Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 152129 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 429 PM EST Mon Jan 15 2018 .SYNOPSIS...
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High pressure centered in eastern Canada will lose its influence on our region tonight and Tuesday as a clipper low approaches from the Great Lakes region. The low will weaken to our north as another area of low pressure forms off the Mid-Atlantic coast Tuesday night. The coastal low is then expected to move northeast toward Cape Cod on Wednesday and the Canadian maritimes Wednesday night. High pressure builds in from the Southeastern states late in the week before progressing offshore this weekend. A cold front may move in from the west by next Monday.
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&& .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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An interesting ocean-effect snow event occurred (is occurring) today in Sussex County, DE, and adjacent areas. With reasonably strong onshore flow and cold waters, the snow machine cranked up late this morning and has continued with a general progression inland this afternoon. Visibilities have reduced locally in the snow to around 1- 3 miles, and some accumulation has occurred on grass and untreated surfaces this afternoon. However, impacts appear to be minimal, with temperatures around/above freezing and snow rates generally light. Nevertheless, retained PoPs in southern Delmarva through the afternoon. Though recent simulations of the HRRR have caught on to this event, guidance has overall not been impressive from a verification standpoint...so nowcasting is the main method of forecasting in the very near term. A couple of weak midlevel perturbations appear to be aiding in large-scale lift across the region this afternoon, and there also appears to be some enhanced convergence in the low levels just downstream of the perturbation moving through the northern Mid-Atlantic at this time. These processes will likely diminish by sunset or shortly thereafter, at which time the precipitation should be on the wane. Deep southwesterly flow becomes established across the region overnight, with a strong (135+ kt) 250-mb jet streak stretching from the Ohio/Tennessee Valleys northeastward into New England. A positively-tilted trough will move into the Great Lakes region overnight, with warm-air advection occurring across much of the Mid- Atlantic downstream of the trough. With pre-established onshore flow, the aforementioned warm advection and low-level cloud deck should prevent much cooling from occurring. Overnight low temperatures are generally about 5-8 degrees cooler than max temperatures today. Several perturbations embedded in the flow will aid in large-scale lift across the region, though the strongest source(s) will be in the low levels. Models are in good overall agreement regarding the development of snow overnight to our west with a general progression into the southern Poconos, Lehigh Valley, and northwest New Jersey late tonight (generally after around 3 am). Lift is generally weak, and moisture will not be plentiful to begin with. Model QPF through 12Z Tuesday is generally under a tenth of an inch in these areas (which maybe would muster an inch). However, this will be a long- duration light snow that begins during the overnight period (with totals reaching advisory levels by tomorrow evening). Thus, the decision was made to start a winter weather advisory for the aforementioned areas at 5 am. The main impacts from this storm will likely be after this period, however.
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&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...
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A long-duration light snow should continue through much of the day in the southern Poconos, Lehigh Valley, and northwest New Jersey. There will be little progress eastward of the precipitation during the day as the approaching trough pivots to a more neutral tilt during this period (and reloads in the southern stream). It looks like the next tier of counties/zones (western Chester County, PA, northeastward to Somerset County, NJ) will see precipitation begin by around noon, though temperatures will be near/above freezing. Nevertheless, given the onset of precipitation will be during this time and the antecedent cold, started the winter weather advisory for these areas at noon. At this point, it looks like the main impacts will be after the daylight hours, however. Indeed, temperatures may approach or exceed freezing even in Reading and Allentown during the afternoon, so some melting of snow (at least on treated surfaces) may occur during this time. Total QPF through 00Z Wednesday in the advisory area will be around a tenth to a quarter of an inch, which may generate snow up to an inch or so in the southeast portions of the advisory up to 2-3 inches in the southern Poconos (without considering melting). During the day, a coastal low will develop near or off the Carolina coast. This will reinforce onshore flow to the north, with strong isentropic lift as surface flow surrounding the surface low becomes increasingly rotational. This will likely begin to enhance precipitation rates late in the day (and should deepen the associated lift, especially as the low approaches), so the snow rates may increase near the evening commute...something to monitor closely. Temperatures have trended upward given the westward trend of the aforementioned coastal low (closer proximity to stronger warm/moist advection). Bumped temperatures up 3-5 degrees in most areas from inherited grids during the day tomorrow, though this is uncertain owing to the plethora of competing factors regarding surface warming and the large spread in the guidance for a 24-hour forecast (generally 7-12 degrees among statistical guidance). Tricky forecast, to say the least.
