Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 232245 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 545 PM EST Thu Feb 23 2017 .SYNOPSIS... A cold front will move into the Northeast tonight before stalling, then moving northward as a warm front Friday night. A strong cold front will move through the Mid Atlantic on Saturday. High pressure will pass through the region Sunday and Monday. A front will set up near the area for much of next week, with several lows expected to move along it. A strong low will move through the Great Lakes and southeast Canada next Wednesday and Thursday, which will pull another strong cold front through the region by late in the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... High pressure will continue to remain anchored over the western Atlantic Ocean. Persistent return flow around the backside of the high has resulted in another unseasonably warm day across the region. Temperatures this afternoon are in the mid/upper 60s in northeastern PA and northwestern NJ, mid 70s in central Delmarva and low 70s in between (except much cooler in upper 50s-mid 60s at the beaches due to the influence of the colder ocean waters). High temperature records were set today at several of our climate sites (see RERs and climate section below for more information). A cold front was analyzed well to our west across the eastern Great Lakes and Midwest region this afternoon. Recent visible satellite imagery shows a CU field developing out to our west in the pre- frontal warm sector as well as across the higher terrain in the southern Poconos. 20Z mesoanalysis from SPC shows 250-500 J/kg of SBCAPE over the area, but LAPS soundings show that the atmosphere is capped by a mid-level subsidence inversion. Hi-res CAM models are in good agreement, showing pre-frontal convection becoming more robust just upstream of us after sunset this evening across the higher terrain as lift strengthens and the cap is temporarily broken. Showers may become increasingly organized along a pre-frontal trough in our far northwestern zones of the CWA (RDG-ABE-MPO-FWN north and west) during the mid to late evening hours. A slight chance for thunderstorms was also mentioned for these far northwestern zones given the potential for slightly elevated convection to tap into 400-700 J/kg of MUCAPE. This activity will weaken overnight. Debris clouds from this evening`s convection contribute to forecast uncertainty regarding the development of stratus/fog overnight. This cloud cover would likely be more extensive north and west of I-95 (closer to the showers), which will like at least delay stratus/fog development. However, the clouds may dissipate by early morning, allowing for at least patchy dense fog to develop closer to sunrise, particularly where it rains. The S-SWly wind field at the top of the boundary layer looks slightly stronger tonight compared to last night, which favors more stratus vs. fog compared to last night. Nonetheless, there is a potential for dense fog again late tonight in most of the region, but confidence in it becoming more extensive than just patchy is not high enough to issue a Dense Fog Advisory this far out. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/... The cold front will dissipate inland by the morning. Similar to today, the day will start with low clouds and fog. The low-level flow is forecast to be ever-so-slightly backed tomorrow (Sly) compared to today (S-SWly), which may allow the stratus to hang on an hour or two later into the day than today. Regardless, there looks to be an opportunity for temperatures to spike to near record levels in the mid to late afternoon hours with clearing skies during peak heating. Trended warmer for tomorrow (closer to the RGEM), yielding highs in the low to mid 70s south of I-78 and mid to upper 60s farther north. && .LONG TERM /FRIDAY THROUGH THURSDAY/... The long term period looks active, as several systems are likely to affect the area through the middle part of next week. At 00Z Saturday, a strong vorticity maximum will approach the Great Lakes and Ohio Valley, with a 140-kt 250-mb jet streak positioned across much of the southern U.S. As the nose of this jet enters difluent flow near the Atlantic Coast, considerable upper-level divergence will collocate with differential cyclonic vorticity advection downstream of the aforementioned vort max. Strongly forced convection will develop along an associated cold front in the Midwest moving east into New York and Pennsylvania by late Saturday morning. A downstream jet streak will develop in the high-amplitude ridging nosing into eastern Canada, providing additional lift via jet coupling to produce widespread precipitation in Quebec, New York, and