Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 240201 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1001 PM EDT Thu Mar 23 2017 .SYNOPSIS... High pressure across the middle Atlantic will move offshore by Friday, while a warm front moves through the Ohio Valley. This frontal boundary will become nearly stationary near our region through Tuesday, as several waves of low pressure move along it. A cold frontal passage is expected Wednesday, followed by Canadian high pressure building into our area on Thursday. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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High pressure was centered near the southern Mid-Atlantic coast this evening. Nearly ideal radiational cooling under light winds, clear skies and a very dry nocturnal boundary layer are allowing temperatures to drop fairly quickly this evening. At 9 PM, temperatures were highly variable ranging from the mid 20s in the southern Poconos, NJ Pine Barrens and rural valleys to 40 degrees in the City of Philadelphia. Forecast temperatures were updating mainly using a blend of LAV/LAMP since this guidance were verifying better so far this evening. However, none of the available guidance were able to capture the large variations in temperatures (sub- mesoscale) very well.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
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The forecast was updated with a focus on hourly temperatures, dewpoints, winds and PoPs since these grids will help determine ptype and ice accrection. Differences from the previous forecast were overall minor, negating the need to make changes to the Winter Weather Advisory headline. Compared to the forecast from the day shift, the onset of precip in the morning has trended slightly slower (8-10 AM in the southern Poconos). Any extra time will provide extra time for temperatures to rebound after daybreak. Wet-bulb cooling look to allow a brief period of sleet at the onset along and north of I-80. Sleet accumulations will be minimal. Ice accretion of up to a few hundredths are expected for Carbon and Monroe Counties during the morning where the advisory is in effect. Light freezing rain may extend beyond the county borders but should mainly be confined to the highest ridges in the Lehigh Valley counties in PA and Sussex County, NJ. At this point, cannot rule the need to expand the advisory into Berks Co, Lehigh Valley and northwestern NJ if precipitation arrives earlier than forecast. This will be a short duration event as precipitation will last for 3-5 hours and temperatures rise steadily to above freezing through midday. See WSW for more information on impacts. By early afternoon, expect all locations to be above freezing and all precipitation to be rain.
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&& .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... An unsettled period of weather is in store this weekend thru the middle of next week. At the mid-levels across the conus, a generally zonal northern stream flow will be in place, while several shortwave disturbances traverse the southern stream. Meanwhile at the surface, Canadian and sub-tropical high pressure systems will maintain a frontal boundary in the vicinity of our region until a more substantial frontal passage on Wednesday. The primary uncertainty will be the location of the aforementioned boundary, along with the timing of several areas of low pressure that ride along it, and accompanying areas of precipitation. Of most concern is the potential of mixed precipitation Sunday night. On Saturday, the NAM and EC are most extensive with the overrunning precipitation associated with the stationary boundary to our north. The GFS looks more reasonable with the placement of the precip in relation to the frontal boundary, confining PoPs to the northern half of our CWA, and our forecast reflects this. As the frontal boundary moves southward in response to Canadian high pressure building to the north, scattered showers are possible across our entire area Saturday night into Sunday. As low pressure moves through the Ohio Valley into the Great Lakes from Sunday night into Monday, more substantial overrunning precip is likely during this time frame. As mentioned previously, there is some potential for mixed precipitation Sunday night into Monday morning. A review of plan view 2-meter and 850 hPa temperatures, 1000 to 500 hPa and 1000 to 850 hPa thicknesses, model p-type, ens plume diagrams, model soundings, and CIPS Analogs points to freezing rain as the primary concern. There is a high amount of uncertainty, especially at this range in the forecast, which translates into low confidence. The probability for freezing rain is low at this time, therefore it was not mentioned in the HWO (per our directives), but this will need to reassessed with future forecast packages. Also, please note that ice map on our website is for Friday, March 23. Expect showers to continue Monday night into Wednesday, and another round of more substantial precip likely centered on Tuesday night into Wednesday, associated with a cold frontal passage. There is uncertainty with the timing of this front, particularly given the time range, but a gradual return to fair weather should occur on Wednesday and continue into Thursday. Temperature-wise, at or above normal through the period. && .AVIATION /02Z FRIDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions will continue through at least 12Z. After 12Z, there is an increasing risk for precipitation (expected to be all rain at the TAF sites, but freezing precipitation will be possible N and W of KABE and KRDG). With any rain, brief MVFR conditions will be possible. The rain should move out by mid afternoon bringing a return to VFR conditions. Light and variable winds may continue through much of the overnight hours before settling in out of the south by 12Z Friday. OUTLOOK... Saturday and Sunday...A period of MVFR is possible in low clouds, especially northwest of an ACY-MIV line. Otherwise, VFR. Southwest winds 10 to 15 knots on Saturday will shift to the east Saturday night into Sunday, increasing to 10-20 knots. Sunday night thru Tuesday...Periods of MVFR possible in low clouds and fog. South-southwest winds Sunday night into Monday may shift to the east-southeast Monday night and Tuesday, generally at or below 20 knots. && .MARINE...
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Winds and seas should stay below SCA criteria tonight into tomorrow morning. By midday tomorrow, winds will increase. Gusts above 25 kt will be possible especially for the New Jersey Coastal waters through the afternoon hours. OUTLOOK... If confidence increases for elevated winds and seas to linger into Saturday night, the Small Craft Advisory (SCA) may need to be extended further into this time frame. As a front moves south across the waters on Sunday, winds and seas may increase to SCA levels during the day, mainly across the northern NJ waters. Looking ahead to Monday and Tuesday, wave heights may build in excess of five feet during this period. Therefore, a SCA may be needed.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...Winter Weather Advisory from 8 AM to 1 PM EDT Friday for PAZ054-055. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 10 AM to 8 PM EDT Friday for ANZ450>453. && $$ Synopsis...Franck Near Term...Klein Short Term...Johnson/Klein Long Term...Franck Aviation...Franck/Johnson/Klein Marine...Franck/Johnson/Klein

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