Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 091154 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 654 AM EST Sat Dec 9 2017 .SYNOPSIS... Low pressure near the North Carolina coast will intensify as passes east of New England tonight and becomes a large storm in the Maritimes on Sunday. A cold front settles down into Pennsylvania and New Jersey on Monday. That front moves northeast as a warm front Tuesday morning in association with low pressure crossing New York state. A very strong cold front follows in its wake Tuesday night. Another low pressure system will move east from the Ohio Valley Thursday night and pass south of Long Island Friday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 630 am update: The dry air near the surface continues to be a hindrance for snow making the move northward, but it looks like the process is beginning now thanks to substantial large-scale lift approaching the area. As a potent vort max approaches the Mid-Atlantic today, jet dynamics combined with considerable differential cyclonic vorticity advection will permit widespread precipitation to develop north into the area. Timing of the vort max appears to have slowed by 3-6 hours...another reason for the slower onset of precipitation across the area. However, the dry air near the surface will continue to pose problems early on in the event for much if any snow to accumulate north of the Mason-Dixon Line. The trend of the HRRR and HRRR-X has been downward with QPF today, though not substantially so. The 06Z NAM Nest continues rather bullish in comparison but has shifted the snow banding somewhat eastward. The dry look of the GFS is worrisome, particularly with considerable precipitation ongoing in the western Atlantic (which may prevent as much northward advection of more substantial moisture). Using an updated consensus blend of the 06Z GFS, 06Z NAM Nest, several HRRR cycles, the 00Z RGEM/WRF-ARW/WRF-NMM, the latest WPC guidance, and some continuity, QPF went down a few hundredths of an inch across the area, with most places receiving up to an inch less of snow than the forecasts this past evening. This warrants no change to any of the winter headlines at this point, though the winter storm warning for the counties bordering the Delaware River looks pretty borderline. General thinking is aligned fairly similarly to the previous forecast, with the ramp-up in snow during the next 6 hours and the best period of accumulation for the CWA this afternoon (into early this evening). Did slow the increase of PoPs north of the Mason-Dixon Line a little bit more, but based on latest radar trends, expecting to see some flakes in the urban corridor within the next couple hours. Given the rapidly evolving nature of the forecast given some short-fused model uncertainty/discrepancies, expect frequent updates/amendments to the forecast today. Sometimes, higher- confidence forecasts can deteriorate as the event nears, and this appears to be a classic case of that. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... Vort max will be moving through the region tonight, and the large-scale lift ahead of it will be shunted eastward during the evening hours. This should mean a fairly quick west-to-east progression of the western fringe of the snow this evening, with the region likely drying out after midnight. End times for the winter storm warnings and winter weather advisories look OK at this time. Snow totals will likely be under 3 inches in the Lehigh Valley and Poconos, but given the fairly aggressive 00Z NAM output, may see some marginal advisory-level totals here. Will keep an eye on this tonight. Meanwhile, 2-4 inch totals look possible for the urban corridor, with 5+ totals expected south of I-195 and southeast of I-295 (with increased confidence the farther south and east you go, at least until the beaches). Confidence in snow totals is around average, but slightly less than that in the urban corridor, where the sharpest gradient in snow totals exists. There will likely be some improvement in the sky cover late tonight, but I suspect any influence on temperatures will be minimal, especially with the winds on the upswing as this occurs. Nevertheless, it will be cold, particularly with some fresh snow on the ground. For this reason, went slightly below consensus statistical guidance for lows tonight, though confidence is not particularly high. && .LONG TERM /SUNDAY THROUGH FRIDAY/...
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500 MB: A trough over the northeast USA Sunday, briefly recedes north Monday and then redevelops southward and even stronger (-4sd) into the northeast USA Tuesday night. It then dominates our weather into next weekend with one more significant short wave rolling eastward through the bottom of the trough into our area Thursday night. Temperatures: Presuming we have some snow cover by the end of today, calendar day averages will mostly likely at or below normal, daily, from yesterday-Friday through next Saturday. The best chance for a normal max/min calendar day is Tuesday, but the downside payoff wont be pleasant as calendar day averages Wednesday and Thursday should be 9 to 13 degrees below normal (max temps Wednes`day` up to 15F below normal with that days high temperature possibly at 1201am). The cold should ease toward next weekend. Forecast basis: Unless otherwise noted a 50 50 blend of the 00z/9 GFS/NAM MOS was applied Sunday-Monday, the 00z/9 GFS MEX was used Monday night and Tuesday and then the 05z WPC gridded elements of 12 hr pop/max-min temp, and 6 hourly sky/td/wind were used for D4-8 which takes us to next Saturday. The dailies... Sunday...Brisk with west northwest wind gusting to 30 mph. some flurries possible in the Poconos. Overall high confidence. Monday...Partly sunny and nice (except flurries still possible over the Poconos) as a cold front approaches. West wind. Moderate to high confidence. Monday night...per WPC qpf, blended partial thickness and blended snow ratios we have fcst a 1-3" snow event north 1/2 NJ and much of E PA but less than 1 inch Philly. Its conceivable there`d be spotty freezing rain as the pcpn event ends with the warm frontal passage toward dawn. This event may require a winter weather advisory. Uncertainty on QPF amount and so, average confidence. Tuesday...Brisk with a shower or flurries developing e PA and nw NJ, in assn with a cold frontal passage. Gusty west wind 25-35 mph in the afternoon. High confidence. Tuesday night and Wednesday...Windy and turning much colder as very cold trough aloft swings down into and through our area. Temperatures Wednesday will be much below normal with sub- freezing highs (teens Poconos) except lower 30s southeast coasts. Flurries or scattered snow showers possible, mainly e PA e MD and nw NJ. West to northwest wind gusts 30-40 mph, especially Wednesday where isolated gust to 45 mph are possible. Wind chill probably down into the single digits at daybreak Wednesday. High confidence. Thursday...Still quite cold. Snow potential, especially afternoon and night as an arctic wave heads east from the upper Ohio Valley. The question...does it track north of our area or close to the Mason Dixon line. This needs to be monitored for a snow advisory (fluff) event possibility. Thursday morning low temps in Philly down into the upper teens too! Wind chill forecast down to about -9 in the Poconos and single digits most of the remainder of the area. Average confidence. Uncertainty is regarding the low track north and milder non event for us, or down across MD out to LI which would be a fluffy snow event, especially I78 north. Friday...Fair as per WPC guidance. Average confidence on the outcome.
