Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 081117 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 617 AM EST Thu Dec 8 2016 .SYNOPSIS... An area of low pressure will continue to slowly drift across eastern Canada today through Friday, and end up south of Greenland by Saturday. With high pressure to our west, a strong west to northwest flow will remain through the end of the week. High pressure briefly affects the area Saturday night, then moves offshore Sunday. A warm front is expected to lift across the area Sunday night, before an area of low pressure is forecast to move near or just north of the area around Monday. This system will pull a cold front across the area Monday, then another quick moving frontal boundary may move across the area around Tuesday night. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... There are two features we will be keeping an eye on today. For portions of Delmarva and far SE NJ: This morning, light rain will be possible in response to a low and mid level short wave trough lifting over the region. Lift is rather weak, so even if any rain develops, amounts are expected to be very light. Across the region: Northwesterly flow will be increasing through the day today in response to an increasing pressure gradient as the positively tilted trough continues to progress east. For most of the region, this will mean little more than breezy and cooler conditions through the day today. Temperatures today will be around 5 degrees lower than yesterday (in other words, near or slightly below normal). The Southern Poconos and NW NJ though may see flurries mid day into the early afternoon if lake effect snow showers are able to propagate this far south and east. A negating factor will be the dry air advection that we have already seen, so no accumulation is expected. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/... Colder air will continue to settle in over the region thanks to the continued northwesterly flow (and associated cold air advection). The pressure gradient across the region will continue to increase as the trough moves further east, so winds are not expected drop off much overnight. This will mean we won`t have efficient radiational cooling conditions, but given the cold air advection, lows tonight will as much as 10 degrees lower than what we will see this morning. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Low pressure continues to drift across eastern Canada Friday, then moves offshore and ends up south of Greenland by Saturday. Meanwhile high pressure remains across the central part of the country. This will keep a steady west to northwest flow across the area. On Friday, although the strong vorticity impulse is expected to be east of the area, there will remain steep low-mid level lapse rates across the area with enhanced moisture as well. With the strong northwest flow, it is possible that some lake effect streamers could set up and bring some snow showers/flurries to portions of the area. The best chances would be across the northern portions of the area, so we keep scattered/isolated snow showers/flurries there. Elsewhere we will keep isolated flurries/sprinkles. On Saturday, the steep lapse rates and moisture combination lifts northward some. We`ll keep isolated snow showers/flurries for the far northern counties, but will keep flurries/sprinkles for the middle half of the area as the northwest flow begins to weaken some, so the flurries may not have as great of a reach southward as on Friday. While most areas probably won`t see much snow accumulation Friday and Saturday, if any snow bands or squalls do develop, it would not be surprising to see some accumulating snow in some areas. High pressure briefly affects the area Saturday night, providing dry conditions. Friday and Saturday will be a cold and windy period, with Friday being the windiest, and Saturday being the coolest of the two. The high builds offshore Sunday ahead of the next storm system. As this happens, it is possible that some showers could develop during the day across the area as a short wave passes just to our north and spreads an area of moisture across the area. However, the best chance of precipitation looks to be Sunday night into Monday. A warm front is forecast to lift across the area late on Sunday night into Monday as an area of low pressure is forecast to move near or just north of the area. Conditions should be cold enough overnight Sunday into Monday for a period of snow to affect a good portion of the area, before much of the area warms up and precipitation turns to rain during the day Monday. There is the potential for an accumulating snow for much of the area, with the northern half having the greatest potential. As the low continues to lift northward, a cold front is expected to move across the area late in the day Monday and overnight. Precipitation chances should diminish behind the cold front. On Tuesday and Tuesday night, the GFS is indicating another cold front moving across the area as low pressure passes to our north. It in turn brings a period of rain and snow to the area late Tuesday into early Wednesday. The ECMWF does not have as strong of a cold frontal passage, so it does not bring any precipitation to the area. For now, we`ll introduce a small chance into the forecast. && .AVIATION /11Z THURSDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions are expected through the remainder of the TAF period. Mid level ceilings between 5000 and 9000 ft AGL will be possible at times through the day time hours. And towards the very end of the TAF period, a cloud deck of 4000 to 5000 ft AGL will propagate in from the west. Otherwise, only high clouds are expected through the TAF period. West northwesterly winds will increase by 18Z in response to an increasing pressure gradient. Gusts near 20kt will be possible primarily between 18 and 03Z. After 03Z, gusts may drop off (following typical nocturnal trends), but sustained winds will stay near or above 10kt. OUTLOOK... Friday-Saturday...Generally VFR, although CIGS may occasionally lower to MVFR. Scattered showers or flurries/sprinkles are possible during the daytime of each day which may temporarily lower conditions. Gusty west to northwest winds 15-25 knots, strongest winds Friday which may approach 30 knots at times. Sunday...VFR early, possibly lowering to MVFR by the afternoon. Rain or snow possible late in the day. Sunday night-Monday...Conditions continue to lower to MVFR and then IFR. Rain or snow becoming likely. Accumulating snow possible for eastern Pennsylvania, much of New Jersey, and the northern portions of the Delmarva.
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&& .MARINE... Winds will increase around mid day today for what will be a prolonged small craft advisory event. The strongest winds are not expected until this evening, when gusts near or just above 30 kt are possible. OUTLOOK... Friday-Saturday...Small Craft Advisory in effect through Friday night, which may need to be extended into Saturday. Winds may approach gale force at times on Friday. Saturday night...Winds expected to drop below Small Craft Advisory levels, but pick up again late Sunday. Sunday-Sunday night...Winds may increase and approach Small Craft Advisory levels at late in the day into the overnight. Monday...Small Craft Advisory conditions possible. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from noon today to 6 AM EST Saturday for ANZ430-431-450>455. && $$ Synopsis...Robertson Near Term...Johnson Short Term...Johnson Long Term...Robertson Aviation...Johnson/Robertson Marine...Johnson/Robertson is the U.S. government's official web portal to all federal, state and local government web resources and services.