Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 151509 AAB AFDPHI Area Forecast Discussion...UPDATED National Weather Service Mount Holly NJ 1009 AM EST Fri Dec 15 2017 .SYNOPSIS... A surface low will move from the Carolinas early this morning to offshore the Mid-Atlantic tonight. High pressure will build into southeast Canada, allowing a backdoor cold front to move into or through the area Saturday night and Sunday. This front will then stall before lifting northward as a warm front Sunday night and Monday. A cold front will progress through the region around the middle of next week. Another system may affect the area near the close of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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930 AM ESTF...Focus at the beginning of the shift has been on timing (both onset and ending) of snow and messaging impact for this quick hitting event this afternoon and evening. Based on upstream observations and the latest available guidance (12Z guidance is starting to come in as we speak), snowfall is expected to develop right over the area (Delmarva/SE PA/SW NJ) between 1 and 3 PM, then quickly expand north/eastward toward the I-78 and possibly I-80 corridor between 3 and 4 PM. The experimental wintry precipitation onset graphic has been updated and sent to our winter weather webpage. The back end of the snow looks to progress rather quickly eastward between around 4 PM (near Reading, PA) and 8 PM (coastal NJ). Here is the key message- The ingredients are there for the snow to have a high impact on travel regardless of snowfall amounts (1-3" in the Winter Weather Advisory area; less to the north and west): (1) The period of steadiest/heaviest snow is expected to occur during the 3-6 PM time frame, aligning with the afternoon/ evening rush hour. (2) With cold air in place (both road and air temperatures will be at or below freezing across most of the area except southern DE), snow will stick on untreated roads and possibly even treated highways in a few spots where heavier snowfall rates occur. (3) There is a potential for locally higher snowfall rates of 1" per hour and poor visibility to occur underneath a narrow band of heavier snow. There is high confidence that a heavy snow band (brief in duration) does develop given the pattern but there is still uncertainty on the exact location. The greatest signal for mesoscale banding is across the southern half of NJ between the NJ Turnpike and the Garden State Parkway but it could develop as far west as the I-95 corridor and as far east as the NJ/DE coast). A Special Weather Statement was issued for most of the forecast area that is outside of the Winter Advisory area since some impacts on the commute home this afternoon/evening will likely occur. The next focus with the forecast late this morning will be on snowfall accumulations and updates to the Winter Weather Advisory headlines. Locally higher amounts of 3-4" are not out of the question underneath the mesoscale band (again, most likely to occur in southern/central NJ). Even though Winter Weather Advisory criteria is higher, there is also a potential need to expand the advisory northward into central NJ (especially Monmouth County).
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/... The previously mentioned surface low quickly lifts away from our region, with snow ending from southwest to northeast. A surface high is expected to build over the southeastern U.S. As a result over our region, low and mid level northwesterly flow will increase. However, through the overnight hours, it does not look like a favorable fetch for lake effect snow to reach the southern Poconos, so have kept the region dry after midnight. Temperatures should be higher than what we saw this morning, thanks to lingering cloud cover. Lows are expected to be in the teens and 20s across the region. && .LONG TERM /SATURDAY THROUGH THURSDAY/... The large-scale pattern in the long term will change somewhat with fairly zonal flow across much of the eastern U.S. next week. However, the flow will remain progressive, with numerous systems passing near or through the region during the period. This weekend`s forecast is a little tricky in a few respects. Though two surface highs will build into eastern North America (one in southeast Canada and the other in the southeastern U.S.), a deformation axis will form in between, with onshore flow equatorward of the Canada high allowing for a backdoor cold front to move into the Mid-Atlantic Saturday and Saturday night. The differences among the models are rather pronounced, with statistical guidance varying between the warmer Saturday/cooler Sunday (ECS) and vice versa (MET/MAV). The warm look of the ECS on Saturday looks questionable given the antecedent cold near- surface air and potential for additional snow cover in portions of the area. Furthermore, the low levels will feature warm-air advection as a weakening midlevel perturbation rapidly progresses to the Mid-Atlantic by Sunday night. Nevertheless, the source region of the surface flow on Sunday will likely be onshore, and this makes me hesitant to go with considerably warmer values this day. I went on the low side of consensus both days, especially Saturday (with residual effects of snow cover and heights that do not rise significantly until dark). With backdoor cold fronts, there is always a worry of fog/stratus, which is not readily apparent in much of the guidance. The pattern is not overly favorable, with the surface high a bit west of where it needs to be for more favorable (and stronger) onshore flow. Then there is the matter of a northern-stream perturbation moving through New England on Saturday, which may generate enough lift (with favorable northwesterly low-level fetch) to bring some snow showers to the Poconos. The 00Z ECMWF even suggests this could spread east-southeast into the Lehigh Valley and northern New Jersey. For now, kept the PoPs confined to mainly Carbon, Monroe, and Sussex (NJ) Counties with the caveat that these may need expansion southeastward should the ECMWF be on to something. The aforementioned weak perturbation moving into the region Sunday night may generate some light rain or snow in the area, especially north of the Mason-Dixon Line. The