Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 051156 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 656 AM EST Mon Dec 5 2016 .SYNOPSIS... A weak disturbance will exit the region this morning. High pressure briefly builds into the area this afternoon and tonight. An area of low pressure lifts northeastward through the Mid-South states Tuesday morning before redeveloping near the North Carolina coast Tuesday afternoon. The low will pass just to our south Tuesday night and then out to sea by Wednesday. Another area of low pressure will progress move across eastern Canada during the second half of the work week. A cold front with this system will move through the area late Thursday. The coldest air mass of the season is expected to arrive behind the front for Friday and Saturday while high pressure builds in. The high moves off the coast late in the weekend when the next disturbance approaches from the northwest. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... As of about 700 am, the rain (light snow far north) has pretty much ended across our forecast area. We have received reports of 0.5 to 1.5 inches of snow from the Pocono region and far northwest NJ. A lot of low clouds remain over the area however and the GOES low-cloud imagery shows cloudiness extending all the way back to the MS valley. Winds will shift to the west later this morning but not too confident that this will be enough to clear things out. Still hoping for partly sunny this afternoon. Previous discussion below: A band of light precip is moving quickly across the mid-Atlantic region early this morning, about as expected. Snow appears to be confined to the Poconos and far northern NJ. Total snow amounts have been reduced geographically and quantitatively. Areas where it is snowing could still pick up a quick inch or two. Radar loop shows the trailing edge of precip already moving into eastern PA and most precip should end across the forecast area by around 10Z. The surface low pressure trof and shortwave aloft which caused the precip will swing E/NE of the area later this morning with high pressure then building in from the Ohio Valley. Some low clouds will hang around this morning after the rain ends but by late morning drier air at all levels is forecast to push in from the west. Thus skies should mostly clear out by mid-day. With considerable sunshine and rather weak if any could advection, max temps this afternoon should be a few degrees warmer than yesterday. There will be a bit of a westerly breeze, however. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/... For tonight, high pressure will move over the area resulting in light winds and mostly clear skies. Temperatures will cool quickly under good radiating condition and overnight mins will reach the mid 30s over Delmarva and upper 20s to low 30s over most of east PA and NJ. By late night some high clouds will spread over the area from the SW ahead of the next weather system. && .LONG TERM /TUESDAY THROUGH SUNDAY/... 635 AM...With the current snow exiting the area early this morning, storm total snow and ice graphics as well as probabilistic snowfall info for the Tuesday-Tuesday night event were sent to our winter webpage. 600 AM...A forecast update was sent to correct bad RH/apparent temperature values (wind chills as low as -120F) in the long-term portion of the forecast. Low pressure forming over the northern Gulf of Mexico will track northeastward through the Mid-South region Tuesday morning. This system is forecast to transfer its energy to a secondary low that develops farther east (lee side of the southern Appalachians). The secondary low should then track up the Southeast coast late Tuesday and then turn out to sea as it passes just to our southeast Tuesday night. Models seem to be in very good agreement with regards to large-scale structure and evolution of this storm as well as timing/amounts of QPF, which adds confidence to the forecast. Compared to yesterday`s 00Z runs, models have trended a few hours slower with the arrival of precipitation on Tuesday as the high is a bit slower to retreat into New England. Precipitation is forecast to start in Delmarva early to mid morning but may struggle to advance northward as it moves into a increasingly dry airmass closer to the high. PoPs were lowered across the northern third of the CWA on Tuesday, with precip likely holding off until late in the day. The Tuesday-Tuesday night system looks to predominately be a rain event for 90 percent the CWA (the exception is southern Poconos and far northwest NJ) although the rain could briefly mix with snow and/or sleet at the onset north and west of the Fall Line. There were still subtle differences between the models with respect to the thermal fields, which factor into the ptype uncertainty for these northern-most zones, particularly along and north of I-80. The most likely scenario at this is for precip to start out as a wintry mix along and north of I-80, possibly changing back over to snow in the evening as precip intensity increases and both evaporative/dynamical cooling becomes more of a factor. A quick burst of snow could yield accumulations of 1-3 inches in Carbon and Monroe Counties in PA and Sussex County, NJ. An elevated warm layer should surge northward in concert with the arrival of SW low-level jet, turning precip into all liquid later Tuesday night. The big question is surface temperatures. Some of the colder model solutions, including the NAM, keep temperatures near freezing north of I-80 at higher elevations (above 