Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 161152 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 652 AM EST Fri Feb 16 2018 .SYNOPSIS... A cold front will move slowly southeastward through our area today. High pressure builds into the Mid-Atlantic states tonight, then shifts offshore on Saturday. Low pressure develops just off the Mid- Atlantic coast Saturday night and speeds away early Sunday morning. High pressure briefly builds across the area Sunday, then moves offshore Sunday night. A strong warm front will slowly lift north of the Mid-Atlantic region early next week, followed by a cold front late Wednesday and then high pressure starts to build in on Thursday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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A 180 kt 250 MB jet core over NYS at 18z today combined with height falls at 500 MB (vorticity maxima) passing through n PA appear to be primary driver for continued periodic showers today. Periods of showery rains will end from northwest to southeast later this afternoon. Turning colder with light winds becoming northwest and eventually gusty 20-30 mph near sunset. Storm totals for this event will probably exceed 1 inch in parts of southeast PA. There may still be a few fog patches in parts of our area early this morning. There is a small chance of pcpn ending as ice pellets vcnty KMPO mid aftn? Forecast basis was use of multimodels for qpf and near 100% south and categorical pops north with special consideration of the hrrrx. Temps were a multimodel but again relying closely on a blend of the 00z/16 GFS/NAM MOS.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/...
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Tonight...any leftover rain ends by 8 PM extreme se DE, otherwise becoming clear and much colder with lows near normal for this date! Gusty northwest winds 15-25 mph during the evening becoming northerly and diminish after midnight. This part of the forecast was a 50 50 blend of the 00z/16 GFS/NAM MOS blended with the colder EC 2m temps.
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&& .LONG TERM /SATURDAY THROUGH THURSDAY/... Summary...A quick moving but potent storm Saturday evening into very early Sunday is expected to bring a swath of accumulating snow; Turning much warmer for awhile early next week with potentially some record warmth; Cooling for late next week. Synoptic Overview...As a closed low remains nearly stationary near Hudson Bay Canada through the weekend, a stronger short wave will allow for a quick moving but potent Mid-Atlantic storm late Saturday into very early Sunday. Given the progressive flow, cold air will not follow this storm, then the pattern shifts into a much more amplified one with a significant trough in the West with a strengthening ridge in the East. This will bring much warmer air to our region during the first half of next week, however a cold front settles through later Wednesday with some cooling in its wake. For Saturday and Sunday...The guidance continues to show a quick moving but potent system developing just off the Mid-Atlantic coast later Saturday then quickly tracking northeastward away from our area early Sunday. While the guidance is in good agreement that precipitation will occur, there continues to be more uncertainty with the precipitation amounts northwestward and also the precipitation type especially near and east of I-95. We start with clouds increasing during the day Saturday and if these thicken rather quickly, surface heating will be slowed. The airmass in place is forecast to be chilly but also rather dry in the lower levels especially. The guidance differs some with the large scale ascent to the northwest of the surface low. This will be key in determining not only the precipitation amounts but the rates. Some guidance suggest that a ribbon of enhanced lift occurs associated with a rapid increase in frontogenetic forcing between about 850 mb and 700 mb. This may become enhanced for a time Saturday night as our region is within the right entrance region of a potent 250 mb jet. This will be a quick hitting storm, however there is the potential for a swath of snowfall rates of near 1 inch per hour. As of now, we are thinking this is near and just north and west of I-95. However despite the surface high well east-northeast of our area, some hints of cold air damming lingering and this could be enhanced some due to wet bulbing effects. Therefore, a thumping of snow may very well occur for parts of our region namely near and west of I-95. Farther to the northwest across our area, there looks to be less QPF but snow to liquid ratios should be the highest in the vicinity of the Poconos. Farther south and east across Delmarva and central to especially southern New Jersey, enough warming should result in a zone of some mixing (mainly rain/snow but some sleet is possible briefly) with any snow going over to rain closer to the coast. In between these zones, a wetter snow if enough accumulates at a fast rate could result in some power outages. We continue to not include any ice amounts as freezing rain may be limited and this is much less certain given marginal surface temperatures where it could occur. Given the above, the quick hitting nature of this event and our latest forecast snow amounts, we opted to go with a Winter Storm Watch for Saturday night for all of eastern Pennsylvania (except Delaware and Philadelphia counties) and northern New Jersey. It is here where we have the highest snow amounts which could approach warning criteria for some areas but this is uncertain, as is the western fringe due to potentially less QPF. Since there is the potential for some higher totals and enhanced rates and to also help with our messaging efforts, we felt a watch was a good way to go at this point. To the east of the watch area, a tier of advisories will be possible. Our precipitation types were derived from a blend of several models with some adjustments then made. The system quickly moves away early Sunday and all the precipitation should be done by 5 AM. As