Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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404 FXUS61 KPHI 151112 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 612 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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The biggest weather story for our area today is the potential for snow through the afternoon. Changes from the previous forecast: Based on the latest high resolution model runs, adjusted the PoPs primarily west of the I95 corridor up as it looks like the snow could continue further west than previously forecast. However, the further west from the coast, the drier the boundary layer, so still expect snow totals near or below one inch. Therefore, no plans to expand or change the winter weather advisory at this time. Synoptic and mesoscale pattern: In the mid and upper levels, a positively-tilted short wave trough will be propagating east through the day at the same time that the low over northwestern Mexico transitions to a cutoff low. At the surface, a low currently over the Carolinas, will lift northeast as it deepens and intensifies (in response to the approaching mid and upper level trough). At this point there doesn`t appear to be a strong signal for significant mesoscale banding as the jet in the upper level is off set significantly to the north from the mid level jet. Additionally, the theta e ridge and max frontogenesis on the north side of the surface low look to stay well off shore of our region. Locations: It generally looks like the favored area will still be along and southeast of the Interstate 95 corridor. There will likely be a sharp drop off in snow along or northwest of 95, but given the potential timing, have erred on the side of caution with including Delaware and Philadelphia counties in the winter weather advisory. Precipitation type: Mostly snow except along coastal Delaware. Some models indicate that there may not be ice crystals in the clouds to start the event, but given the satellite trends, it looks like seeder-feeder high clouds should arrive over our region well before precipitation begins, leading to a negligible freezing rain threat. Along the Delaware coast, the center of the surface low is expected to get close enough to the coast to result in a rain/snow mix and periods of all rain. Temperatures: with the exception of coastal Delaware, most of the area will see high temperatures near or below freezing, which is about 10 to 15 degrees below normal.
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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 /11Z 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 from 1 PM this afternoon to 9 PM EST this evening for PAZ070-071. NJ...Winter Weather Advisory from 1 PM this afternoon to 9 PM EST this evening for NJZ016>027. DE...Winter Weather Advisory from 1 PM this afternoon to 9 PM EST this evening for DEZ001>003. MD...Winter Weather Advisory from 1 PM this afternoon to 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...Johnson Short Term...Johnson Long Term...CMS Aviation...CMS/Johnson Marine...CMS/Johnson is the U.S. government's official web portal to all federal, state and local government web resources and services.