adjacent New England Saturday afternoon. As the vort max impinges upon the high- amplitude ridging, it will make a shift northward, leaving our region on the southern fringes of the stronger dynamics/lift and well within the warm sector of the system. Given sufficient diabatic heating, marginal boundary-layer based instability is expected by afternoon in our area, which may act to compensate for the somewhat weaker large-scale ascent. Vertical wind profiles will be quite strong, with deep-layer shear 40-60 knots. Unidirectional wind profile and orientation parallel to the initiating boundary will support a line of convection moving through the area during the afternoon. Strength of the wind profile and steep lapse rates near the surface suggest isolated damaging wind gusts are possible as the line moves through the region. The key to the extent of this threat will be the degree to which the boundary layer destabilizes during the day and the ability for updrafts to withstand the strong vertical wind profile -- the usual players in a high CAPE-low shear environment. Today`s simulations suggest there will be a window of at least partial surface heating just downstream of the cold front, with MUCAPE near 500 J/kg during peak heating. Maintained a threat of thunderstorms in the grids for Saturday afternoon. Models have sped up the timing of frontal passage a tad, and reflected this in the grids somewhat by increasing PoPs a bit earlier in the day and decreasing them a bit more Saturday evening/night. Temperature forecast on Saturday is a big challenge, as sky cover, precipitation, and timing of the frontal passage are all highly complicating issues. I went pretty close to statistical guidance, which was generally in good agreement despite the complex scenario. Nevertheless, there is large uncertainty and potential for error for highs Saturday, which currently are around five degrees lower than the values in place for Friday. A sprawling but progressive high will move in behind the front for Sunday and Monday, so a period of dry weather and much cooler (though still near to slightly above average) temperatures are forecast during this period. By Monday afternoon, model agreement becomes quite poor. These large discrepancies exist owing to an unusually high degree of spread associated with an upper-level low digging off the California coast this weekend and progressing eastward early next week. The GFS is very quick to move a perturbation from this upper low into the central plains by 00Z Monday and to the Mid-Atlantic States by 00Z Tuesday. Widespread precipitation breaks out on the East Coast Monday night in advance of this system. The ECMWF, meanwhile, is much weaker with this perturbation and basically shows no precipitation at all with its passage, but is more aggressive in breaking out scattered/light precipitation on Tuesday afternoon/evening as isentropic ascent increases downstream of a stronger system that develops in the central plains into the Great Lakes by midweek. As the low rapidly develops and lifts into southeast Canada by Thursday, a cold front sweeps through the East Coast Wednesday night and Thursday, bringing widespread precipitation through the area. Meanwhile, the GFS moves this low/associated precipitation mostly to the north of the area. Other guidance provides little insight, as the CMC becomes a strong outlier with the depth/orientation of the longwave trough beyond this weekend, and the ensemble guidance shows that the GFS/ECMWF solutions are nearly equally plausible. As a result, broadbrushed PoPs/sky cover/temps/winds Monday night through Thursday, with generally slight chance to chance PoPs during this period. There is at least some chance for a mix of rain and snow in the southern Poconos Monday night if the GFS solution pans out. Then again, if the ECMWF solution is closer to reality, there may be no precipitation at all... Temps during this period should be well above average. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions thru the remainder of the afternoon and into this evening for most locations. However, convective showers and perhaps an isolated thunderstorm may organize ahead of a cold front this afternoon, possibly reaching as far southeast as RDG-ABE during the late evening or overnight (02Z-06Z). We may need add a TEMPO group in there for RDG and ABE (currently advertised as PROB30). Another round of stratus and fog is likely late tonight (after 06Z), with the potential for LIFR conditions with 2-3 hours on either side of sunrise. Confidence is low for the formation of stratus and fog but low-medium for coverage and magnitude of the fog. It could certainly become dense with vsby near 1/4SM early in the morning. S-SW