Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 172030 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 330 PM EST Tue Jan 17 2017 .SYNOPSIS...
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Low pressure over the Great Lakes will track towards northern New England tonight. Meanwhile, a secondary low will begin to develop over the Mid-Atlantic, and that will approach southern Long Island late tonight. This entire system will move offshore on Wednesday. High pressure then briefly builds across the Eastern Seaboard through the end of the work week. Weak low pressure will pass through the region this weekend, and then a stronger system will impact the area early in the new week.
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&& .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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A mid level short wave trough was located over the eastern Great Lakes this afternoon. The feature will progress to the east and it should pass across New York State this evening, then New England overnight. At the surface, low pressure is forecast to pass over the eastern Great Lakes region with a secondary low expected to develop off the coasts of New Jersey and Long Island tonight. Areas of rain will continue to move from west to east across eastern Pennsylvania, northeastern Maryland, Delaware and New Jersey into this evening. As the axis of the short wave moves to our northeast, the chance for rain should begin to lessen. Some drying is anticipated in the mid levels in the wake of the short wave. However, abundant low level moisture will remain and we will likely see areas of fog and patchy drizzle in our region. Pockets of near freezing temperatures are holding tough on the Pocono Plateau and in the highest terrain of Sussex County in New Jersey. The lingering cold air will be slow to scour out of that area due to a very subtle northeast surface flow there. As a result, we have extended the Freezing Rain Advisory until 10:00 PM for the elevated terrain of Monroe County and Sussex County. Valley locations in those counties should remain above freezing. Temperatures are forecast to remain steady or rise slowly overnight. The wind should be light and variable at most locations.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
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Low level moisture is forecast to linger into Wednesday morning with low clouds and some patchy fog. Also, there may be some spotty light rain or drizzle for a while. A mid level short wave trough that was located over Iowa and vicinity this afternoon will progress to the east and it should pass off the Middle Atlantic coast on Wednesday afternoon. The enhanced mixing ahead of and with the short wave and drying in its wake should result in some breaking of the cloud cover, especially during the afternoon hours on Wednesday. A northwest wind is anticipated to increase around 8 to 12 MPH. Maximum temperatures are expected to range from the lower 40s in the elevated terrain of the Poconos and northwestern New Jersey to the upper 50s in southern Delaware.
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&& .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
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Overview: The long term period will feature a series of low pressure systems that will bring unsettled weather to much of the Northeast and Mid-Atlantic. This period also features a jet stream that will keep the Arctic air bottled up to the north. Although a series of storm systems will impact the area during this time, they do not have origins from the Arctic, so any cold air that funnels into the Northeast and Mid-Atlantic will generally return temperatures closer to normal. With deep S-SW flow that will dominate the time frame due to a Bermuda high that will set up over the western Atlantic, temperatures will run well above normal, with high temperatures generally 10 degrees above normal, for this time of the year. Although a bit beyond the time frame for this forecast period, the Climate Prediction Center 6-10 day outlook starting on Monday indicates a high probability, 60-70% for NJ and Delmarva and 70-80% for SE PA and the Poconos, of above normal temperatures next week. Low pressure over the western Atlantic will continue to drift offshore Wednesday night as high pressure over the TN/OH Valleys slowly presses east. Not anticipating much in the way of lingering precip, so will carry a dry forecast for the CWA Wednesday evening, and conditions will further clear through the night. The high will continue to slowly press east on Thursday, and will be nearly overhead along the East Coast late Thursday night, and then will move offshore on Friday. Skies clear out during the day Thursday, and generally expecting mild conditions with highs in the low to mid 40s across the Poconos, and in the low to mid 50s across the rest of NJ, SE PA, and into the Delmarva. The next system is rather weak and disorganized, and