Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 241710 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1210 PM EST Sat Feb 24 2018 .SYNOPSIS...
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A warm front south of Delaware into central Virginia will edge north into southern New Jersey Sunday morning where low pressure will develop on this front and scoot out to sea in the afternoon. The trailing cold front will stall in the Carolinas Monday awaiting the passage of another wave of low pressure to our southeast. Strengthening high pressure builds into the mid Atlantic states Tuesday. A cold front will drop down through the region on Wednesday. Low pressure crosses our area Thursday, probably intensifying into a large storm somewhere off the mid Atlantic or New England coast Friday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... The primary forecast challenges today will be lingering fog across portions of far southern NJ and Delmarva, the extent of precipitation moving in from the west and high temperatures. Indications are that fog will continue into the early afternoon from Cape May County into the southern half of Delaware and the Eastern Shore of Maryland. We will extend the Special Weather Statement, as visibilities have not dipped below 1/4 mile. Precipitation will initially be concentrated along the PA Turnpike into early afternoon as drier air has worked in just north of this area, as evidenced by the sun breaking out this morning. As the atmosphere gradually saturates, the precip will fill in north of this area later in the afternoon, especially north of I-78. Temperature-wise, no major changes. Areas mainly north of the PA Turnpike did see an upward bump this morning with the sun breaking out but will soon cloud over, so this should not have a big impact on the forecast. Previous discussion... Several changes were made to the forecast today. Fog is getting dense in Chester County and the Maryland Eastern Shore. Have issued a dense fog advisory through 8 am for these areas, and there is at least some chance the advisory may need to be carried eastward into Delaware given short-term model forecasts and recent trends in DOV/GED surface observations. Visibility should improve by mid to late morning, but patchy fog and even some drizzle look to remain through much of the day across the area. This leads to concern number two, which is the forecast highs today. Inherited forecast simply looks too warm for the given pattern. Despite potential for rising cloud bases (albeit gradually) during the day, the overcast will very likely stick around. Though it has been a warm February, it is still February -- and with my pessimistic sky forecast today, I am awfully skeptical temperatures will reach the upper 50s in portions of SE PA/S NJ. Generally, I lowered temperatures 3-5 degrees across the area, and I am wondering if this is enough, especially if this morning`s observations agree with the colder NAM. Forecast temps may require more fine-tuning in future updates, but I felt that starting the trend downward was warranted. Several midlevel perturbations move northeastward into the northern Mid-Atlantic today, and with favorable positioning of the northern- stream 250-mb jet max (right-entrance region dynamics in play) combined with low-level isentropic ascent along/north of a baroclinic zone stretching zonally across the region, periodic bouts of rain look to occur in the CWA beginning this afternoon, especially north of the Mason-Dixon Line. There is still some variability in timing of the precipitation, so I kept hourly PoPs in the 50-80 percent range for the afternoon until there is more solid agreement amongst the hi-res models. Nevertheless, rain is a pretty good bet in PA/NJ this afternoon with diminishing chances in southern MD/DE. QPF is generally a quarter inch or less through 00Z Sunday, though amounts may be a little higher than that in the Poconos and Lehigh Valley if the more aggressive models are correct. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... One round of rain should be diminishing and moving away from the region during the evening hours, but conditions will likely once again be favorable for low clouds, fog, and drizzle during this time. Trended PoPs downward during the evening hours, but maintained categorical PoPs after midnight as the next round of rain is expected to move in. Kept mention of fog through the night, but generally kept drizzle confined to the evening hours, as steadier rain is more likely after midnight, especially north of the Mason- Dixon Line. This next round of rain will occur as yet another vort max approaches the region in midlevel southwesterly flow between 06Z and 12Z Sunday. Isentropic ascent poleward of the quasi-stationary baroclinic zone will allow for precipitation to break out again (with help from the steady 250-mb jet max to our north). The enhanced ascent provided by the stronger vort max, the slowly poleward-moving warm front, and the considerable upper-level divergence aided by the jet max will likely lead to more formidable rainfall totals during this period. Generally, amounts should be approaching a half inch to an inch north of the Mason-Dixon Line by night`s end with lighter amounts to the south. Fairly decent agreement among hi-res models on QPF, so confidence is slightly above average. Temperatures will not fall too much as the warm front makes its move (slowly) northward, overcast conditions continue, and warm advection curtails nocturnal cooling. Forecast lows are in the mid to upper 30s in the Poconos and northwest New Jersey to the mid to upper 40s southeast of the Fall Line. Will need to keep an eye on temperatures in the Poconos, as models tend to have a warm bias in this area north of warm fronts, and even a couple degrees cooler could mean at least a chance for some isolated slick spots. For now, chances are too low for mention. && .LONG TERM /SUNDAY THROUGH FRIDAY/...
