Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 151410 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 910 AM EST Sun Jan 15 2017 .SYNOPSIS... High pressure over the Midwest will build eastward into the region today. The high will settle overhead tonight and Monday before moving offshore Monday night. An area of low pressure over the Great Plains will track northeastward into the Great Lakes region on Tuesday. Meanwhile, a warm front with this system will lift northward through the northern Mid Atlantic on Tuesday followed by the passage of a cold front late on Wednesday. High pressure generally looks to be in control Thursday through Saturday. However, a disturbance may pass close by to our south Friday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Drier air was moving into the area. High pressure west of the Great Lakes will move eastward today. Expect a mostly sunny sky today. Temperatures will climb into the upper 30s across the north and low/mid 40s elsewhere. These readings are a few degrees above normal for this time of year. Winds will be mostly North to Northwest at 5 to 10 mph. No appreciable changes were made to the fcst this mrng, with quiet wx in place.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... High pressure will settle across the area tonight. Mostly clear skies will be across most of the area but some cloudiness will be across Delmarva. Low temperatures tonight will be close to normal for mid January with teens up across the southern Poconos and northern NJ and low/mid 20s elsewhere. && .LONG TERM /MONDAY THROUGH SATURDAY/... The period will start off quietly with a 1032 mb surface high centered over the region Monday morning. The high will move offshore Monday afternoon and night while an area of low pressure tracks northeastward from the Central Plains to the Midwest. The low, which is responsible for the major freezing rain event across the central Plains, is then forecast to cut well to our northwest across the Great Lakes, putting the Mid Atlantic on the warm side of the system. This is looking like mostly a rain event for the Tuesday-Wednesday time frame except for northern areas at the onset. The majority of guidance indicate precipitation along a warm front starting out on Tuesday as freezing rain in northeastern PA and northwestern NJ with a shallow (1-2 kft deep) layer of sub-freezing air trapped beneath a well-defined warm nose aloft. Models favor a short duration of freezing rain (around 6 hours in the southern Poconos and even less farther south) as temperatures rise above freezing rather quickly with the warm fropa. However, as discussed continually in recent months, models often scour out the low-level cold air across the higher terrain N/W of the Fall Line too quickly in these setups so the duration of icing may be longer. Timing, in terms of when the lower clouds arrives Monday night and when the precipitation moves in, will help determine the extent of the freezing rain. The denser cloud cover will limit help prevent temps from dropping much Monday night across most of the forecast area. RH fields from the NAM/GFS/ECMWF show the northern edge of the cloud cover slow to progress north of I-78. This setup would allow the airmass to radiate effectively (especially in the evening) and for temperature to fall into the mid or even lower 20s across northeastern PA and northwestern NJ. Conversely, lows in these northern zones may only be in the upper 20s or lower 30s if the clouds arrive earlier in the night. Additionally, the earlier the precipitation arrives on Tuesday, the greater the threat exists for freezing rain. The faster solutions (e.g., 00Z ECMWF ops) brings in the precipitation overnight while the slower solutions (e.g., 00Z NAM) hold off the onset until afternoon. Spread in the GEFS and SREF ensembles provide additional evidence for timing uncertainty. The timing details will hopefully become clearer over the next 24 hours. Nonetheless, there is high confidence that areas north of I-80 (Carbon and Monroe Counties in PA and Sussex County, NJ) see at least some freezing rain. Our latest ice accumulation forecast graphic shows light as far south as the I-78 corridor and as far east as I-287. This ice forecast can be considered a bit bullish as it lies closer to the 90th percentile of the latest PWPF ice guidance from WPC, but after taking into account model bias and local terrain, this approach seemed warranted. PoPs are highest Tuesday afternoon and night. Rain could become widespread for a period as a secondary low developing along the system`s triple point passes through the region. A cold front most likely moves through sometime late Tuesday night or Wednesday morning, which would give us an opportunity to dry out during the day Wednesday. The 00Z GFS is much slower to develop the low and is subsequently later with the cold fropa, resulting in a soggy Wednesday. Although the GFS cannot be ruled out at this point, the official forecast, which was weighted more toward WPC/SuperBlend guidance, is more optimistic for Wednesday. The cold air behind Wednesday`s cold front mainly has Pacific origins and thus is not very cold for mid winter standards. Therefore, temperatures should remain above normal during the second half of the week and into next weekend. Thursday should be dry with high pressure builds into the area. Confidence is not as high for a dry Friday with a disturbance organizing over the Gulf of Mexico and possibly passing close by to our south. High pressure looks to regain control of our weather pattern heading into next weekend. && .AVIATION /14Z SUNDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. High pressure will build over the area today then move to the east/northeast by early Monday. The last of the MVFR fog has all but dissipated and VFR conds are expected for the balance of the day. Wind will be north to northwest at 8-12 knots today and light tonight. There may an increase in high clouds late Sun night and Monday ahead of the next system. OUTLOOK... Monday and Monday evening...VFR. Monday late night through Tuesday night...Lowering to MVFR and then IFR as precip moves in from the west. There is considerable uncertainty regarding the onset timing of precip and when CIGs/VSBYs lower...It could be as early as late Monday night or as late as Tuesday afternoon. Expect mainly rain. However, freezing rain may occur at the onset late Monday night or Tuesday at KABE and perhaps in the vicinity of RDG. Wednesday...Sub-VFR conditions and SHRA in the morning should improve later in the morning or afternoon. Wednesday night and Thursday...Predominately VFR.
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&& .MARINE... High pressure will build across the waters today before moving away Monday. A gentle North to Northwest flow is expected today and this evening before becoming Northeast later tonight. Seas on the ocean should remain in the 2 to 3 ft range. A little early morning low clouds and fog then fair weather is expected today and tonight. OUTLOOK... Monday through Thursday...Winds will be less than 20 kt. Seas may build close to 4 or 5 ft late Wednesday afternoon or night as low pressure moves off the coast and strengthens offshore. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Klein Near Term...Nierenberg/O`Hara Short Term...O`Hara Long Term...Klein Aviation...Klein/Nierenberg/O`Hara Marine...Klein/O`Hara

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