Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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828 FXUS61 KPHI 101128 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 628 AM EST Sun Dec 10 2017 .SYNOPSIS... Low pressure will move well northeast of the region today. A weak low will move into New England tonight, while a stronger low will move from the southern Canadian prairies tonight to New England on Tuesday. The cold front associated with the Tuesday system will bring in the coldest air of the season for the middle of the week. Another surface low may affect the northeastern U.S. by the end of the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... A very interesting first winter storm of the season. With the aid of very strong and deep large-scale ascent from the right entrance region of an intense upper-level jet streak and broad frontogenetical forcing in the 850-700 mb layer, bands of snow readily developed during the day across the area and persisted with little movement for quite some time. Last night`s NAM Nest identified one of these regions quite well (i.e., the Lehigh Valley into northwest New Jersey), though it was overdone with totals to the south and east. Unfortunately, there was little evidence of this in much of the other guidance until shortly before the event, with the coarser GFS (among others) much too dry in this area with the 00Z/9 simulation. The trends of consensus were generally in the right direction (higher totals to the north; lower to the south) last night, but still way too dry/low north/west of Philly and a little on the wet/high side to the south and east. One thing that most of the hi-res models appeared to pick up on reasonably well was the character of the snow through the event, with a period of partial melting of the snow during the afternoon (denoted by lower snow ratios and a lack of dendritic snow, with even a little sleet observed in some locations) straddled by two more dendritic periods along and east of the urban corridor, the latter of which certainly aiding snow totals along and east of the Delaware River. Farther to the west, the character was mostly dendritic for a longer portion of time, which (along with the favorable mesoscale banding), allowed for healthy snow ratios and resultant snow accumulations. The vort max responsible for the winter storm will be moving northeastward away from the area today with quasi-zonal midlevel flow in its wake. Models indicate little in the way of smaller- scale perturbations moving across the region today. As a result, opted to reduce PoPs across the northwest today, as there is fairly reasonable agreement favorable ascent will be too far to the north and west, though sky cover may be more prevalent in the northwest CWA. Skies could be variable today, but most model soundings suggesting at least partial (if not substantial) sunshine for most of the area, save for the Poconos and vicinity. There may be some development or stubborn maintenance of scattered to broken low to midlevel clouds (which models have tended to underforecast this season so far) with the low-level cold air advection allowing for decent boundary-layer mixing today and some residual moisture aloft from the passing vort max. With a strong surface pressure gradient from the deepening coastal low pulling away today, west winds will be elevated today, with gusts 20 to 25 mph or so possible across the area during the late morning and afternoon hours. Temperatures will struggle to warm with fresh snow cover and cold air advection (at least to begin the day). If clouds are more prevalent, this would slow warming even more. For now, kept temperatures below a consensus statistical blend by about 1 to 2 degrees across the area. Am a little worried this is still too optimistic in the northern CWA, where cloud cover may be more persistent and the thicker snow cover may not be accounted for adequately via model calculations of surface fluxes and boundary layer processes. && .SHORT TERM /6 PM THIS EVENING THROUGH TUESDAY NIGHT/... A vort max will be moving through the region tonight, but it will be moisture-starved, and the strongest ascent should be well north of the area. Nevertheless, the 06Z NAM has increased precipitation output versus coarser guidance (06Z GFS and 00Z ECMWF), generally north of I-76. There are also some timing discrepancies during this period, with the GFS notably faster than the ECMWF. Whatever precipitation that would occur during this period would be snow (or flurries), with temperatures/thermal profiles certainly cold enough for it. However, any precipitation would be light and effects generally minimal or negligible. For now, opted to increase PoPs in the northwestern CWA to slight-chance during this period, but kept them unmentionable in the urban corridor eastward. However, with the recent 06Z NAM output indicating a threat of some precipitation across more of the area, this may require bumping up PoPs farther east in subsequent forecasts. The 12Z simulations will be interesting to watch today. There are subtle hints of QPF in the GFS/ECMWF overnight, both of which tend to underforecast smaller-scale lift and light/showery precipitation in these regimes. As with today`s highs, went somewhat below a consensus statistical blend for tonight`s lows. Monday should be generally quiet as the weak vort max moves rapidly eastward and the Mid-Atlantic is downstream of a digging northern-stream perturbation moving into the Great Lakes during this period. By Monday night, this perturbation will begin phasing with a southern-stream perturbation moving east through the Midwest. This should allow precipitation to break out in the Great Lakes region eastward into the northern Mid-Atlantic and New England Monday night. How far southward this precipitation makes it is the big question of the forecast, as the NAM keeps most precipitation north of I-78 while