Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 210806 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 406 AM EDT Mon Aug 21 2017 .SYNOPSIS... High pressure over the area moves offshore tonight, then slowly drifts out to sea through Tuesday. A cold front approaches from the west on Tuesday, then works its way across the region on Wednesday. Canadian high pressure builds into the region late in the week and into next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Quite a few adjustments to the forecast for Monday, with more likely on the way. Biggest change was to slow the onset of cloud cover a little bit during the day, especially from the Delaware River eastward. Latest model guidance shows a somewhat slower onset with increased clouds, both with the developing cumulus field during the day and with high-level cirrus as upper-level moisture progresses eastward. That would be good news for solar eclipse fans, but confidence is not particularly high with the sky cover forecast given the uncertainties associated with the extent of the cumulus field that develops (a planetary boundary layer process, which in forecast land is typically low-skill, even in the short range). However, confidence is increasing that storms will break out to our west in the afternoon and move into western portions of the area by late afternoon or early evening (likely after eclipse time). With aid from a low-amplitude vort max rapidly progressing through Pennsylvania during the afternoon, smaller- scale lift provided by differential heating (perhaps via increasing cloud cover to the west versus little to the east), terrain effects, etc. will likely be sufficient to develop scattered convection. Storm-scale environment is not too shabby either, with MUCAPE (SBCAPE) approaching/exceeding 2000 (1500) J/kg in the western CWA by late afternoon and effective shear increasing to 30-40 kts. With a deep layer of strongly positive buoyancy (i.e., relatively "thick" CAPE profiles) and wet bulb zero heights potentially as low as 10-11 kft during peak heating, such magnitudes of shear may generate updrafts sufficiently strong for hail in addition to strong winds. Will evaluate the environment further, but may add some enhanced wording to the Wx grids during the afternoon hours west of Philadelphia.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/...
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Some mid-level shortwave energy will pass through the region Monday night. Onshore flow will usher a warm and humid airmass into the region, as surface dewpoints will climb into the upper 60s to low 70s, and into the mid 70s across the Delmarva and extreme southern NJ. This will result in isolated to scattered showers and thunderstorms in the evening, gradually tapering off after midnight. Patchy fog may form in the pre-dawn hours going into Tuesday morning.
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&& .LONG TERM /TUESDAY THROUGH SUNDAY/...
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On Tuesday, a longwave trough associated with an upper low near James Bay is forecast to amplify over the Great Lakes region as shortwave energy digs around the backside of the trough. The surface low will deepen in response to increasing height falls as it tracks northeastward across Ontario and Quebec. The attendant cold front will progress eastward through the Midwest. Looking farther downstream and closer to our area, models show development of a pre- frontal/lee trough over central PA and Potomac Highlands during the afternoon hours. Tuesday will clearly be the hottest of the next 7 days with forecast highs in lower 90s (except cooler 80s near the coast and at higher elevations in NE PA/NW NJ). The humidity will also be a factor with dewpoints reaching the mid 70s across Delmarva and southern NJ and lower 70s just about everywhere else. Peak heat indices in the upper 90s to near 100F are likely for the urban I-95 corridor and points south/east. With the expectation that focused lift will remain to our west near the pre-frontal trough on Tuesday, convection over the forecast area will likely be disorganized and generally isolated in coverage (perhaps widely scattered across NE PA where the terrain could locally enhance lift) during the afternoon. Slightly better chances for showers and storms arrive Tuesday evening as convection along this trough propagates eastward into our area. PoPs are highest in our far western zones (west of the Delaware Valley and near the eastern shore of MD). Showers and storm are then expected to move eastward into the Delaware Valley and coastal plain at night as the cold front catches up with the pre-frontal trough and moves into the area. Even though the environment should be supportive of showers and storms continuing into the night owing to the arrival of deep synoptic lift, PoPs decrease slightly with eastward extent due to some uncertainty concerning the intensity and coverage of convection later with the nocturnal boundary layer tending to stabilize further into the night. The approach of a low- and mid-level jet ahead of the cold front will yield increasing shear