Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 141256 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 756 AM EST Thu Dec 14 2017 .SYNOPSIS... Low pressure will move quickly to the north and east of the area this morning. High pressure will build in from the west for the rest of today through Friday. A low pressure system will move up the Mid Atlantic coast on Friday, remaining well offshore. High pressure then returns for the weekend. A weak low pressure system will move through Sunday night into Monday with another system expected around midweek. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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745 AM update: The remainder of the Winter Weather Advisory has been cancelled with snow moving out of northeastern PA and northern NJ. There are additional snow showers farther south across DE and southern NJ but with precip moving quickly to the east and with temperatures above freezing, do not expect much in the way of snowfall accumulations and impacts on travel. The forecast (PoPs and Wx) has been adjusted accordingly. Previous discussion... A decently strong clipper low is moving through the northern Mid- Atlantic this morning, with most of the area receiving a dusting to around an inch and a half of snow with warm-air advection generated precipitation that moved through the area earlier. Lingering light snow persists north of I-78 as the surface low remains to the west. However, it will quickly redevelop offshore as synoptic-scale lift quickly increases in favorable left-front quadrant jet dynamics combined with differential cyclonic vorticity advection from the attendant midlevel vort max. This means that the zonally-oriented band of snow across most of Pennsylvania should generally diminish with time while rain/snow develop offshore during the next few hours. However, as the surface low to the west propagates closer to the western CWA, there should be a final push of snow across northern portions of the CWA, where up to another inch or so is possible through the morning rush hour. As such, the winter weather advisory was not changed with this update, though snow totals themselves will likely be below criteria (at least in an areal average sense) with this event. I may chip away at the advisory from south to north during the next few hours should the snow be as far north as the latest HRRR simulations suggest (versus the NAM). For context, the precipitation as simulated yesterday was underdone and too far north in the WAA regime (by about 100 miles), so I do not have much confidence in the placement of the final push of snow this morning. With subfreezing conditions continuing, expect slick roads for the morning commute, especially those that remain untreated. To the south, the dry slot has effectively ended the precipitation (mainly along/south of I-76). Models are much too cold in this area, and hourly temperatures have been a real struggle as a result. Statistical guidance has trended warmer today, but I suspect this is too aggressive where accumulating snow has fallen. Nevertheless, given the cold bias in the southern areas this morning, I inched up temperatures today in general. Once the low passes this morning, west winds should pick up and become gusty again, especially this afternoon. Gusts should be much lower today versus yesterday, and with warmer temperatures, it will simply be uncomfortable versus bone-chilling.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/... High pressure moves into the Mid-Atlantic tonight, and this will cause the winds to subside after dark. Mostly clear skies to start the night will set the stage for a sharp drop in temperatures during the evening, likely exacerbated in areas where remaining snow exists (i.e., especially in the north). Generally went below guidance for lows, though with increasing cloudiness likely to occur late, there is some uncertainty here. The increasing cloud cover will be aided by a 250-mb jet streak encompassing much of the eastern U.S. downstream of another midlevel vort max entering the Great Lakes region. The main question is timing, with faster approach of the clouds likely resulting in warmer lows. Current forecasts are for an increase rapidly after midnight, which should be ample time for temperatures to plummet in the evening. Speaking of lows, they are forecast to range from single digits in the Poconos to around 20 in the Philly metro area. Bundle up. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Friday through Friday night...High pressure will make its way into the area overnight and cross the region on Friday. A coastal low develops along the Carolina coast and makes its way up the Mid Atlantic coast. The models continue to show this system remaining offshore and out of our area. However, some moisture may reach into our area and we could see some snow/rain fall towards the afternoon and into the evening. Additionally, a couple of shortwaves will rotate through the area on Friday with a another clipper system staying to the north of the area. Guidance shows the moisture from the clipper drying out before it reaches the region but with a strong northwest flow, we could see some streamers make it from the lakes to our area. Saturday through Sunday morning...High pressure slides across the eastern United States Saturday, bringing some beautiful late fall weather. The weather should remain quiet through at least early Sunday before the next system arrives. Saturday may be a bit windy, especially along the coast. Warmer air filters in from the southwest but we won`t really start to feel the difference until Sunday. Temperatures are expected to be in the 30s to lower 40s on Saturday and in the upper 30s to upper 40s on Sunday. Sunday afternoon through Monday...A weak low pressure system will approach the region sometime around Sunday night into Monday. While the models are not too robust with moisture, it seems more likely underdone and we expect a period of rain or mixed precipitation through the region. However, with differences in timing and strength of the low, confidence remains low. Tuesday through Wednesday...The models diverge with how to handle a strong low pressure system to our north and the attendant cold front. Indications are we will see some precipitation occur sometime in the Tuesday/Wednesday timeframe but confidence is low with respect to coverage and timing. && .AVIATION /13Z THURSDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. At 1245Z, conditions are improving to VFR with the SN already exiting the area. However, RDG and ABE are still reported MVFR CIGs in SN (though it is more likely BLSN). W-NW winds will increase later this morning, which could potentially lead to BLSN and localized reduced visibility to MVFR in spots where fresh snow fell early this morning. Gusty west or northwest winds will occur from 15Z to 22Z for most of the area (speeds 10 to 15 kts with gusts to 20 kts or so). High confidence. Thursday night...VFR with northwest winds around or below 10 kts. Increasing mid and high clouds late. High confidence. Outlook... Friday...Mainly VFR conditions expected. MVFR or lower possible in snow showers Friday afternoon and evening. Light winds. Confidence: Moderate Saturday...Mainly VFR conditions expected. West winds around 10 to 15 knots possible. Gusts up to 20 knots possible, mainly at KACY and KMIV. Confidence: Moderate Sunday through Monday...Mainly VFR conditions expected. MVFR conditions in snow/rain showers possible Sunday afternoon through Monday. Southwest winds around 5 to 10 knots. Confidence: Low
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&& .MARINE... 630 am update: Gale warning continues for the Delaware coastal waters, with models indicating peak winds between now and 10 am. Latest model simulations show continued advisory-level gusts for the New Jersey coastal waters until around 10 pm tonight. Extended the advisory here, and will replace the gale warning with a small craft advisory for the Delaware waters once the gales come down. Previous discussion... Gales have developed at buoy 44009 this morning, so I have issued a short-fused gale warning for the Delaware coastal waters through noon today. Elsewhere, strong small-craft advisory conditions will exist this morning before a slow diminishing trend occurs this afternoon. Although winds may linger near/slightly above advisory criteria early this evening, not confident enough to extend the advisory at this time. Seas should remain elevated through the day, but as northwest flow develops this afternoon, there should be a diminishing trend this evening. Light snow should continue across the New Jersey coastal waters this morning, with visibility restrictions likely. Fair weather should set in this afternoon and tonight. Outlook... Friday...Sub-advisory conditions are expected. Friday night through Saturday night...Small Craft Advisory conditions possible. West to northwest winds with gusts around 25 to 30 knots. Gale force gusts possible but confidence is low at this time. Seas will increase and near 5 feet on Saturday. Sunday through Monday...Sub-advisory conditions expected. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 10 PM EST this evening for ANZ450>453. Gale Warning until noon EST today for ANZ454-455. Small Craft Advisory until 6 PM EST this evening for ANZ430- 431.
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&& $$ Synopsis...Meola Near Term...CMS/Klein Short Term...CMS Long Term...Meola Aviation...CMS/Meola/Klein Marine...CMS/Meola

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