models are uniformly unimpressed with the system, which is unsurprising given its weakening phase. One thing to watch will be its track, though, which is a little uncertain given the influence of a northern-stream kicker and the always unknown position of the backdoor front (typically farther south than progged this far out). With a slightly southward track (one reason the ECMWF is colder on Sunday), the isentropically-generated lift/precip on Sunday night spreads over more of the area. Meanwhile, the GFS looks rather sparse save for areas north of I-78. Plenty of uncertainty here, so kept fairly low/broad PoPs. The 00Z CMC, notably, is more in agreement with the ECMWF. A surface low develops and moves through southeast Canada Monday night and Tuesday, but models generally keep the strongest lift north of the region. The 00Z GFS looks suspiciously wet so far south of the system, though it is noticeably deeper with the attendant trough. There also appears to be more phasing with a southern-stream perturbation near the Gulf Coast. There is a lot of uncertainty here, given that the southern-stream perturbation stems from a trailing upper low in the Southwest, and the evolution of these systems is generally of low predictability. At 18Z Tuesday, the 00Z ECMWF has two distinct southern-stream vorticity maxima in association with the Southwest upper low, whereas the GFS has a single (stronger) entity (with typical long-range timing errors, too), with the disparities between the two giving me little confidence forecasts will remain like they are now. With all of this said, I do not have a lot of confidence to change the forecast from what I inherited until model agreement improves. Main changes were to bring in a distinct dry period on Monday (after the first perturbation passes) and keep low PoPs in Tuesday with this second system. QPF/impacts from both of these systems look low/minor at this time. There should be a distinct warming trend Monday and Tuesday given the midlevel ridging that develops in advance of the Canada surface low. Temperatures on Tuesday could be around ten degrees above seasonal averages. Another deep trough enters the western U.S. near the end of next week, which should regenerate zonal flow or even slight ridging east of the Rockies during this period. This will bring a period of dry and seasonable weather (cooling after frontal passage midweek, with warm advection returning thereafter). A potentially strong surface low looks to develop in the central plains during this period and will move east or northeast via a strong southwesterly jet streak. The models are all over the place with effects to the Mid-Atlantic, with potential for plenty of warm-sector precip near the end of the week (GFS) or frontolysis limiting the precipitation generated in our area (ECMWF) or a frontal wave developing and generating a low that crosses much closer to the area (CMC). At this point, kept Wednesday and Thursday fairly dry, but there may be another chance for some precip near the end of the week. && .AVIATION /15Z FRIDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Starting VFR through much of the morning, but MVFR ceilings are expected to move in around 18Z. In addition, snow is expected, primarily along and east of the Delaware Valley which will result in MVFR, and localized IFR, visibility. Confidence is moderate on both flight categories and timing. Tonight...Conditions should improve back to VFR between 00 and 03Z. Northwesterly wind gusts up to 20 kt will be possible at KACY. For the rest of the TAF sites, northwesterly winds will generally be near or below 10 kt. Outlook... Saturday: Generally VFR, with snow showers possible in the Poconos and vicinity. West winds 10 to 15 kts with gusts around 20 kts or so. High confidence. Saturday night: Generally VFR with light and variable winds. There is a low chance of sub-VFR CIGs/VSBYs if stronger onshore flow develops. Medium confidence. Sunday: Generally VFR, with increasing cloudiness late. Light east or southeast winds. Medium confidence. Sunday night and Monday: Generally VFR, though brief sub-VFR conditions are possible with light rain or snow, especially north of PHL. Light winds generally transitioning from southeast to southwest during the period. Low confidence. Monday night and Tuesday: Sub-VFR conditions possible, with scattered showers during the period, especially on Tuesday. Winds primarily between south and west at 5 to 15 kts. Medium confidence. && .MARINE... Today...winds and seas should stay below small craft advisory criteria. Tonight...northwesterly winds will increase, primarily after midnight. On the Atlantic Coastal waters, wind gusts right around gale force are possible, but still uncertain. Therefore, have issued a gale watch for these locations. On the Delaware Bay, small craft advisory conditions are likely. Outlook... Saturday: Westerly gales possible in the morning, with advisory- level winds likely during the afternoon. Fair weather. Saturday night: Residual advisory-level northwest winds in the evening will diminish overnight. Seas below criteria. Fair weather. Sunday and Sunday night: No headlines anticipated. There may be some light rain off the New Jersey coast, with potential for some visibility restrictions. Monday and Monday night: No headlines anticipated. Mainly fair weather, but there could be some showers off the New Jersey coast at times during the period. Tuesday: Southwest winds increasing to near or above advisory levels by afternoon, with seas also building. A chance of showers and attendant visibility restrictions. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Winter Weather Advisory until 9 PM EST this evening for PAZ070- 071. NJ...Winter Weather Advisory until 9 PM EST this evening for NJZ016>027. DE...Winter Weather Advisory until 9 PM EST this evening for DEZ001>003. MD...Winter Weather Advisory until 9 PM EST this evening for MDZ008- 012-015-020. MARINE...Gale Watch from late tonight through Saturday morning for ANZ450>455. Small Craft Advisory from 1 AM to 6 PM EST Saturday for ANZ430- 431. && $$ Synopsis...CMS Near Term...Klein Short Term...Johnson Long Term...CMS Aviation...CMS/Johnson Marine...CMS/Johnson

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