Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 251441 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1041 AM EDT Sun Sep 25 2016 .SYNOPSIS... High pressure centered north of the Great Lakes will build southward along the eastern U.S. seaboard today and tonight. The high then moves off the New England coast Monday as a cold front approaches from the west. The front is expected to pass through the area Monday night and then stall off the coast Tuesday. A wave of low pressure may develop to our south and move up the coast sometime during the middle to late week. The low eventually departs to our northeast and high pressure builds in either late in the week or next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... No changes needed to the forecast today. High pressure will continue to build across the northeast states, keeping fair conditions across the area. RH values may fall into the upper 20s to low 30 this afternoon. However, winds will be light with the high pressure system in place over the region. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... High pressure will continued to be anchored across the region tonight. This will allow for one more night of clear skies and efficient radiational cooling. Temperatures should fall into the 40`s overnight for many of us. However, some of the higher elevations of the southern Poconos and NW NJ may see lows in the upper 30`s. With light winds, some patchy frost looks possibile. However, 850 mb temperatures look several degrees warmer than ideal coupled with dry ground conditions should limit the coverage of any frost to patchy. && .LONG TERM /MONDAY THROUGH SATURDAY/... On Monday, high pressure centered over New England in the morning will retreat offshore in the afternoon while a cold front advances eastward toward the western side of the Appalachians. Low-level southerly flow in between these two systems will develop along the eastern seaboard. Forecast high temperatures on Monday are only a degree or two warmer than today since ensuing WAA pattern over the region looks to be modest. Mid and high clouds are expected to stream in ahead of the front during the day, resulting in periods of clouds and sun. The cold front is expected to move through the area Monday night. The juxtaposition of a strong (40 kt S-SWly) low-level jet, a well-defined atmospheric river (PWAT values peak between 1.75-2.00 inches) and low-level theta-e ridge provides support of an organized band of showers ahead of the front Monday night and Tuesday morning. There will be additional forcing for ascent higher aloft that arrives when the forecast area becomes positioned in the right-entrance region of a 100+ kt upper-level jet streak that is located on the eastern side of the Great Lakes low. With minimal spread noted in the guidance for the fropa timing, felt it was time to raise PoPs to categorical. The progressive nature of the front will limit our rainfall potential. QPF amounts range between one-half and one inch, but there could be locally heavier totals if deeper convection (embedded thunderstorms) develop in the warm sector. We`ll take any rain we can get at this point, especially across the north, to help narrow the rainfall deficit...even if it`s by a small fraction. The front may slow down as it moves off the coast Tuesday in response to the primary low that hangs back to our northwest becoming cutoff over the Great Lakes region. Therefore, showers will likely be slow to exit the coastal plain and may even linger well into the afternoon or evening near the coast and lower Delmarva region. There continues to be sizable uncertainty in the forecast for the second half of the week with models having a difficult time capturing the evolution of the cutoff low over the eastern CONUS and secondary cyclogenesis along the East Coast. There is increasing support for a coastal low to form to our south Wednesday and slowly track up the mid-Atlantic coast into the end of the week. This would result in two or more days of below normal temperatures and rainy conditions across much of the region. The last three runs of the ECMWF (including the operational run and about one-third of its ensemble members) and now the UKMET operational keeps the unsettled pattern right into next weekend with the low stalling nearby to our south for several more days. Trended toward a more pessimistic (cool/damp) scenario for the second half of the week but changes to the previous forecast were conservative given the relatively limited skill inherent with forecasting cutoff lows (i.e., nothing is set in stone yet). && .AVIATION /14Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR today and tonight with northerly winds of 10 knots or less becoming lighter and variable this evening. OUTLOOK... Monday...VFR. Southerly winds around 10 kt. Gusts to 20 kt develop late in day. Monday night...Lowering to MVFR from west to east as showers move in ahead of a cold front. IFR possible late in heavier showers. A 40-kt southwesterly low-level jet moves overhead, but model forecast soundings do not indicate it meeting criteria to warrant LLWS in the TAFs. Tuesday...Showers and MVFR restrictions to start the day until the front moves through. Fropa will be marked by a wind shift from southerly to westerly. Conditions will first improve to VFR in western terminals (e.g., ABE/RDG) sometime in the morning and then eastern terminals (e.g., MIV/ACY) during the afternoon. Tuesday night and Wednesday....Predominately VFR. Wednesday night and Thursday...Sub-VFR conditions and showers may return but forecast confidence is lower than normal. && .MARINE... Small Craft Advisory will continue as seas remain 4-6 feet across the Atlantic waters. WaveWatch guidance shows a slow decrease of the waveheights today to around three feet tonight. Winds will be northerly around 10-15 knots at times, then become lighter and variable tonight. OUTLOOK... Monday...No marine headlines anticipated. Monday night and Tuesday morning...SCA possible. Southerly winds ahead of a cold front increase to around 20 kt with gusts near 25 kt. Seas build to 4-5 ft in our coastal waters. Tuesday afternoon through Thursday...Winds and seas below SCA threshold. RIP CURRENTS...A long period swell coupled with 3-5 ft seas are both still factors that the rip current risk will be best represented by the moderate risk category today. The probable risk for the formation of dangerous rip currents for Monday is low. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until noon EDT today for ANZ450>455. && $$ Synopsis...Klein Near Term...Gaines/Robertson Short Term...Gaines Long Term...Klein Aviation...Gaines/Klein Marine...Gaines/Klein

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