 by this afternoon. Showers and isolated embedded tstms MD e shore and w DE into extreme sw NJ with diminishing intensity so the 1040 AM pop adjustment was to account for the mid level rain dying out midday/early this afternoon. while was not exactly sure how to handle this CHES Bay organized system that dumped 3-4.5" on the other side of the Bay and 1-2" parts of the MD e shore, it is on the wane. We have seen the SPC expanded marginal risk into MD E shore but no action at this time for enhanced wording. Will reevaluate with 12z GFS guidance. NAM has not handled this system very well so am inclined to favor GGEM/GFS/EC/UK/WPC blend. 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. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM 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 NIGHT THROUGH WEDNESDAY/... Unsettled weather continues for the first portion of the extended fcst, with some improvement by the end. A large area of upr lvl low pres will be located to our s and w at the start of the extendd pd. This low will spin in place then gradually retrograde to the Grtlks and sern Canada by the weekend. At the sfc, an area of low pres will be located over the midwest, with high pres to the ne over New Eng and sern Canada. The sfc features will keep a persistent moist nely flow over the area at the sfc with swly flow aloft. As the sfc low moves by to the nw, one pd of rain is expected and as several other s/wvs rotate thru the upr low several other pds will occur. As is usually the case with upr lows, it will not rain the whole time. It will be cloudy and dreary with pds of rain. The mdls differ on which times will be rainy and where those rainy pds will occur. For example on Friday, the GFS is mainly dry, while the ECMWF is fairly wet and the NAM somewhere in the middle. Have tried to make adjustments to pops where there was a decent signal in the mdls, but these times were limited and overall confidence on which pds will be rainy are low. By Sat and Sun, the upr low moves nwd into the Grtlks area and begins to weaken and fill. Overall precip chcs shud decrease. However, guid today is wetter than it was yda and once again it will depend on individual w/wvs. Sat shud be wetter than Sun with precip chcs decreasing with each passing day. Some guid now even wants to linger precip chcs into Mon until the upr low finally passes to our e and high pres builds in behind it. By Wednesday, all eyes turn to the tropics and any potential impacts from what is now Tropical Storm Matthew later in the week. Please see NHC advisories for updates. && .AVIATION /19Z WEDNESDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. This afternoon...VFR conditions to start. Cigs 1000-2200 feet advect- develop southwestward fm the NJ now and spread west to PHl by 18z. showers vcnty KILG and KMIV may reach PHL but these showers are tending to dry out as they move northeast. drizzle may develop KACY late in the day. light n-ne wind early this morning becomes gusty northeast 20-28 kt by this afternoon. Tonight...widespread cigs 700-1700 ft with vsby occasionally aob 3 mi in stratus, mist and fog with showers. ne wind with scattered gusts 20 kt. OUTLOOK... Thu night-Fri...A prolonged period of MVFR or IFR conditions with pds of RA expected to affect the TAF sites through the period. E to Ne winds are expected to gust 20-25 knots for much of the area, especially KMIV and KACY. Moderate confidence. Fri night-Sun...Some improvement, possibly to VFR by Sun. SHRA remain psbl. Moderate confidence. && .MARINE... SCA in effect for the NJ 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 through tonight along the NJ coast. Seas in the coastal Atlantic waters will subsequently build to 5-7 ft by tonight. SCA continues with a 6 PM start for De Bay and the Atlantic DE waters though could see a sooner start. OUTLOOK... Thursday night...Gale warning in effect early, will likely need to be extended. Fri...Gale wind may remain across the northern half of the coastal waters. SCA may be needed elsewhere. Fri night-Sat...Wind expected to drop below SCA, however, seas will likely remain above 5 feet thru the pd. Sat night-Sun...Wind and seas expected to drop below SCA levels and no marine flags anticipated. Rip Currents... There continues a moderate risk for the formation of dangerous rip currents today as per a recheck of the ene sustained wind ~18kt and dominant 4 second short period swell. Thursday: The probability for the formation of dangerous rip currents is at least moderate to perhaps high. && .TIDES/COASTAL FLOODING... 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. && .CLIMATE... Will add extensive climate information late this morning or this afternoon as time permits. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...Flash Flood Watch through Thursday afternoon for MDZ008-012- 015-019-020. MARINE...Gale Warning from 4 AM Thursday to 2 AM EDT Friday for ANZ430- 431-452>455. Gale Warning from 4 AM Thursday to 6 AM EDT Friday for ANZ450- 451. && $$ Synopsis...Drag Near Term...Drag Short Term...Drag Long Term...Nierenberg Aviation...Drag/Nierenberg Marine...Drag/Nierenberg Tides/Coastal Flooding... Climate...

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