the GFS/ECMWF extend at least some precipitation down to the Mason- Dixon Line. As with tonight`s system, thermal profiles are favorable for wintry precipitation, with the added complexity of a potential warm nose just above the surface. The NAM, in particular, is suggestive of some freezing precipitation at the far southern fringes, which looks to be along the I-76 corridor (plus or minus). Precipitation amounts would be light, but given the potential for some sleet or freezing rain, this bears some watching. Farther north, snow is likely, with light accumulations a reasonable bet for the Poconos and adjacent areas. Here, the QPF is in stronger agreement amongst the models, with snow ratios likely allowing for 1-3 inches in portions of the area to the north of I-78. Winter headlines (of the advisory type) may be required during this period, but rather substantial model discrepancies remain. It looks like impacts would be on the minor side with this event, however. A cold front associated with the northern-stream perturbation will blast through the Mid-Atlantic on Tuesday, and cold air will rush in behind it. Winds may howl after passage, and precipitation should rapidly wind down during the day. Kept temperatures close to a statistical blend Tuesday and Tuesday night for now, but confidence is a bit lower with the precipitation/cold frontal timing uncertainty. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... Had no time to update the long term forecast overnight. However, a brief look at the latest model simulations suggests previous forecast needs little amending. Wednesday will be windy and cold, with potential advisories required for the winds on Wednesday and for the wind chills Wednesday night in the Poconos. With the degree of cold air and the digging/strengthening nature of the associated vort max, I suspect models are underplaying the winds/chill at the moment. Forecast becomes quite uncertain Friday and Saturday, with models showing highly variable solutions during this period. In general, another northwest-flow system looks to move through during this period, but its track and its strength are big question marks, which has rather large impacts on sensible weather. Broadly speaking, looks like some temperature moderation will occur during this period as midlevel flow becomes somewhat flatter/more zonal. There is a chance of some precipitation during this period, though capping PoPs at slight chance seems to be a reasonable way to go at this point. && .AVIATION /11Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR with west winds 10 to 15 kts with higher gusts, especially late this morning through the afternoon. Some VFR CIGs may exist, especially northwest of KPHL. Medium confidence. Tonight...Generally VFR, though CIGs may approach MVFR thresholds north of KPHL. There is a small chance of flurries or light snow to the north of KPHL, but chances are too small for mention at any TAF sites currently. West winds should diminish after dark and may become more southwesterly with time. Medium confidence. OUTLOOK... Monday...VFR. Generally light west or southwest winds. High confidence. Monday night...Sub-VFR conditions possible, especially north of PHL, with a wintry mix of precipitation possible. Winds generally light and predominantly southerly. Low confidence. Tuesday...Sub-VFR conditions possible, especially early in the day, with wintry mix possible generally north of PHL. Winds becoming west and increasing to 15 to 25 kts with higher gusts after cold frontal passage. Confidence medium in general evolution but low on timing. Tuesday night and Wednesday...Generally VFR with very strong west winds likely. Speeds 15 to 25 kts with gusts to 40 kts possible. Medium confidence. Wednesday night and Thursday...Generally VFR with west to southwest winds 5 to 15 kts with potentially higher gusts. Medium confidence. && .MARINE... Made some changes to the small craft advisory for today. Extended the New Jersey Atlantic coastal marine zones through tonight, given an uptick in wind gust forecasts and favorable boundary-layer profiles through the period. Extended the Delaware Atlantic coastal waters through 7 pm, with the expectation that winds and seas will diminish here more rapidly than farther north (though maybe persisting longer than previously expected). Did not change the advisory for Delaware Bay at this time. Recent observations from buoy 44009 and Brandywine indicate gusts approaching gale force. Though prolonged gale-force gusts are not expected, an occasional gust on the low end of criteria is certainly possible today. Should winds trend upward, a short-fused gale warning is not out of the question. OUTLOOK... Monday...Sub-advisory conditions and fair weather expected. Monday night and Tuesday...Possible visibility restrictions with rain and/or snow, especially for the New Jersey coastal waters. Generally sub-advisory conditions Monday night, but advisory conditions likely by Tuesday as southwest winds increase and seas build. After cold frontal passage late on Tuesday, winds will switch to west or northwest and become quite strong. Tuesday night and Wednesday...Gale-force west or northwest winds quite probable. Wednesday night...Residual gale conditions becoming advisory- level conditions during this period. Thursday...Sub-advisory conditions are forecast at this time. && .CLIMATE... Philadelphia recorded a daily record snowfall on December 9. The daily snowfall total of 4.1 inches broke the previous record of 2.9 inches set in 1942. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EST Monday for ANZ450>453. Small Craft Advisory until 7 PM EST this evening for ANZ454- 455. Small Craft Advisory until 5 PM EST this afternoon for ANZ430- 431. && $$ Synopsis...CMS Near Term...CMS Short Term...CMS Long Term...CMS Aviation...CMS Marine...CMS Climate...CMS

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