Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 250753 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 253 AM EST Sun Feb 25 2018 .SYNOPSIS...
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A warm front will lift north and east of the area by early afternoon. Then a cold front will pass through the area late this afternoon and evening. Strengthening high pressure is then forecast to build into the Mid-Atlantic states Tuesday. A cold front will drop down through the region on Wednesday night while low pressure develops over Illinois. That low slides to the Mid- Atlantic coast by Friday morning, then intensifies into a strong storm over the western Atlantic Friday night before heading slowly out sea over the weekend.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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An elongated band of rain with embedded convection is moving slowly northward through the area early this morning. A few of the convective cores have produced lightning, so have added a slight chance of thunder to the grids to account for this. Model soundings have been showing this potential for a while now, with a nose of 200- 400 J/kg MUCAPE impinging upon the southern fringe of the precipitation shield. With strong lift in association with both synoptic and subsynoptic processes during the next few hours, think the midlevels will continue to cool sufficiently to allow for parcels rooted above the near-surface stable layer to become buoyant. Not only will this provide an environment favorable for isolated lightning strikes, but it will also permit fairly decent rain rates with the stronger downdrafts, given anomalous precipitable water values in place (1-1.4 inches) and deep/warm cloud depths. The HRRR is on the higher side of model consensus with QPF, and it has done a far better job than the NAM Nest and WRF- ARW/NMM in depicting the overall evolution of the precipitation the past few hours. Though I think the HRRR is somewhat overdone (which it may have a tendency to do in warm-front setups in this region), I did bump up the QPF a little bit through the day, with potential for 0.5-1 inch of rain in much of the region north of the Mason-Dixon Line. There should be a sharp drop-off south of there, with very little precipitation expected in far southern Delaware, which will be well displaced from the elevated isentropic lift atop the retreating warm front moving through the area today. The warm front is the main forecast challenge today. The song remains the same in forecast land, with experience suggesting models are way too gung-ho in moving the warm front north of the I-76 corridor by afternoon. Despite the intense surface low in the Midwest moving rapidly north-northeastward today, I am not convinced the near-surface stable layer will give quite so readily. For one thing, the layer is deeper than the models were depicting last night at this time, which suggests there is more work to be done in sweeping this denser air poleward. In addition, the model trends are in the colder direction, both operational and statistical. The MAV and MET have dropped 3-5 degrees for today`s highs in 24 hours, and the hi-res guidance is exhibiting similar trends. I followed suit, keeping highs in the 50s in Philly and the 40s north of I-80. On the other hand, it is quite possible I am a little too cool in Delmarva, where stronger mixing and a longer period without rain may allow for temperatures to exceed 70 this afternoon. The evolution of rainfall today is the other concern, with the consensus showing the main show between now and noon, with a good period of moderate precipitation around to a few hours after daybreak. Rain should wind down from west to east this afternoon, but there have been a couple model simulations suggesting the development of instability showers upstream of the main shield of rain this afternoon across the area, in closer proximity to the surface cold front. Certainly cannot rule it out, but the infrequency of this in the simulations suggests this scenario is fairly low-probability at this time.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
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As a deep trough lifts northeastward into Canada tonight and a vort max moves off the East Coast this evening, any remaining precipitation early this evening should cease quickly after sunset. However, skies may have a hard time clearing as residual upper and midlevel moisture remains, thanks in part to an intense 250-mb jet streak stretching from the Ohio Valley into New England. However, drier air near the surface will filter into the area as the cold front moves through the region. Dew points should drop from the 50s in the evening to the upper 30s by daybreak, but residual cloud cover and at least some mixing should keep temperatures reasonably mild. Forecast lows are in the low to mid 30s in the Poconos and northwest New Jersey, upper 30s to around 40 in the Lehigh Valley and the Pine Barrens of New Jersey, and mid to upper 40s in the urban corridor, the beaches, and most of Delmarva. The main concern tonight is for development of some patchy fog in the valleys, especially if winds can remain light and especially during the evening hours when residual surface moisture will be more abundant. At this point, given the substantial drying expected in the post-frontal regime, I think the odds are too low for inclusion in the forecast. Nevertheless, would not be surprised to see a few local spots with reduced visibility for a time after dark.