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This section will update by 4Pm but here is a draft of the thinking, taking into the 12z/24 cycle of NAM, GFS, GGEM and UKMET. Hazards: None explicitly and maybe there wont be any this long term period but potential exists for a small stream hydro hazard somewhere in the CWA Friday, and possible winter wx hazard, a possible coastal flood hazard depending on the position of the western Atlantic cyclone as well as its intensity and a Gale event, again dependent on the latitude of the cyclone. 500 mb: a short wave in the mid Mississippi Valley 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 Wednesday morning starts reenergizing in the Ohio Valley Thursday, becoming a large cyclonic circulation system along the mid Atlantic coast Friday, slowly weakening seaward next weekend. Temperatures: Please see the climate section for updated February climate stats. Calendar day averages should be around 10 degrees above normal Monday, nearly 8 degrees above normal Tuesday, around 10 degrees above normal 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. Forecast basis: Unless otherwise noted, a 50 50 blend of the 12z/24 GFS/NAM MOS will be used Sunday night - Monday night, the 12z/24 GFS MEX MOS for Tuesday, and the 15z/24 WPC 12 hr elements of max/min temp and pop, as well as 6 hourly dewpoint/wind/sky for Wednesday- Saturday. The dailies... Sunday night...Variable high cloudiness. Northwest wind. Confidence: Well above average. Monday...Northwest wind gust 15 mph during the day and possibly 15 to 20 MPH at night as instability transfer deepens with cold air advection aloft. 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 southern DE for a time during the day. Confidence: Above Average. Tuesday...Sunny and beautiful! Light wind. Confidence: Well above average. Wednesday...Partly sunny (lots of cirrus) and a few degrees warmer. Southwest wind may gust 20 mph in the afternoon. Confidence: Above Average. Wednesday night-Thursday...In association with either a cold frontal passage or a warm front east-southeastward extension from the Midwest low pressure system, a period of rain is possible, primarily within 4 hours of sunrise. Winds becoming east, but may shift west late Thursday? Confidence: MUCH Below Average on any details. Friday into Saturday...Gusty north to northwest winds and periods of precipitation. For now rain most of Friday and thereafter unknown. Confidence: below average
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&& .AVIATION /18Z SATURDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...IFR to LIFR conditions in low clouds and fog were beginning to lift this morning, albeit temporarily at some locations. Still some uncertainty with regard to timing the improvement in CIGs/VSBYs with MVFR or even VFR possible this afternoon. The exception will be ABE and RDG, where periods of rain this afternoon should maintain MVFR ceilings. Winds becoming northeast under 10 kts. Moderate confidence. Tonight...Prolonged sub-VFR conditions likely as fog/drizzle are expected this evening followed by steadier rain after midnight. Winds generally east or northeast under 10 kts. Moderate confidence. Outlook... This section will further update by 4PM. Sunday...IFR conditions expected with limited improvement through the day. Confidence is low that we will return to VFR during the day. East to southeast winds will become southwest to west by the afternoon. Moderate confidence. Sunday night...Improving to VFR cirrus by midnight. Any rain showers will taper Sunday evening and conditions are expected to improve. Northwest wind. Confidence: well above average. Monday...VFR bkn-ovc clouds aoa 8000 ft, clearing late. Northwest wind gusting 15-20 kt, especially late day. Confidence: above average. A little concerned about low pressure forming vcnty N Carolina and spreading rain into southern DE for a time during the morning. 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...will be added at 4PM.
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&& .MARINE...
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Generally sub-advisory winds/seas expected through this evening. However, winds will begin to increase on the northern/central NJ coastal waters late tonight as east to northeast winds begin to crank up in response to an approaching storm system. Have issued a small craft advisory for these areas beginning at 3 am Sunday. Visibility restrictions are expected to continue through this evening for Delaware Bay and the Atlantic coastal waters of southern New Jersey and Delaware, so the Dense Fog Advisory was extended through 7 PM. Another round of dense fog is possible tonight. Rain chances will increase this afternoon and tonight, especially off the NJ coast. Outlook... This section will further update by 4PM. Sunday...Seas are expected to build and exceed 5 feet on the ocean on Sunday. Winds also pick up through the day Sunday with gusts of 25 to 30 knots possible. Confidence: Average. Sunday night...Lingering SCA seas possible. Confidence: Average. 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...will be added at 4 PM.
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&& .HYDROLOGY... Periods of rain through the weekend could result in minor and poor drainage flooding. The heaviest rain is expected Saturday night and Sunday, when up to additional inch of rain will fall. By the time the rain ends Monday morning, 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 Sunday night and Monday, there will be rises on areas 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. && .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 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... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 7 AM Sunday to 7 AM EST Monday for ANZ452>455. Dense Fog Advisory until 7 PM EST this evening for ANZ430-431- 453>455. Small Craft Advisory from 4 AM Sunday to 7 AM EST Monday for ANZ450-451. && $$ Synopsis...Drag 1210P Near Term...CMS/Franck Short Term...CMS Long Term...Drag 1210P Aviation...CMS/Franck/Miketta Marine...CMS/Franck/Miketta Hydrology... Climate...

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