Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 210759 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 259 AM EST Tue Feb 21 2017 .SYNOPSIS... High pressure will build over the Mid Atlantic states through tonight, before shifting offshore. A warm front will slide through our region Wednesday morning. A cold front may edge down into eastern Pennsylvania and New Jersey early Friday before retreating northward Friday night. Low pressure over the Great Lakes region Saturday morning will send a cold front off the east coast Saturday evening, as the storm heads across Quebec. High pressure follows across the mid Atlantic states Sunday night. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Mid-level ridge traversing our region today begins to attenuate, in response to shortwave energy approaching the Appalachians late this afternoon. This will lead to an increasingly west flow aloft, allowing mid and upper level moisture to overspread the area. Expect increasing cloudiness, but the sun will manage to make it through. As high pressure moves to the south coast of New England, winds will veer from easterly this morning to southerly by this afternoon. As modest warm air advection aloft gets underway, an inversion will develop around 900 hPa, which will limit mixing. In addition, the low-level air mass will moisten, due to the increasing maritime influence. Both of these factors, in addition to increased clouds, will limit the maximum temperature potential. As such, went more in line w/ guidance, generally a MAV/MET blend. Also checked this against EC 18Z 2-meter temperatures. Grids reflect warmer temperatures in the PHL-TTN urban corridor and interior Delmarva, trending down southeastward to the coast, particularly the barrier islands. Overall, temperatures still running 5 to 10 degrees above normal. Winds will gust 10 to 15 knots at times late this morning into the afternoon.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
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A diffuse shortwave will traverse our region overnight, while its associated cold front essentially washes out (frontolysis) across the Appalachians. A southerly flow will be maintained across the area, but model cross sections indicate low-level dry air remaining, with saturation confined closer to I-80. Aloft, weak mid-level positive vorticity advection occurs, within the left-rear quadrant of the upper-level jet, along with some weak isentropic lift. The lifting mechanisms are generally diffuse and weak, so the added moisture and terrain forcing, generally northward of I-78, leads to the greatest chance for light precipitation across this area. Thus, highest PoPs (chance) were focussed closer to I-80, with lower (slight chance) Pops south to I-78. Low temperatures around 10 degrees above average across our region.
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&& .LONG TERM /WEDNESDAY THROUGH MONDAY/... 500 MB: A ridge persists over the mid Atlantic states through most of Saturday before a strong short wave departs the Great Lakes for eastern Canada later in the weekend. The ridge reemerges a little further south, off the mid Altantic coast, early next week. Temperatures: As our forecast area proceeds to a 1st or 2nd warmest February on record and a top 10 warmest winter in the period of record dating back to the late 19th century, calendar day averages Wednesday are expected to be 10 to 15 degrees above normal, 17 to 22 degrees above normal Thursday, nearly 25 degrees above normal again Friday (see yesterdays average temperatures in our area), around 15 degrees above normal Saturday depending on the cold air advection subsequent to the late Saturday CFP, still 2 to 7 degrees above normal Sunday and Monday. Forecast basis: Unless otherwise noted, a 50 50 blend of the 12z/20 GFS/NAM MOS guidance Tuesday night; adjusted warmer guidance Wednesday per the 4 Pm max`s on GGEM 2m temp and NAM boundary layer temp; warmer adjusted mav/met guidance, then thereafter close to the 15z/20 WPC guidance Thursday - Monday. The continuing daily cool bias in the daytime max temperature guidance of the last several days is probably related to the lack of snow cover and the persistent and climatologically anomalously warm jet stream pattern. The dailies including any possible significant insight... Tuesday night...Chance of light showers during the night ahead of a warm front. Low probability there could be a touch of freezing rain highest terrain above 1500 feet north of I-80. This ice risk isn`t in the HWO or zones since confidence is below average and guidance favors above freezing min temps during any rain. Confidence: average or below average regarding the occurrence of any rain. Wednesday...Considerable morning cloudiness, then becoming partly sunny and warmer during the afternoon. Near record warmth still (vulnerable records) possible though some of the cooler 2m temp and MOS guidance does raise a cautionary flag. Our 330 PM forecast temps may end up a degree or warmer than now predicted. This fcst is above all available guidance and reflects the recent cool bias in max temps the past several days... the warm regime with a 550DM 1000-500mb thickness. Max temps