Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 151313 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 913 AM EDT Sun Apr 15 2018 .SYNOPSIS... Low pressure will traverse a stationary front located south of the area from the Ohio Valley today through the Middle Atlantic on Monday, when its accompanying cold front will sweep through the region. This will be followed by several weak troughs of low pressure Monday night and Tuesday, until another area of low pressure organizing over the Midwest and its associated fronts move through the area on Thursday, and slowly offshore of New England on Friday. Canadian high pressure will move into the area for next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 845 am update: Backdoor cold front is on the southern edge of the CWA at this time, with temperatures in Georgetown dropping into the 40s while Cambridge, MD, remains above 60. Several adjustments to the forecast have been made. Temperatures and dew points were running too high north of the front, so I lowered these using a blend of colder hi-res guidance (the HRRR and NAM Nest seem to be doing particularly well). These adjustments also resulted in a better depiction of frontal placement and expected evolution through the day. Lowered PoPs (for measurable precipitation) this morning but kept them fairly similar to what they were this afternoon based on a clear ramp-up in large-scale lift and model-generated precipitation this afternoon. Based on increased reports of fog/drizzle/mist from spotters and mesonet observations, kept mention of these in the grids everywhere today. Moreover, given the verification of hi-res guidance (versus warmer/coarser guidance) this morning, I maintain concern of a freezing drizzle/rain threat in the southern Poconos this afternoon. This will be monitored closely. Though any glazing will likely be on elevated surfaces, I cannot completely rule out some impacts here. Should this be observed/expected to occur, a short-fused advisory or special weather statement may be required. Previous discussion... After slowing down, it looks like the front is making steady fast progress once again as seen on the KDOX radar. At this rate, it could be south of DE by sunrise. Dry air so far has been winning out behind the front. Aside from fog reported at Mount Pocono (likely terrain enhanced fog), and isolated reports of drizzle in northeastern NJ and right along the NJ shore, we haven`t seen widespread drizzle, fog, or rain across the area. High resolution models depict this changing by 15Z or so with light rain/drizzle across the coastal Plains and 95 corridor. I am suspicious of this given how poorly many of the high resolution models handled the dry air further north. I think it is more likely we will see mostly dry conditions through the day until we get to late this afternoon or this evening. As mentioned by the previous shift, there is a chance if enough precipitation develops in the southern Poconos and NW NJ, that we could have freezing rain develop, primarily along the ridges. It would be dependent on wet bulb effects to get the temperatures to actually fall through the day time hours. At this point, I don`t have enough confidence on precipitation developing to issue a winter weather advisory, but we will continue to monitor this closely. As for temperatures, we could see in many areas, temperatures 40 degrees or more below yesterdays high temperatures! The one wild card is in southern DE and adjacent parts of MD. Eventually the front should stall once again, if it stalls far enough north, those areas could see highs near 60. However, given the recent trends of the front, I may need to adjust temperatures down further, as so far the front shows no signs of slowing down. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... 845 am update: Made some adjustments to the overnight forecast. Included patchy fog mention most of the night around/north of the Mason-Dixon Line given increased confidence on the placement of the front`s surge back northward overnight. Included chances of thunder generally south of the I-195/I-276 corridors starting this evening. This was based on hi-res model soundings of increasing elevated instability as the low-level jet stream gets cranking in advance of the surface cold front progressing into/through the Appalachians. Hi-res simulations (WRF-ARW/NMM, NAM Nest, RGEM) suggest convective showers with sufficient instability (in general) for some lightning. Will need to watch the threat of wind gusts with any convection given the strong winds aloft, but overall, threat will be mitigated north of the backdoor front by a pronounced and reasonably deep stable layer (frontal inversion). Think the threat of more substantial surface wind gusts with the convection will commence near/after sunrise. Finally, changed temps/dew points to increase the gradient across the retreating front. Should see substantial warming in the southern CWA overnight, but this will not be as pronounced farther north. Speaking of which, think near-/sub-freezing temperatures will persist this evening in the southern Poconos. With measurable precipitation chances increasing during this time, increased the time window for which freezing drizzle/rain remains possible/likely in this area. Again, the main threat is in the highest elevations on elevated surfaces, so expected impacts are minimal (for now). Previous discussion... Overview: we will see a brief return of the warm front, including some light rain showers, before the main event, convection along a cold front arrive very late tonight into Monday morning. With this convection, damaging winds and heavy rain will be possible. Details:There remain a lot of what ifs and questions