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&& .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR before the snow begins...sub-VFR once the snow begins, with IFR VSBYs likely, especially this afternoon and especially from the urban corridor southeastward. Exact timing of the more substantial restrictions is somewhat low confidence; however, midlevel precipitation is beginning to develop at this time, so the onset of snow is probably within a couple of hours at the city terminals. Light/variable winds today. Confidence: High with sub-VFR conditions, though low on timing/severity; very high with winds. Tonight...Gradual improvement to VFR after snow moves northeast of the area by late evening. Residual sub-VFR CIGs may continue through/after midnight, especially along/east of the urban corridor. Light/variable winds becoming west or northwest 5 to 15 kts late. Overall confidence in evolution is high but with exact timing is low. OUTLOOK... Sunday...VFR. West wind wind gusty to near 30 kt. A few snow showers possible Poconos. High confidence. Monday...VFR. West wind. High confidence. Monday night...Restrictions psbl with snow except possibly rain south. Average confidence. Tuesday...VFR with a possible flurry or sprinkle north. West wind gusty near 30 kt. High confidence. Tuesday night and Wednesday...VFR WIND with flurries or snow showers possible e PA E MD and nw NJ. West-northwest gusts 35 kt with isolated 40 kt possible, especially Wednesday. High confidence. && .MARINE... North winds are picking up on the waters early this morning, and this trend should continue through the day. A small craft advisory goes into effect at 9 am for the Atlantic zones of New Jersey and at 5 am for the Atlantic zones of Delaware. This timing looks quite good at the moment, so made no changes to these products. There may be a lull in winds this afternoon and this evening before they pick up again tonight (with a more westerly flow by this time). Expect a few visibility restrictions in rain and/or snow through the day with a gradual tapering trend tonight (generally south to north). OUTLOOK... High confidence on all forecasts for the days listed below. Sunday...SCA flag with psbl short fuse upgrade to Gale in a few zones with WNW winds. Sunday night...SCA possible. Winds and seas decreasing. Monday...Sub-SCA. Fair weather. West wind. Mon night..rain snow likely and sub sca. Tuesday...Westerly SCA probable. Tuesday night and Wednesday...Gale northwest wind 35-40 kt gusts as the coldest air of the season arrives. && .CLIMATE... Record daily snowfalls for the date are listed below for Saturday December 9. Note...compaction and melting on contact may limit amounts below our forecast in some of our forecast area. ACY 3.5-1933 PHL 2.9-1942 ILG 5.0-1928 ABE 9.1-2005 && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Winter Weather Advisory until 1 AM EST Sunday for PAZ070-071- 101>106. NJ...Winter Weather Advisory until 1 AM EST Sunday for NJZ008>010- 012>015-025-026. Winter Storm Warning until 1 AM EST Sunday for NJZ016>024-027. Winter Weather Advisory until 1 AM EST Sunday for NJZ001-007. DE...Winter Weather Advisory until 7 PM EST this evening for DEZ001- 004. Winter Storm Warning until 7 PM EST this evening for DEZ002- 003. MD...Winter Weather Advisory until 7 PM EST this evening for MDZ008- 012. Winter Storm Warning until 7 PM EST this evening for MDZ015- 019-020. MARINE...Small Craft Advisory until 5 PM EST Sunday for ANZ450>453. Small Craft Advisory until 5 PM EST Sunday for ANZ454-455. Small Craft Advisory from 6 AM to 5 PM EST Sunday for ANZ430- 431. && $$ Synopsis...Drag Near Term...CMS Short Term...CMS Long Term...Drag 653 Aviation...CMS/Drag Marine...CMS/Drag Climate...

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