the flow turns northwest and increases some with drier air arriving, clouds are expected to clear out for much of the area. The flow is progressive however and therefore Sunday afternoon will be on the milder side with plenty of snow melting occurring. For Monday and Tuesday...A pattern change results in much more amplified flow, with a large trough settling in across the West while a ridge strengthens in the East. This allows for significant warming in the East, with potentially some record warmth on Tuesday. High pressure offshore Monday will allow for a return flow along with warm air advection. Low pressure in the Central Plains allows a warm front to extent eastward Monday. This warm front is forecast to be more pronounced with time aided by downstream warm air advection. As this occurs, the bulk of the model guidance shows an area of ascent with a warm air advection burst which results in a swath of showers/rain Monday afternoon before lifting northward. The warm front should work northward Tuesday with possible showers with it. The extent of the showers on Tuesday will depend on how quickly the warm front advances northward and the associated lift shifts to our north. It is possible that we start Tuesday with some areas of low clouds/fog, however the overall pattern is rather warm and therefore the warm front should progress steadily northward. Based on the northward advancement of the warm front with at least partial clearing, Tuesday afternoon could get quite toasty for much of the area. It is during this time where some locations could be near record warmth. Regarding the chance for showers, we continued with a time of chance PoPs and Tuesday could end up being dry as we get more into a true warm sector. For Wednesday and Thursday...As an upper-level trough slides eastward from the Northern Plains Wednesday, an elongated surface cold front will be shifting eastward with some ripples of low pressure riding along it. This boundary will be running into a lingering ridge near and east of the southeast U.S. coast, therefore the eastward progression of the front may be slowed some. The northern side of the ridge however is forecast to flatten some and this would allow the cold front to arrive in our later Wednesday. The front may get hung up to our south Wednesday night and Thursday as strong high pressure builds to our north and a ridge remains across the Gulf Coast states. We therefore continue with a mention of showers Wednesday (no thunder at this time), and some slight chance to low chance PoPs on Thursday as some guidance shows a wave of low pressure riding the front. Wednesday is expected to be another rather mild day ahead of the front and it could be warmer than currently forecast, then cooling occurs at night and Thursday. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. This morning...Generally VFR cigs but with patches of MVFR or even IFR near showers. Wind trending northwest. This afternoon...Generally VFR CIGS in last bands of showers asstd with possible MVFR VSBY, ending late in the day from nw to se. Northwest wind increases considerably toward 22z when gusts should become widespread 15-25 kt. Tonight...VFR. Gusty northwest winds 15-25kt early become northerly toward 09z/17 and max gusts diminish to 15 kt or less. OUTLOOK... Saturday and Sunday...Clouds increase and lower during Saturday, then MVFR/IFR Saturday night through very early Sunday morning as snow moves through. There could be some mixed precipitation near and and east of the I-95 terminals, with any snow or rain/snow mix going to rain at MIV and ACY. Some accumulating snow is forecast for most terminals with the higher amounts just west of the I-95 terminals. Conditions are expected to quickly return to VFR Sunday as clouds clear. High confidence on precipitation, lower confidence on where mixed precipitation occurs. Monday...MVFR/IFR possible as showers move through. Southwest winds may gust to around 20 knots during the afternoon. Tuesday...Possibly some low clouds and/or fog to start, otherwise mainly VFR. Southwest winds may gust up to 25 knots during the day.
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&& .MARINE...
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SCA Atlc waters of SNJ and DE is marginal and mainly for seas today. Northern waters was delayed till later in the day as was De Bay. Fog may be a marine hazard this morning. Northwest wind gusts 25-30 kt should develop late this afternoon or evening and continue much of tonight. Thereafter, winds trend north and gusts diminish to 20 kt toward sunrise Saturday. OUTLOOK... Saturday and Sunday...The conditions are anticipated to be below Small Craft Advisory criteria despite low pressure tracking to our south and east. Monday and Tuesday...The conditions are anticipated to be below Small Craft Advisory criteria overall, however southerly winds start to increase late Monday through Tuesday. Seas may start to build close to 5 feet on the outer waters late Tuesday.
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&& .CLIMATE... Some record high temperatures could be challenged on Tuesday, February 20th. These records are listed below. Atlantic City...71 in 1930 Philadelphia....70 in 1939 Wilmington......71 in 1930 Allentown.......68 in 1930 Trenton.........70 in 1930 Georgetown......68 in 2002 Mount Pocono....59 in 1930 Reading.........72 in 1930 && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Winter Storm Watch from Saturday evening through late Saturday night for PAZ054-055-060>062-101>106. NJ...Winter Storm Watch from Saturday evening through late Saturday night for NJZ001-007>010-012-015. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EST Saturday for ANZ452>455. Small Craft Advisory from 10 AM this morning to 6 AM EST Saturday for ANZ450-451. Small Craft Advisory from 4 PM this afternoon to 6 AM EST Saturday for ANZ430-431. && $$ Synopsis...Gorse Near Term...Drag 652 Short Term...Drag 652 Long Term...Gorse Aviation...Drag/Gorse 652 Marine...Drag/Gorse 652 Climate...652

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