Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 182337 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 637 PM EST Sat Feb 18 2017 .SYNOPSIS... Low pressure in southeast Canada will send a cold front southward into and through our area Sunday or Sunday night. Canadian high pressure will extend south into the Mid Atlantic States Monday, then move into the western Atlantic late Tuesday. A warm front passes eastward through our area early Wednesday. A cold front may sag southward into Pennsylvania and New Jersey Thursday night. It should move north of our area Friday night as strong low pressure moves from the Great Lakes into southeast Canada. Its trailing cold front is expected to move off the East Coast later Saturday. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... 630 pm update: Surface analysis valid at 23Z indicated a 988-mb low in southwest Quebec with a trailing (weak) cold front pushing through the eastern Great Lakes as warm air surges northeastward downstream of it in the warm sector. An associated vorticity maximum in southeast Canada will progress southeastward to the Canadian Maritimes by 12Z Sunday as a reinforcing shortwave trough digs into southern Quebec. Meanwhile, a southern stream upper low in vicinity of the Ohio and Tennessee Valleys will trudge eastward to the Carolinas by 12Z Sunday. Weak isentropic ascent just downstream of this upper low will aid in the generation of light precipitation, possibly not much more than sprinkles, in the Mid Atlantic west of Chesapeake Bay this evening and tonight. The eastward extent of these sprinkles is the main forecast question overnight, with high-resolution models consistently showing sprinkles or very light rain showers progressing northeast into central/southern New Jersey and Delmarva in the 03Z-15Z time frame. Precipitation will have a hard time reaching the ground given the relatively dry low levels (and weak overall ascent). Nevertheless, did expand the sprinkle mention northward generally to the south and east of I-95 and increased the duration of this mention through most of the night. Tonight will be considerably warmer than last night -- probably by 15-25 degrees -- given the residual warm air advection in place downstream of the aforementioned cold front. Current temp forecast looks OK. However, did lower dew points this evening given recent obs. Previous discussion below... A spring-like afternoon in progress as low-level warm advection has increased as a warm front continues to lift north of our area. The 12z Sterling, VA raob showed +14C at the top of a strong low-level inversion (about 1500 feet). Some cooler temperatures are noted right along the coast though with a more southerly wind component off the chilly ocean water. Through tonight, our region is mostly between systems. One tracks well to our north as the parent trough slides from Hudson Bay toward the Canadian Maritimes. This will however push a weak cold front into our area later tonight. The second system is a closed low that will open up/weaken as it tracks eastward from the Tennessee Valley to the southern Mid Atlantic overnight. It is mostly the southern system that is driving an increase in mostly high level cloudiness across our area this afternoon and especially tonight. There is a low-level jet around 40 knots forecast to develop close to our northern zones for a time tonight before shifting into coastal New England. This will aid in some moisture advection; however, forcing with the incoming cold front is weak and moisture looks rather limited. As a result, perhaps some sprinkles across mostly our southern zones overnight. Given the expected cloud cover on the increase tonight in combination with a southwesterly wind, will result in milder temperatures. Low temperatures are mostly an even blend of MOS and continuity. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... Surface low pressure will move into the Canadian Maritimes Sunday, with a weakening upper-level trough sliding offshore of the southern Mid Atlantic region. A weak cold front associated with the northern surface low will settle across our area but then tend to stall or even wash out. There is a secondary cold front that is forecast to stay to our north during the day Sunday, and colder air is found behind it. The initial front has little change in temperature with it with just some weak cold air advection forecast in the afternoon; however, the boundary layer remains warm. The low- level flow turns more from the west and northwest during the day which will add a downslope component for the coastal plain. As a result, another warm afternoon is expected although the far north should be a bit cooler given the weak cool air advection settling in. Some spots, mainly from Philadelphia on south and east, may be very close to their record high temperature for the date. An area of mainly mid level clouds associated with the southern system will exit to the east in the morning, with an increase in sunshine expected. High temperatures are a MOS blend, however these were boosted a bit especially for the coastal plain (I-95 south and east) given a downsloping component to the flow. It should be warmer at the coast given enough of an offshore flow. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... 