1000 ft). The NAM tends to handle thermal profiles in the boundary layer better in these types of setups when the residual cold-air damming wedge can be stubborn to erode. Forecast temperatures were taken from a blend of various models/previous forecast but the NAM was had more weight. The top-down method was used to determine ptype, resulting in a potential for pockets of freezing rain Tuesday night in the coldest spots. Using the FRAM with input from our forecast database, several hundredths of an inch of ice are possible across the far north. A Winter Weather Advisory may be needed as we get closer to the event. Precipitation will taper off from SW to NE late Tuesday night and Wednesday morning as the coastal low moves farther out to sea and a weak trough progresses offshore. Expect some breaks in the cloud cover to develop Wednesday afternoon with high temperatures ranging from the mid 40s in NE PA/NW NJ to mid 50s across the coastal plain in S NJ and Delmarva. Shortwave energy rounding the base of the longwave trough will move in from the west on Thursday. Low chances for light showers were kept in the forecast for Thursday. Thermal profiles support a mix of light rain/snow in the Poconos and rain elsewhere during the day. Precipitation could briefly mix with or even change over to snow if showers lingers into the evening when much colder air moves into the area. At this time, we are not anticipating much in the way of snowfall accumulations. The Mid-Atlantic region will be positioned in between low pressure over eastern Canada and strong high pressure over the Great Plains late in the week. will tighten over the region late Thursday and Friday. A brisk northwest wind develops late Thursday and Friday in response a tightening pressure gradient. CAA pattern intensifies, marking the arrival of the coldest airmass of the season so far. Forecast highs are only in the 30s and lower 40s both Friday and Saturday. Wind chills look to dip into the teens across most of the forecast area (single-digits in the Poconos) both Thursday night and Friday night. The next storm system could affect the area sometime either Sunday or next Monday. Even though the low looks to track well to our north, there may be enough residual cold air in place for a mixed bag of precip at least at the onset. && .AVIATION /12Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. As of 700 am, IFR or LIFR ceilings have developed at all our TAF sites, even though the rain has ended. The latest TAFs show cigs improving around midday as winds shift to the west, but confidence in the this timing is not really high. Previous discussion below: IFR conditions have developed at ABE/RDG where rain has changed over to snow. Radar loop indicates that the snow should end shortly there, with improving vsbys but probably still IFR cigs. Otherwise mostly VFR or MVFR conditions are noted. Reduced cigs/vsbys will likely persist through mid- morning by should improve to VFR at all sites by mid day as high pressure builds in from the west. Winds will shift to the W or NW at around 10 kt. VFR conditions will continue through tonight but winds will die down as high pressure moves overhead. OUTLOOK... Tuesday and Tuesday night...VFR conditions early, lowering to MVFR/IFR by midday and continues through the night with periods of rain/fog/drizzle likely. Winds likely become gusty 15-20 kt out of the northwest late in the day. Wednesday and Wednesday night...Rain tapers off early in the day. MVFR CIGs may linger through the day and night. Thursday and Thursday night...Mainly VFR but SCT SHRA may develop during the day which could briefly lower conditions in a few spots. NW wind gusts 20-25 kt develop in the afternoon and evening. Friday..VFR. Brisk NW winds gusting 25-35 kt. && .MARINE... Sub SCA conditions are expected through today and tonight. There will be some restricted visibility in areas of rain and fog early this morning. The current south winds will shift to the WNW this afternoon as a low pressure trof swings by and then high pressure builds in from the west. Winds may gust to 20kt or so this afternoon and evening but should subside some and veer to the N as high pressure moves over the area. OUTLOOK... Tuesday and Tuesday night...A SCA was issued for our coastal waters off DE and southern NJ as well as the lower Delaware Bay for Tuesday afternoon and night. Confidence is high in these southern zones where onshore flow is expected to be strongest. The strongest winds are expected to develop late in the afternoon and continue in the evening. Wind gusts near gale force are possible for a brief period near or just after after sunset. The SCA may eventually need to be expanded northward in our coastal waters for Tuesday night but marginal conditions do not warrant a fourth period headline. Wednesday...Winds will weaken but seas remaining at or above 5 ft may necessitate a SCA during the day. Wednesday night...Both winds and seas look to drop below advisory threshold Wednesday night. Thursday and Thursday night...SCA will likely be needed as NW winds increase late in the day and into the night. Friday...Winds and seas exceeding SCA criteria. Winds may get close to gale force. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 1 PM Tuesday to 6 AM EST Wednesday for ANZ452>455. Small Craft Advisory from 4 PM Tuesday to midnight EST Tuesday night for ANZ431. && $$ Synopsis...Klein Near Term...AMC Short Term...AMC Long Term...Klein Aviation...AMC/Klein Marine...AMC/Klein

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