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&& .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
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Tuesday night and Wednesday...Primary surface low associated with a clipper system is expected to weaken over the Great Lakes region as it transfers its energy to a secondary low just off the Mid-Atlantic coast Tuesday evening. The coastal low will continue to deepen as it tracks northeastward to about 150 miles east of the NJ coast by early Wednesday morning and then close to Nantucket by late Wednesday afternoon. Today`s models have trended notably westward with the track of the secondary low (a common model bias we have seen with just about every coastal storm so far this winter). With support from today`s 12Z guidance, snowfall amounts with the latest forecast sent this afternoon are notably higher than previous iterations. Models are in reasonably good agreement regarding the synoptic pattern with some important differences on the mesoscale (i.e., strength and placement of frontogenetical lift to support mesoscale snow banding on the N-NW side of the developing coastal low). Accordingly, there is moderate to high confidence that snowfall totals for this event will increase across the CWA from SE to NW (with the heaviest snow falling along and N/W of the Fall Line where Winter Weather Advisories are in effect). There is also higher certainty that a period of moderate to locally heavy snow (snowfall rates 1"+ per hour) will fall north of I-78 Tuesday night (this is when low- level convergence becomes focused along a sharpening trough that extends N-NWwd from the coastal low). Snow will end quickly from west to east Wednesday morning, although it may linger around into the afternoon or early evening near the coast and across southern DE. The greatest uncertainty in the snowfall forecast is for east- central NJ. The majority of the models indicate the low will not organize quickly enough to be able to expand the deeper lift/ Atlantic moisture and heavier snowfall back westward into coastal NJ Tuesday night. However, if the abovementioned model trends continue, we will have to increase accumulations across Middlesex-Monmouth- Ocean and perhaps nearby counties. Wednesday night...Gusty NW winds behind the coastal low will advect colder air into the region. A chilly night is expected with forecast lows ranging from the single digits in the snow-covered southern Poconos to the upper teens in Phila and near the coast. We do not expect a second coastal low to impact the area Wednesday night with all models backing off on this scenario. Thursday through Sunday night...Broad high pressure builds across the southeastern CONUS late in the week before gradually moving off the Southeast coast next weekend. Our region will initially be situated in westerly flow on the northern periphery of the high that eventually becomes more southerly as the high starts to move offshore, resulting in a warm up that starts Friday and continues into the weekend with highs in the 50s possible by Sunday. Expect a dry stretch through the weekend. Monday...Low pressure is forecast to track to our northwest across the Great Lakes. Rain chances increase from west to east ahead of a cold front on Monday. Highs well into the 50s are forecast for Monday.
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&& .AVIATION /22Z MONDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...MVFR to low VFR CIGs will likely continue through the night, with a subtle trend downward with time. Light snow is forecast to develop in the Lehigh Valley and southern Poconos by daybreak. Sub-VFR VSBYs are likely at RDG/ABE at times once the light snow begins. Light winds are forecast through the period, which should be generally east or northeast, though speeds may become light enough to be somewhat variable after midnight. Medium confidence. Tuesday...Sub-VFR CIGs should continue through the day, with most precipitation (mostly light snow) remaining confined northwest of I- 95. Kept any precip mention out of Philly terminals, though there is some chance of precipitation occurring by late afternoon (which may be either rain or snow). Where snow occurs, sub-VFR VSBYs should be observed at times. Winds generally light and variable, with perhaps a southeast component becoming dominant by late in the day. Medium confidence. OUTLOOK... Tuesday night and Wednesday morning...A chance of light snow with low ceilings and reduced visibility possible. Wednesday afternoon through Saturday...Mainly VFR.
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&& .MARINE...
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Ocean-effect snow is occurring in portions of Delaware and the adjacent coastal waters of the Atlantic and Delaware Bay. These should diminish after dark, but there will likely be some locally reduced visibilities in the heavier snow bands late this afternoon. Small craft advisory conditions continue on the Atlantic waters with strong easterly flow and seas between 5 and 8 feet ongoing. The trend will be downward with winds and seas tonight, but models have been much too low with seas today (as usual with onshore flow events). Thus, expect at least the seas to be above criteria through most if not all of the night, though winds will likely diminish after dark. Did not make changes to the expiration time of the advisory at this time. For Delaware Bay, sub-advisory conditions are expected tonight. Tomorrow...Sub-advisory conditions are expected on all of the waters. OUTLOOK... Tuesday night...No marine headlines are anticipated. Wednesday through Thursday night...SCA conditions are expected with NW wind gusts of 25-30 kt expected in wake of a coastal storm. Light freezing spray is expected. Friday and Saturday...Winds and seas below SCA thresholds.
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&& .HYDROLOGY...
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An ice jam continues along the Delaware River at Trenton, and flooding has been reported adjacent to the river in Bucks and Mercer Counties. Several road closures are in place between Morrisville and Yardley in PA and along NJ State Highway 29 in Trenton. The ice is having an impact on the ability to accurately forecast the river level in the Trenton vicinity. Please use any forecast levels with caution. If you live near the Delaware River in the Trenton and Yardley vicinity, pay special attention to any additional information that is issued regarding the Delaware River. Additional ice jams may form next weekend when milder conditions arrive.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Winter Weather Advisory from noon Tuesday to 10 AM EST Wednesday for PAZ101-103-105. Winter Weather Advisory from 5 AM Tuesday to 6 AM EST Wednesday for PAZ054-055-060>062. NJ...Winter Weather Advisory from noon Tuesday to 10 AM EST Wednesday for NJZ009-010. Winter Weather Advisory from 5 AM Tuesday to 6 AM EST Wednesday for NJZ001-007-008. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EST Tuesday for ANZ450>455.
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&& $$ Synopsis...Klein Near Term...CMS Short Term...CMS Long Term...Klein Aviation...CMS/Klein Marine...CMS/Klein Hydrology...CMS/Klein

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