500 MB: A southeastward moving strengthening short wave in Quebec Sunday night moves past the Maritimes Tuesday while a strong ridge in the Great Lakes slides east. A weak short wave cuts eastward into the ridge across New England Wednesday. In the wake of the short wave, a ridge dominates over the mid Atlantic States Thursday into Saturday before a strong short wave moves into the Great Lakes over the weekend. Temperatures: As our forecast area proceeds to a top 5 warmest February on record and a top 12 warmest winter in the period of record dating back to the late 19th century, the coming week of Monday through Saturday should average 10 to 15 degrees above normal. The coolest day is expected to be Tuesday...only several degrees above normal, but thereafter, some model guidance points to a three day period of temperatures 15 to 20 degrees above normal between Thursday and Saturday. BUT... there may be a a wrinkle developing. Dependent on the strength of the short waves exiting the western USA late this week and weekend, there is a chance of a low level cooler-colder flow setting up near and north of I-78 Friday onward. Forecast basis: Unless otherwise noted, a 50 50 blend of the 12z/18 GFS/NAM MOS guidance Sunday night-Monday night, thereafter the 12z/18 GFS MEXMOS Tuesday, and then the 15z/18 WPC guidance Thursday - Saturday, except used the warmest avbl guidance for Wed-Thu which was the 12z/GFS MEXMOS which am confident is onto the warmth...and our forecast may be 5 degrees too cool Wed and Thu. As an example of guidance...todays high temp at phl 67... warmer by 3 degrees than ydys warmest guidance which was the highres NMM and WRF and against this mornings 00z/18 guidance was warmer than the warmest guidance which were the ADJUSTMET and HIRESWRF. Modeled MOS southwest flow warmth seems to have a cool bias. The dailies including any possible significant insight... Sunday night...Mostly clear. Small chc some strato cu toward dawn Monday (NAM has a layer of high rh near 950MB). Northwest wind gust 15 mph. Confidence: above average. Monday...After any low strato cu dissipate, mostly sunny with a cooler north northwest flow gusting near 20 mph. Still above normal temps. Confidence: above average. Tuesday...WAA increasing high clouds limiting max temperature potential. Chance showers may invade E PA late in the day in the leading portion of the instability burst. Wind light southeast to south. Confidence: average on max temps which have favored the milder of the avbl guidance. Tuesday night...Chance of light showers ahead of a warm front. Low probability there could be a touch of freezing rain high terrain north of I-80. This ice risk isn`t in the HWO or zones since confidence is below average and guidance favors min temps during any rain, above freezing. Confidence: average or below average on the occurrence of any rain. Wednesday...Becoming Sunny and warmer. Forecast temps may end up several degrees warmer than now predicted. West wind. Confidence: above average except confidence on max temps remaining this cool is below average. Thursday...mixed clouds and sun and warm. Max temps may end up 5f warmer than now predicted. Southwest wind. Confidence: above average. Friday...early morning stratus/fog along a cool front that has settled into our area. maybe a shower north of the boundary or maybe the boundary doesn`t make it down into our forecast area. otherwise variable clouds. Confidence: below average on the scenario. This ECMWF colder soln will need to be monitored for future impact in our area. Confidence below average on the temps and chance of any rain. Friday night...a good chance of showers ahead of the cold front moving east into our area or the ECMWF solution of a warm front moving north from I-78. Shower timing and temperature uncertainty as well as wind uncertainty, though probably turning southeast to southwest, depending on latitude in our area. Saturday...Maintained continuity despite the faster GFS solution. EC and GGEM operationals are slower and at this time, did not want to raise hopes for the GFS blustery dry westerly flow on Saturday. Confidence below average. && .AVIATION /23Z SATURDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. For the 00Z TAFs...VFR through the period. High ceilings this evening should rapidly lower to around or slightly below 10 kft by 06Z. BKN CIGs expected to continue through the 12Z-15Z time frame before dissipating during the day tomorrow. A sprinkle is possible, especially at KILG/KACY/KMIV, overnight. Light southwest winds will slowly veer to northwest by morning. Speeds should increase to around 7-12 kts during the day tomorrow, perhaps with an occasional gust to 20 kts. OUTLOOK... Sunday night...VFR northwest wind. MVFR sc may develop toward dawn. Overall confidence: above average. Monday...Possible MVFR sc early otherwise a VFR northwest to north wind... gusty 15 to 20 kt. Confidence: above average. Tuesday...VFR lowering high cigs develop during the day. chance of showers late in the day vcnty KABE KRDG. southeast to south wind. Confidence: above average. Tuesday night...Conditions may briefly lower to MVFR overnight into early Wednesday morning with a chance of showers and a southwest wind. Confidence: average. Wednesday...Becoming VFR and a west wind develops. Confidence: above average. Thursday...VFR. Southwest wind. Confidence: above average. && .MARINE... The conditions are expected to remain below Small Craft Advisory criteria through Sunday. A southwesterly flow through tonight will gust to around 20 knots at times, then turn west and northwest late tonight and Sunday as a weakening cold front moves through. OUTLOOK... Sunday night - Monday...Sub Small Craft Advisory conditions expected, although northwest to north winds may approach advisory levels across the northern New Jersey coastal waters overnight Sunday into Monday morning. Confidence: average. Monday night-Thursday...Sub Small Craft Advisory conditions expected although winds may gust up to 20 knots at times. Confidence: above average. && .CLIMATE... Allentown tied their record high for the date at 67 degrees (originally set in 2011). Record max temps Sunday that may be approached (within 2f) or equaled. PHL 68-1948 ILG 67-1997 ABE 63-1997 TTN 68-1948 GED 74-1976 && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Drag Near Term...CMS/Gorse Short Term...Gorse Long Term...Drag Aviation...CMS/Drag/Gorse Marine...Drag/Gorse Climate...CMS/Drag

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