Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 090915 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 415 AM EST Sat Dec 9 2017 .SYNOPSIS...
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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.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... Did not change much to the inherited forecast for today. General thinking is that snow should rapidly spread north from the near-steady axis that has persisted overnight in the far southern CWA in the 12Z-18Z time frame as large-scale lift rapidly increases via favorable jet dynamics and approach of a very strong midlevel vort max. The process has begun already, but the near-surface dry air will take quite a bit of time to saturate, so very little snow accumulation will occur to the north of the Mason-Dixon Line until after daybreak. Snow should spread into most of the CWA by afternoon, which will be the most favorable time for accumulating snow across the area. Main changes to the forecast are as follows: 1) Added Sussex (NJ) and Warren Counties to the winter weather advisory, as the consensus QPF from high-resolution model output was around or just above 3 inches of snow for the event. Though on the marginal side, this is the first event of the year, so think it is wise to expand the advisory to these areas, particularly given the slight upward trend (overall) during the night. 2) Added coastal Cape May County to the winter storm warning for southern NJ. Models are consistent in developing/maintaining a nearly-stationary axis of light to moderate snow through the day from the Delmarva northeastward to far southeast New Jersey. Though northeast surface flow may make temperatures marginal here, especially for a time this afternoon, the heavier snow (which has already begun in this area overnight) during the morning hours may tip the scales to warning criteria here. Was far less confident for the beaches of Sussex County, though totals have already approached 4 inches (including at Lewes). Here, the snowy part of the event is drawing to a close, with more of a mixture expected after daybreak. Confidence is simply not high enough for an upgrade to a warning here at this time. 3) Extended the winter weather advisory for coastal Atlantic and coastal Ocean Counties until 1 am Sunday. High-resolution guidance keeps precipitation going in these areas through the evening hours, with temperatures close enough to keep snow in the picture (at least as a mixture) through this time. This also provides some spatial continuity with the warning just to the west. 4) Added some mention of freezing rain and sleet to the far southeast CWA, where near-freezing temperatures and profiles indicating an isothermal layer near (or even somewhat above freezing) above the surface at times during the day may lead to some ice pellets with the primarily rain and/or snow precip type. 5) QPF was pivoted, with higher totals somewhat farther west in the northern CWA and somewhat farther south and east in the southern CWA, which matches the more NNE-SSW orientation of the mesoscale bands hi-res models produce today as jet dynamics improve rapidly during the morning hours. In general, however, there were few changes to QPF and snow totals for the event overall. 6) Temperatures were generally lowered today, with wet-bulbing effects early on likely keeping temperatures somewhat lower than previous forecast. The dry near-surface air currently in place north of the Mason-Dixon Line has been stubborn to moisten, so when snow begins to reach the ground, a noticeable drop in temperatures should occur. && .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. High confidence. some flurries possible in the Poconos 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 /09Z SATURDAY THROUGH WEDNESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of tonight...Generally VFR outside of snow, but conditions will deteriorate to MVFR CIGs and likely IFR VSBYs once the snow arrives. Generally, periodic snow should occur at MIV and especially ACY through 12Z but may struggle to reach the other TAF sites the rest of the night. Light and variable winds. Overall confidence medium. Saturday...Prolonged sub-VFR conditions likely, though CIGs may stay generally MVFR outside of heavier snow. VSBYs, on the other hand, will likely reach IFR, especially along/southeast of the urban corridor, for much of the day. Snow should begin to diminish from southwest to northeast around or just after sunset, but sub-VFR conditions should continue for a time after the snow ends. Winds generally light and variable. Overall confidence medium. 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.
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&& .MARINE...
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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.
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&& .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...
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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 from 9 AM this morning to 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 from 9 AM this morning to 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.
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&& $$ Synopsis...Drag 414 Near Term...CMS Short Term...CMS Long Term...Drag 414 Aviation...CMS/Drag 414 Marine...CMS/Drag 414 Climate...

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