through this period. The first issue is the timing of the warm front, and how far north it will reach tonight. This will be a key factor for moisture return which will play an important role in the hazard potential with the main line of convection. It will also have an impact on how strong the cap will be ahead of the main line of convection. Another issue is with the main upper level low, which will stay well to our west through this time, but will be in transition becoming more negatively tilted. This could have impacts with the timing of the front, as the eastward progress of the surface low could also be slowed. That being said, the models seem to be in agreement of a frontal passage between 09 and 15Z (a secondary front or trough is expected later in the day, which could bring another wind shift). As for the hazards: Heavy rain: precipitable water values look to be quite high (in the 90th percentile for this time of year), and mean layer RH is exceptionally high. However, precipitation ahead of the front is expected to be light, and front front should move through quite fast, limiting the window of opportunity for heavy rain. The main threats look to be flooding in poor drainage and urban areas. Plus any heavy rain along the coast, if it falls close to the high tide could exacerbate tidal flooding issues (see coastal flooding section below). However, if the front slows down or if it becomes more likely to see training storms ahead of the front, the threat for flash flooding could increase. At this time, not enough confidence all these factors will come together, so have held off on any flood watches. Strong winds: Unquestionably a low CAPE/high shear environment, with bulk shear values at or above 60 kt by Monday morning, most of that in the lowest 3 km. The limiting factor remains instability. As mentioned earlier, a strong cap may be in place depending on how quickly and how far north the warm front gets. Surface based instability will likely be negligible. However, there is some very modest elevated instability. Given the strong forcing with this system, would not be surprised if the elevated instability is realized. For now, will continue the mention in the HWO. Monday afternoon: Once the front moves through, could see some showers develop behind the front, though dry air advection will likely be developing very quickly behind the front. Thus, it is still uncertain at this time if we will see any precipitation Monday afternoon. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... A deep, closed mid-level low will move from the Ohio Valley to New England Monday night into Tuesday. The inflection point of the flow moves across the region Wednesday morning, as a brief short wave ridge moves in from the west. The next mid-level trough moves toward the area on Thursday and closes off just to the north on Friday. Ridging attempts to build into the area next weekend. The main uncertainty is beyond Wednesday with the phasing of northern and southern stream energy over the Great Lakes, and this is evident in the ensemble mean and spread, leading to some divergence in model solutions with respect to the timing and track of the next system. The main concerns are snow showers Monday night into Tuesday mainly over the higher terrain, the potential for frost Tuesday Night, and precipitation type (yes, precipitation type) over the higher terrain and timing with the next system Thursday into Friday. Temperature-wise, we`re generally running near seasonal norms Tuesday through Saturday. Considerable cloudiness Monday Night into Tuesday. With the mid- level low aloft, there will be abundant moisture, but lift will be scant. As colder air works in while the precipitation is winding down, there is some potential for snow to accumulate over the higher terrain of Carbon, Monroe, and Sussex (NJ) Counties Monday Night. Generally less than an inch of snow is possible, mainly on grassy surfaces. On Tuesday, while the column remains very moist and there is weak lift, the colder air aloft pulls away as the maximum heating of the day approaches. Convection would be confined to the low-levels where lapse rates are steepest. This setup would favor scattered rain showers, increasing with coverage from late morning into the evening, with some wet snowflakes mixing in across the higher terrain north of I-78. Given the clouds, temperatures ~10 degrees below normal and westerly wind gusts up to around 25 mph, it will certainly feel chilly. For Tuesday night, if the cloudiness exits early enough and the winds become light and variable, there is the potential for frost, especially west of the Delaware Valley. The concern here is for sensitive plants as we enter the growing season generally south of a PA Turnpike to I-195 line. With short wave ridging aloft, a fair weather Wednesday is still in store. Another complex storm system is on the horizon during the Thursday and Friday period, which could present precipitation type issues over the higher terrain, especially Thursday Night. There are three camps of models with this system: the GFS/CMC take low pressure through the St. Lawrence Valley (warm solution), the ECMWF/UKMET develop a fast moving Nor`easter (cold solution) as low pressure redevelops off the NJ coast, while the NAVGEM is in the middle. Given the uncertainty regarding the aforementioned phasing, there are track and timing uncertainties with this system, and we cannot rule out any one solution at this time. While the GFS solution would result in mainly rain across the entire region, the ECMWF solution would introduce accumulating snow across the higher terrain Thursday night, with the potential for advisory level criteria at least on grassy surfaces. Given the uncertainty