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&& .LONG TERM /MONDAY THROUGH SATURDAY/...
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Hazards: None explicitly attm but a few hazards are certainly possible with the potential for a powerful storm in the Thursday to Saturday time frame. These potential hazards include a small stream hydro hazard somewhere in the CWA on Friday, a possible winter wx hazard for elevations of ne PA and nw NJ, and a coastal flood hazard which appears likely but the severity just cannot be pinned down this far out. For now minor tidal inundation flooding expected for at Thursday and Friday morning high tide cycles, with at this time the higher departures (flooding potential) expected in NJ. A Gale event, the first 2 hour or longer gale since Feb 5, is expected but the exact time is not yet known. Storm force gusts are possible. 500 mb: a short wave in the central plains Sunday night will cross the Mid Atlantic coast Monday evening with east coast ridging to follow during mid week. Then a weakening closed low in the desert southwest Tuesday night starts reenergizing in the Ohio Valley Thursday, becoming a large cyclonic circulation system along the mid Atlantic coast Friday, and slowly weakening seaward next weekend. Temperatures: Please see the climate section for updated February climate stats. Calendar day averages should be nearly 10 degrees above normal Monday, Tuesday and Wednesday, 10 to 15 degrees above normal Thursday, before cooling to between 5 and 10 above normal Friday and 5 degrees or less above normal next Saturday. The dailies... Monday...Northwest wind gusts 15 mph during the day and possibly 15 to 20 MPH at night as instability transfer deepens with cold air advection aloft. Expecting considerable mid and high cloudiness into early afternoon Monday then clearing late in the day and at night. The short wave may spread a little rain northward into far southern DE for a time during the day. Highs in the low 50s north, upper 50s south. Confidence: Average. Tuesday...Sunny and beautiful! Light wind. Highs in the low 50s north, mid 50s south. Confidence: Well above average. Wednesday...Partly sunny (lots of cirrus) and a few degrees warmer. Southwest wind may gust 20 mph in the afternoon. Highs in the low 50s north, upper 50s south. Confidence: Above Average. Wednesday night-Thursday...In association with either a cold frontal passage or a warm front extension east-southeastward from the developing Midwest low pressure system, rain will developg either late Wednesday night or Thursday. It may become heavy in the afternoon. Winds becoming east. Highs in the upper 40s north, mid 50s south. Confidence: Below average on any details. Friday into Saturday...Probably gusty northerly winds and periods of precipitation. As of now, rain most of Friday and thereafter unknown, but potential exists for some elevation snow. Highs in the low 40s north, low 50s south on Friday, low 40s north to upper 40s south on Saturday. Confidence: below average Please see the WPC Day 7 winter wx outlook graphic and their day 4-7 qpf graphics issued this Sunday afternoon to gain further appreciation for the upcoming modeled-outlooked potential.
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&& .AVIATION /08Z SUNDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of tonight...Conditions currently deteriorating to MVFR/IFR CIGs and should become even more prevalent with time as rain moves in. Once the rain sets in, VSBYs should lower to at least MVFR (probably lower), especially if fog becomes more widespread near daybreak. However, fairly frequent variations in categories with both CIGs/VSBYs are expected through the night. Some lightning has been observed with the heavier embedded showers, but chances are too low for TAF mention at this point. Winds will remain east-northeast around 5-10 kts. Moderate confidence. Sunday...Prolonged sub-VFR expected through most of the day, with IFR or worse conditions most likely during the morning hours. Rain should begin to diminish during the afternoon from west to east, but sub-VFR CIGs/VSBYs may linger longer, perhaps through most of the daylight hours. Winds will become variable during the afternoon but should remain below 10 kts. Moderate confidence. Sunday night...Conditions should improve to VFR during the evening hours from west to east with any remaining precipitation ending. There is some potential for patchy fog at the susceptible sites (RDG, ABE, TTN, MIV), but confidence is too low for mention in the TAFs at this point, especially as drier (albeit light) northwest surface winds commence during this period. Moderate confidence. Outlook... Monday...VFR. Northwest wind gusting 15-20 kt, especially late day. Confidence: above average. Tuesday...VFR, virtually clear. Light wind. Confidence: well above average. Wednesday...VFR cirrus. Southwest winds may gust 20 kt during the afternoon. Confidence: Above average. Thursday...Conditions should deteriorate to MVFR or IFR in rain. east to southeast wind. Confidence: Average.