winds around 10 kt this afternoon will become light tonight and Friday morning, before increasing again to around 10 kt with gusts to 15 kt during the afternoon. OUTLOOK.. Friday night...Sub-VFR conditions possible with areas of low clouds and fog. Saturday and Saturday night...Line of showers/storms likely will move through the area during the day, with gusty/erratic winds and sub-VFR conditions likely during passage. South winds may gust at times above 20 kts before frontal passage. Winds will rapidly switch to west or northwest after the front passes, with gusts above 20 kts likely. Rapid improvement to VFR after frontal passage. Sunday...VFR with winds northwest 10-20 kts with gusts to 30 kts possible. Sunday night and Monday...VFR with winds near/below 10 kts becoming southerly by Monday. Conditions may become sub-VFR late in the day as another system approaches, but this is very low confidence. Monday night and Tuesday...Potential for sub-VFR conditions with precipitation, low ceilings, and reduced visibilities possible at times. Very low confidence. && .MARINE... No marine hazards expected this afternoon and evening. A S-SW wind around 10-15 kt this afternoon and evening will weaken overnight and into the morning. Another round of dense fog may occur over the waters either late tonight or early Friday morning. After coordination with neighboring offices, we held off on a marine dense fog advisory due to uncertainty in when the dense fog forms and how widespread it will become. There is a potential for the dense fog to persist into the daytime a few miles off the Atlantic coast. OUTLOOK... Friday night through Saturday night...Fog may again develop on the coastal waters, with at least some potential for visibilities less than one mile, as very warm/moist air remains over the relatively cold waters. A line of storms will likely move through Saturday afternoon and evening, with any fog rapidly dissipating as this occurs. Seas are expected to build to small craft advisory (SCA) thresholds on Saturday, and winds will become westerly and become gusty after cold frontal passage. Gusts may approach gale levels late Saturday night. Sunday and Sunday night...SCA northwest winds continue but should decrease below advisory thresholds Sunday evening. Monday through Tuesday...Generally sub-SCA conditions expected, though winds may approach advisory levels for a brief period Monday afternoon. && .CLIMATE... **Many records today contribute to a record warm Feb and a top 10 warmest winter in the period of record for Mount Holly FA** As of 5 PM...records were set at GED 75, MPO 63, RDG 73, ABE 72 As of 5 PM...records were tied ACY 72, ILG 73 For reference, here are/were the high temperature records for today, Friday and Saturday (not including the new records so far today). Location Thursday 2/23 Friday 2/24 Saturday 2/25 ACY 72-1985 75-1985 77-1930 PHL 75-1874 74-1985 79-1930 ILG 72-1985 78-1985 78-1930 ABE 71-1985 76-1985 74-1930 TTN 74-1874 74-1985 76-1930 GED 67-1985/1990 72-1961 76-1975 RDG 72/1932/1922 77-1985 77-1930 MPO 60-1977 60-1984 70-1930 Todays February rankings were updated at 515 PM. We`ll finish the seasonal AT 6 pm. The following are the monthly and seasonal expectations. It is virtually certain that these values will be at or below reality and that our forecast area is experiencing a record warm February and a top 10 warmest winter. Records date back to the late 19th century. Details below. February: PHL 43.9 #1 Normal 35.7 Record 42.2-1925 POR 1874 ABE 38.9 #1 Normal 30.7 Record 38.6-1998 POR 1922 ACY 42.9 #1 Normal 35.3 Record 41.6-1890 POR 1874 ILG 43.1 #1 Normal 35.1 Record 42.3-1903 POR 1895 Note for ABE: There is a pretty good chance ABE will end up warmer and possibly very close to their monthly temp record. Winter (DJF) PHL 40.3 #7 and solid. It wont slip. ABE 35.8 #5 and solid. It wont slip. ACY 39.8 #9 and may rise ILG 39.4 #5 tie Past two years of monthly average temperatures through February 2017, a summary of above normal months listed below: For ABE: 23 consecutive months of above normal temps! FOR PHL: 22 of the past 23 months have been above normal. For ACY: 19 of the past 23 months have been above normal. For ILG: 17 of the past 23 months have been above normal. (Jan Feb March 2015 was the last time we had significant and persistent below normal monthly temps.) Snow: Atlantic City should end up tied for 5th least snowiest February on record 0.3" && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...AMC Near Term...Klein Short Term...Klein Long Term...CMS Aviation...CMS/Klein Marine...CMS/Klein Climate...544P updated seasonal is the U.S. government's official web portal to all federal, state and local government web resources and services.