and extended time frame, it`s just too early to rule out either scenario at this point. A return to fair weather is expected for the first half of the weekend. && .AVIATION /13Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. We continue to experience communications issues with KILG, so have included AMD NOT SKED in the latest TAF. Today...Prolonged MVFR/IFR conditions expected, with CIGs ranging from 500 to 1500 feet and visibilities locally reduced in patchy fog/drizzle/mist. Measurable rain chances increase this afternoon. Winds generally east or northeast 10 to 20 kts with gusts occasionally 25-30 kts. Moderate confidence. Tonight into Monday morning...Through the overnight and into Monday morning, expect continued low ceilings at IFR or lower. Showers are possible through the overnight hours. A low level jet may develop near or after 06Z, resulting in a brief period of low level wind shear. However, the main period we are watching is between 09 and 15Z (still some uncertainty with that timing) when a line of showers and thunderstorms is expected to move from west to east through the region. Strong winds and IFR visibilities may be possible with the strongest storms. Moderate confidence in the overall pattern, but there remains considerable uncertainty in the details. Monday afternoon...Expect a wind shift to westerly winds. By Monday afternoon, we could start to see improvement back to VFR. Low confidence. Outlook... Monday Night...There is some potential for lingering MVFR ceilings, mainly at ABE, RDG, and TTN. Elsewhere, VFR conditions. Westerly wind gusts up to around 25 kts. Moderate Confidence. Tuesday...Predominantly VFR. Brief periods of MVFR ceilings, especially in rain showers. Westerly wind gusts up to around 20 kts. Moderate Confidence. Tuesday Night and Wednesday...VFR conditions. Winds generally around 10 kts or less. Wednesday Night...Ceilings lowering to MVFR, then to IFR late. Thursday...Ceilings gradually improving to MVFR, but IFR will be possible in showers. && .MARINE... For all waters...a line of showers and thunderstorms Monday morning could bring sudden gusty winds and occasional lightning. For the northern and central New Jersey coastal waters...gale force gusts continue. There may be a brief lull in the winds this morning, and again tonight, but for the most part expect gale conditions to continue into Monday afternoon before winds finally start to subside. On Monday, expect winds to shift from easterly to southeasterly to westerly through the day. For the Delaware Bay, Delaware coastal waters, and southern New Jersey coastal waters...winds will continue to ramp up through the day today. Gale force gusts are likely to start late tonight, and continue into Monday before slowly diminishing. Outlook... Tuesday...SCA. Westerly winds gusts around 25 kts with seas AOA 5 ft. Tuesday Night and Wednesday...Sub-SCA conditions. Wednesday Night and Thursday...With the approach of low pressure, at least SCA-level winds and seas are expected. && .TIDES/COASTAL FLOODING...
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Observations associated with this morning`s high tide indicate water levels in many locations along the northern NJ coast are just at or slightly below minor flood category. However...will keep the Coastal Flood Advisory running in that area for a little while longer to cover any problems that might be occurring in the backbays or inlet areas. Previous Discussion... A Coastal Flood Advisory has been issued for this morning`s high tide along the Northern New Jersey coast for minor flooding. For the high tides this evening and Monday morning, a Coastal Flood Watch is in effect for the Northern New Jersey coast, when moderate flooding is possible. Further south, a Coastal Flood Advisory is in effect for the remainder of the New Jersey coast, the Delaware coast, and all of Delaware Bay, for the high tides this evening and Monday Morning. Increasing astronomical tides associated with the approaching new moon in combination with a brisk onshore flow will be the primary drivers of this event. There is also the potential of minor flooding along the Delaware River and upper Chesapeake Bay. For these locals, there is a greater concern for the Monday morning high tide cycle, given an increasing southeasterly flow regime between now and then.
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&& .CLIMATE... Daily Record Low-Maximum Temperatures Site 4/15 ---- ---- PHL 41 (1923) ACY 44 (1975) Period of record Aug 1943 - present ILG 42 (1943) ABE 38 (1943) RDG 41 (1943) TTN 40 (1943) GED 50 (2009) Period of record Feb 1945 - present MPO 40 (2007) Period of record Oct 1999 - present && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...Coastal Flood Watch from 6 PM EDT this evening through Monday afternoon for NJZ012>014-020-026. Coastal Flood Advisory until 1 PM EDT this afternoon for NJZ012>014-020-026. Coastal Flood Advisory from 6 PM this evening to 1 PM EDT Monday for NJZ021>025-027. Coastal Flood Advisory from 8 PM this evening to 2 PM EDT Monday for NJZ016. DE...Coastal Flood Advisory from 6 PM this evening to 1 PM EDT Monday for DEZ002>004. Coastal Flood Advisory from 8 PM this evening to 2 PM EDT Monday for DEZ001. MD...None. MARINE...Gale Warning until 5 PM EDT Monday for ANZ450>452. Small Craft Advisory until 6 PM EDT this evening for ANZ430- 431-453>455. Gale Warning from 11 PM this evening to 5 PM EDT Monday for ANZ453>455. Gale Warning from 2 AM to 11 AM EDT Monday for ANZ430-431. && $$ Synopsis...LF Near Term...CMS/Johnson Short Term...CMS/Johnson Long Term...LF Aviation...CMS/Johnson/LF Marine...Johnson/LF Tides/Coastal Flooding...JJM Climate...LF

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