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&& .MARINE...
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Dense fog appears to be increasing again on Delaware Bay and the adjacent Atlantic waters, with models adamant that visibilities will drop precipitously late tonight, perhaps spreading northward through much of the New Jersey coastal waters as well. Should these trends be observed, the advisory may need to be expanded northward. Will keep a close eye on it for the rest of the night. For now, it remains in effect through mid morning. Regarding the small craft advisory through Sunday night, the wind forecast is trending downward, and I am becoming far less confident winds will mix to advisory levels in much of the advisory area (especially for the southern NJ and DE coastal waters). Will leave the advisory up for now and monitor trends through the next few hours to determine if the advisory can be trimmed for portions of the area. Rain is expected periodically through this evening, with attendant visibility restrictions expected. Fog may linger for much of the day on the waters, and an extension of the advisory in time is possible. Additionally, cannot rule out the possibility of a lightning strike with the stronger/embedded convective showers within the main shield of rain. Outlook... Monday through Wednesday...Sub-advisory conditions are expected on the area waters with northwest winds gusting 15-20 kt Monday becoming light Tuesday and southwest on Wednesday. Confidence: Average. Thursday...an easterly SCA likely with a possible Gale for the NNJ waters. Confidence: Average.
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&& .HYDROLOGY...
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Periods of rain today (Sunday) could result in minor/poor drainage flooding. The heaviest rain is expected across portions of eastern PA and northern NJ, with an additional inch of rain. By the time the rain ends this evening, the region will have received between 1 and 2 inches of rain since Thursday, with the highest amounts falling over the northern third of the forecast area. By tonight and Monday, there will be rises on area rivers and streams, with some possibly approaching bankful. Of note will be how much rain falls over the Susquehanna, as there is the potential for inconvenience flooding below the Conowingo Dam on Monday. We may need to think about a new hydrology section for late this week to cover NJ/PA but just too far in advance with plenty of uncertainty to add any confidently stated information, except that a fairly potent weather pattern appears to evolving for a portion of our forecast area.
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&& .CLIMATE... **Top 3 warmest February on record and top 10 wettest February on record.** February projected climate ranking as of Noon today-Saturday Feb 24, based on our mid shift fcst through the 28th and mins this Saturday morning; and for rainfall, amounts 1201 AM today. RDG and TTN not included due to too much missing data. ABE #3 warmest 38.0 or 7.3F warmer than the 30.7 norm. 39.2-2017 38.6-1998 38.0-2018 36.8-1954 ACY #1 warmest 42.7 or 7.4F warmer than the 35.3 norm. 43.0 -2017 42.7 -2018 40.6 -1954 ILG #3 warmest 41.3 or 6.3F warmer than the 35.0 norm. 43.1-2017 42.3-1903 41.3-2018 41.2-1976 PHL #3 warmest 42.5 or 6.8F warmer than the 35.7 norm. 44.2-2017 42.5-2018 42.2-1925 41.8-1998 41.4-1890 Water equivalent February pcpn as of 1201 AM today. PHL ranked #10 with 5.30". An additional .50 would raise to #5. All time 6.87-1896 ILG ranked #8 with 5.48". An additional .50 would raise to #5 All time 7.02-1979 ABE ranked #13 with 4.47". An additional .50 would raise to #5. All time 7.62-2008 ACY ranked #2 with 6.12". Wettest is 6.50-2010. Our expectation still is that ACY will exceed the previous all time record rainfall for February, by the end of the weekend. It could be a close though. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 7 AM this morning to 2 AM EST Monday for ANZ452-453. Dense Fog Advisory until 10 AM EST this morning for ANZ431- 453>455. Small Craft Advisory until 7 AM EST Monday for ANZ450-451. Small Craft Advisory from 7 AM this morning to 6 PM EST this evening for ANZ454-455.
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&& $$ Synopsis...Miketta Near Term...CMS Short Term...CMS Long Term...Drag/Miketta Aviation...CMS/Miketta Marine...CMS/Miketta Hydrology...Kruzdlo Climate...Drag

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