probably occur around 330 or 4 PM due to the warming southwest flow and boundary layer temps reaching 13C at 21z. southwest wind but much cooler coastal seabreezes. Confidence: average..ie less than yesterday. Wednesday night...Fair (thin high clouds). patchy fog possible in a few countryside locations. Thursday...Mixed clouds and sun. Near record warmth still possible though some of the cooler 2m temp and MOS guidance does raise a cautionary flag. Models may be rain contaminated. So there is uncertainty. Much cooler coastal sea breezes embedded within the southwest gradient wind. BL temp right now only fcst to 14C PHl but 18C Delmarva...all this because of modeled showers which if occurred would reduce max temps to the 60s. Confidence: average...ie less than yesterday and throwing caution to the wind because of a shower risk. Friday...early morning stratus/fog possible along a cool front that may settled into the Nj and PA portion of our area? Maybe a shower north of the boundary during the afternoon or maybe the boundary doesn`t make it down into our forecast area? otherwise considerable clouds. Wind probably southeast turning south to southwest. Confidence: below average on the scenario. Saturday...Models slowing down and warming. It should shower for a few hours along and head of the cold front with a chance for a gusty thunderstorm. Wind fields pretty strong, weak cape but unstable with swi near -2 and tt 50. Dewpoints probably close to 60 in a sliver ahead of the cold front. Could have a squall line and embedded isolated severe thunderstorms if the strong wind field instability maintains. Confidence: average. Sunday...Becoming mostly sunny and cooler but still above normal temps as the sfc low moves northeast into Canada taking the primary chunk of colder air east northeast with it, making it difficult to import too much of the cold air here, from the midwest. Still it will be cold. Breezy. Westerly Gusts to 25 or 30 mph. Confidence: average. Monday...increasing clouds ahead of the next Midwest toward the Great Lakes low pressure system. WAA precipitation may arrive late in the day or at night. Confidence: below average. && .AVIATION /08Z TUESDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR. Easterly winds early this morning will veer to the south-southeast from late morning into the afternoon, generally less than 10 knots with some occasional gusts to 15 knots. Tonight...MVFR ceilings may develop toward daybreak at ABE and RDG, with highest confidence at ABE. As a result, we have inserted a TEMPO group at ABE to reflect this. Otherwise, VFR. OVC100 ceilings across the area. At TTN, PNE, PHL, and ILG, SCT050 ceilings possible toward day break. Winds will maintain a southerly direction, generally less than 10 knots. Outlook... Tuesday night...VFR with a small chance of shower. light south wind. Wednesday and Thursday...Mostly VFR conditions with ocnl cigs aoa 5000 ft. There could be a little MVFR fog late each night. south to southwest wind . Friday...VFR but north of a possible quasi stnry front it might become MVFR in a shower. east southeast wind ahead of the warm front and southerly gusty 20 kt south of it and this front should edge north during the afternoon and at night. Saturday...VFR cigs with a period MVFR conds and gusty southwest winds to 30 kt in showers and a possible tstm ahead of a CFP.
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&& .MARINE...
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Today and tonight...High pressure will continue to move slowly eastward over the northwest Atlantic. Winds initially out of the east this morning will veer to the southeast this afternoon, and continue veering to the south tonight. On the ocean waters, winds less than 20 knots, and seas generally 1-3 feet through the period. On the Delware Bay, southeast wind gusts around 20 knots along with seas of 1-3 feet are expected this afternoon into this evening. Sub-SCA conditions across the coastal waters through the period. Outlook... Tuesday night through Friday...Winds and seas should stay below SCA criteria. A gradual shift from southeasterly to southwesterly is expected Tuesday night. By Thursday night, winds could be light and variable shifting to southeasterly on Friday in response to a weak cool front briefly settling southward into our area. Saturday...SCA south to southwest wind conditions likely ahead of a late day or nighttime cold frontal passage.
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&& .CLIMATE... Records for later this coming week. Most of these records will probably remain intact as is, but there appears to be opportunity for approaching within 2 degrees at PHL/ABE/TTN/MPO on 1 to 3 days. Wednesday 2/22 Thursday 2/23 Friday 2/24 ACY 68-1991 72-1985 75-1985 PHL 68-1997/1974 75-1874 74-1985 ILG 69-1997/1974 72-1985 78-1985 ABE 68-1974 71-1985 76-1985 TTN 66-1997 74-1874 74-1985 GED 72-1997 no data no data RDG 70-1974 72/1932/1922 77-1985 MPO 56-1997 60-1977 60-1984 && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Drag Near Term...Franck Short Term...Franck Long Term...Drag Aviation...Drag/Franck Marine...Drag/Franck Climate...Staff

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