Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 132056 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 356 PM EST Fri Jan 13 2017 .SYNOPSIS...
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Large high pressure will build just to our north tonight, then weaken Saturday into Saturday night as it moves offshore. An area of low pressure will move to our south Saturday night, then high pressure will build back to our north Sunday and influence the weather through Monday. Then, low pressure is forecast to move from the southern plains to the Great Lakes by Tuesday. This will bring a warm front across the area Monday night into Tuesday morning. A warm southwest flow will likely develop across the east coast for the middle to end of the week.
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&& .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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Cold front has progressed well south of the area today with cold high pressure settling into the Great Lakes and the Northeast. Temperatures have been nearly steady today as low-level cold air advection upstream of the front has overtaken the region. Large- scale environment consists of quasi-zonal upper-level flow poleward of a slowly retrogressing subtropical high in the eastern Gulf of Mexico. Several small-scale perturbations embedded in the near-zonal flow will progress across the Mid-Atlantic States through the weekend. The first will move through the region tonight, bringing increased mid and high clouds to the area. Temperatures will steadily cool this evening before the increased cloud cover slows the downward trend late tonight. The damage will be done, so to speak, as temperatures should be near or below freezing across the CWA (from upper teens in the southern Poconos to around freezing at the Delaware beaches). Winds will continue to diminish this evening as the surface pressure gradient relaxes.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
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The next perturbation in the near-zonal upper-level flow moves across the region tomorrow. Though the perturbation is rather weak, it will be sufficiently strong to produce cyclogenesis off the Carolina Coast by 00Z Sunday. Large-scale lift in association with the vorticity maximum will generate precipitation in the Ohio Valley tonight spreading into the Mid-Atlantic during the day Saturday. Owing to the overall weak lift/dynamics in place, precipitation should not be heavy with this system, and the strongest lift will be south and west of the area. Models have trended somewhat drier and warmer with the system in the past 24 hours, with little if any QPF generated north of I-78. Model soundings suggest the profile will be cold enough for snow as precipitation moves in by early afternoon. However, surface temperatures will creep above freezing south of the Mason-Dixon Line, comfortably so south of Dover, so any precipitation here will likely transition to rain by late afternoon. Given the light QPF, marginal temperatures, and antecedent warmth this week, not expecting anywhere to receive more than an inch of snowfall, with the highest axis somewhere between I-95 the central Eastern Shore of Maryland and adjacent Delaware. Meanwhile, vertical profiles will also be drying out in the midlevels as the perturbation passes to the east of the region by late afternoon. This suggests snowfall will be more difficult to generate as the event wears on, and precipitation may turn more to freezing drizzle or light freezing rain where surface temperatures remain cold enough. However, two caveats lead to me not issuing any headlines at this time. The first is that such a change in the vertical profile of moisture occurs when lift progresses away from the region, so the transition to more of a wintry mix is not high confidence (and may not amount to much more than drizzle if it does occur). The second is that where QPF is slightly higher (in MD/DE), temperatures look to be near or above freezing. Although minor winter weather impacts cannot be ruled out tomorrow, there is simply not enough confidence to warrant issuance of any winter weather products at this time. Stay tuned to the latest forecasts, as this may change. One final note: Despite the lack of winter weather products issued at this time, this is certainly not to say that no winter-weather related impacts will occur. With light snow and potentially freezing drizzle/rain in portions of the area tomorrow, impacts are most certainly possible!
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&& .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
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Low pres will move s of the area Sat night. Temps will be cold enough from the I-95 corridor n and w for some wintry precip, but overall qpf and pops are low. Pops have been lowered with this fcst. Will have to keep an eye on this but no headlines will be issued at this time. The low moves out by erly Sun and high pres builds to the n which will bring dry wx for at least the first part of Mon. Then, low pres will develop over the srn plain and will move newd twd the Grtlks thru Tue. The wmfnt assocd with this low will cross the area Mon night. Precip will develop as erly as Mon aftn (GFS) or hold off until Mon night or Tue (ECMWF and CMC). For now, will keep Mon dry. Pops increase Mon night into Tue. There is genly good agreement wrt this aspect, though the mdls disagree on when and where the precip is most likely to occur. The GFS is also faster in moving the precip out. The cdfnt crosses the area erly Wed in the GFS and then Wed into Thu is dry. In the ECMWF, a secondary low forms on the front, it is, therefore, slower to cross and Wednesday is wet, before fropa Wed night and a dry Thu. The CMC is somewhere in the middle, slower (much) than the GFS but not as slow as the ECMWF. So, for now, will hold some pops thru Wed and see how this develops. Needless to say this is a lower than average confidence fcst. The good news is that temps will be quite warm thru most of the week (after Mon)...well into the 50s in many areas. Therefore, any precip that falls Tue into Wed will be plain rain everywhere. Temps look to be 10-15 degrees above nrml for most of next week!!
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&& .AVIATION /20Z FRIDAY THROUGH WEDNESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Northwest winds are beginning to diminish across the area as the cold front moves away from the region. However, cloud cover will increase tonight, with ceilings lowering through tomorrow as the next system moves through the region. Light precipitation is expected to move into KILG and KMIV around 18Z Saturday with KPHL/KPNE/KTTN/KACY/KRDG affected shortly thereafter. For now, thinking is that precipitation will mostly start as light snow but with time, a light wintry mix is possible. MVFR/IFR ceilings/visibilities expected as precipitation occurs. Most of the precipitation should end by late evening, with some improvement to ceilings/visibilities expected thereafter. Winds should be at or below 10 kts by the late evening hours, transitioning from north tonight to east tomorrow morning to southerly by late in the day. OUTLOOK... Sat night...MVFR or IFR conditions Sat night. Wintry mixture of precipitation possible, with mainly snow/sleet for ABE/RDG/TTN, and snow/sleet/freezing rain possible for the remainder of the TAF sites. Precip should be light and pops/qpf not all that high. Low to moderate confidence. Sun-Sun night...Conds improving to VFR. Moderate to high confidence. Mon-Wed... VFR early, lowering to MVFR, then to IFR late Mon night into Tuesday. Chance of precipitation late Mon into Tue. A wintry mix possible, mainly n and w erly Tue. Timing diffs develop after that, with precip over in one mdl by Wed mrng and continuing thru Wed in another. So its psbl conds cud improve on Wed or not. Mdt to high confidence Mon-Tue, low on Wed.
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&& .MARINE...
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Advisory conditions continue in the coastal waters this afternoon but are becoming more marginal with time. Conditions should become sub-advisory levels this evening, and the Small Craft Advisory will likely be allowed to expire as scheduled. Winds will transition to light easterly flow by tomorrow afternoon. A chance for light mixed precipitation is expected during the afternoon and evening tomorrow. OUTLOOK... Sat-Tue..Sub SCA conds are expected, although winds may gust 15-20 knots at times on Sun. Wed...SCA conds psbl as swly flow increases in advance of cdfnt. Wind cud gust to 25 kts and seas could reach 6 ft.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EST this evening for ANZ450>455. && $$ Synopsis...Nierenberg Near Term...CMS Short Term...CMS Long Term...Nierenberg Aviation...CMS/Nierenberg Marine...CMS/Nierenberg

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