is almost falling apart as it tracks from the Southern Plains and into the Great Lakes and OH Valley. Nonetheless, a weak warm front approaches on Friday afternoon, and some light precip will develop over extreme SW portions of the CWA, generally across SE PA, the Delmarva, and southern NJ in the afternoon, and then light precip spreads to the north and east Friday night as that system lifts northeast through the region. High pressure briefly builds through the region on Saturday, but moves offshore by Sunday as a weak cold front passes through. Conditions should generally be dry, but cannot rule out a few passing showers. Things get interesting for the start of the new work week. A trough moves into the Western U.S. early in the weekend, and then a strong closed H5 low moves into the Southern Plains/Gulf Coast states on Sunday, where a storm system become more organized and developed. This system will lift to the north and east, towards the TN/OH Valleys, Sunday night and Monday, and will slowly work its way through the East Coast Monday through Tuesday. Latest GFS/ECMWF in decent agreement with the overall setup, so feel reasonably confident going with high-end likely PoPs for Sunday night through Monday night, with western portions of the CWA bumped up to low-end categorical. With strong onshore flow ahead of this system, can expect abundant low-level moisture to spread into the region. GFS indicating PWATs around 1.5" during this time, so can expect locally heavy rain from time to time. In addition, GFS indicating an environment with CAPE (less than 100 J/kg) and 850-700MB wind speed 60-65 KT. Cannot rule out a rumble of thunder Monday afternoon, especially south of I-76, but given that it is mid-late January, may be a bit too far to include a mention of thunder on Day 7. But will keep an eye out on it. Lower confidence in the forecast by Tuesday. Models indicating secondary low will develop off the Mid-Atlantic coast, and it remains to be seen how that will impact the Northeast/Mid- Atlantic. Will carry chance PoPs for most of the region, with higher PoPs along the coast. Monday looks to be the warmest, but wettest, day of the period with highs ranging from the 40s across the Poconos to near 60 in the Delmarva. Otherwise, as mentioned above, temperatures will be warm.
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&& .AVIATION /20Z TUESDAY THROUGH SUNDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Conditions are forecast to continue lowering into this evening as areas of rain pass across the region from west to east. Conditions are anticipated to be in the IFR range at all eight of our TAF sites for tonight. IFR conditions are expected overnight with abundant low level moisture lingering in our region. The rain should taper to patchy drizzle. Areas of fog are likely. A developing northwest surface flow on Wednesday is forecast to slowly draw dry air into our region. Conditions are expected to slowly improve through the MVFR range and they should reach VFR during the afternoon. A light southeast wind late this afternoon should become light and variable for tonight at most locations. The wind is expected to settle into the northwest for Wednesday and speeds are anticipated to increase around 10 knots for the afternoon hours on Wednesday. OUTLOOK... Wednesday night through Friday morning...VFR. Wind speeds should be 10 KT or less during this time. Friday afternoon through Friday night...Sub-VFR conditions in -RA. Saturday through Sunday. Mainly VFR. Few passing showers with brief sub-VFR conditions possible.
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&& .MARINE...
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Weak low pressure is forecast to develop off the coasts of New Jersey and Long Island tonight before it moves out to sea on Wednesday. The system is not expected to become particularly strong so wind speeds and wave heights should remain below the Small Craft Advisory criteria. The wind is anticipated to veer from the southeast and south this evening to the west and northwest for Wednesday with speeds increasing around 15 knots. Wave heights are forecast to build only to around 2 to 3 feet on our ocean waters. OUTLOOK... Wednesday night through Sunday...Sub-SCA conditions expected. Few passing showers possible on Friday/Friday night. Sunday night through Monday night...Easterly flow 20-30 KT. SCA conditions likely, with gale force winds possible on Monday, mainly on the ocean waters. Rain, possibly heavy at times. Isolated thunderstorms possible Monday afternoon.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Freezing Rain Advisory until 10 PM EST this evening for PAZ055. NJ...Freezing Rain Advisory until 10 PM EST this evening for NJZ001. DE...None. MD...None. MARINE...None.
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&& $$ Synopsis...MPS Near Term...Iovino Short Term...Iovino Long Term...MPS Aviation...Iovino/MPS Marine...Iovino/MPS

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