profiles (0-6 km bulk shear 35-45 kt) by evening. Additionally, weak to moderate amount of instability (MLCAPE 1500 to 2000+ J/KG) is forecast to be available to fuel convective updrafts. Some storms could be strong to severe, primarily during the afternoon and evening hours, with this setup. The latest D2 Convective Outlook from SPC includes the southern Poconos in a slight risk for severe storms with a marginal risk extending southeastward into the I-95 corridor. The latest 00Z models have trended faster with the cold front as it moves through eastern PA and NJ early Wednesday morning. The front will then likely slow down when it reaches Delmarva and perhaps eventually stall just south of the area across southern VA Wednesday afternoon. Accordingly, it is looking more probable that the majority of the forecast area dries out on Wednesday, especially north of the Mason-Dixon line. While PoPs were also lowered farther south, the close proximity to the cold front means that there is still a chance that showers and storms linger into the day. Additionally, some of the models (most prominently, the 00Z NAM) develop a wave of low pressure along the stationary front in southeastern VA. More organized convection could shift back northward toward Delmarva and possible far southern NJ by evening. High pressure over Canada and the Great Lakes region then starts to build southeastward toward the area on Thursday. This expansive high will remain in control through the weekend. The end result will be an extended stretch of pleasant weather Thursday through Sunday. Highs in the upper 70s/lower 80s and and lows ranging from the mid 50s to lower 60s are forecast each day and night, respectively. While the forecast remains dry for these four days, there looks to be increasing cloud cover heading into the weekend and potentially a risk for an isolated shower with 1) the approach of the upper trough and 2) light onshore flow helping to moisten low-levels over time.
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&& .AVIATION /08Z MONDAY THROUGH FRIDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. For the 06Z TAFs...Added some TEMPO fog to KRDG, KABE, and KMIV based on latest trends, with brief sub-VFR conditions possible, especially at KRDG/KMIV, through 12Z. Cumulus field will develop during the day, with bases generally in the 4-6 kft range. Added PROB30 -TSRA to KRDG/KABE given somewhat increased confidence that convection will move into eastern PA late this afternoon into the evening hours. Generally increasing cloud cover from this afternoon through tonight, and latest guidance shows at least some potential for sub-VFR CIGs/VSBYs Monday night. Winds generally light/variable through 15Z, then becoming mostly southerly 5-10 kts. Outlook... Tuesday...Early morning fog should dissipate by mid morning, leading to VFR for most of the day. Showers and storms develop during the afternoon but current thinking is coverage should initially be isolated and mainly confined to western terminals (ABE/RDG). Tuesday night...Potential for MVFR or IFR restrictions with slightly higher chances for showers and storms. This activity should progress to the east-southeast through the night. Wednesday...Scattered showers and isolated thunderstorms mainly for terminals S/E of PHL and earlier in the day. May start off MVFR with lower CIGs but improvement to VFR is likely from NW to SE during the day. A wind shift from SW to NW can be expected early in the day w/ fropa. Wednesday night through Friday...VFR and light winds.
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&& .MARINE...
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Winds and seas remain tranquil across the waters through Monday as high pressure gradually shifts offshore. Seas will remain two feet or less through Monday afternoon, with a few 3 foot seas possible off the northern NJ coast by evening as southerly winds begin to strengthen. Outlook... Tuesday...SCA was issued for the Atlantic coastal waters of NJ and DE for late Tuesday afternoon and night. S-SW winds are expected to strengthen late in the day to 15-25 kt. Isolated gusts to 30 kt are possible mainly during the evening when the winds will be strongest. Seas will quickly build in response to the wind field to 4-6 ft. Wednesday...A wind shift from SW to W-NW should occur on Wednesday behind a cold front. Winds will also decrease as well. There is still a possibility that the SCA may need to be extended into Wednesday morning if seas take a bit longer to subside below 5 ft. Thursday and Friday...Winds and seas below SCA criteria. RIP CURRENTS... At this point we expect the risk for the formation of dangerous rip currents today to begin low. However, with winds increasing out of the south and the dominant period trending longer, the risk could increase as we go through the day depending on how quickly the wind shift occurs. For now we will mention a low risk in the surf forecast but continue to monitor closely especially as we go through the day on Monday. The rip current risk will likely increase to moderate on Tuesday
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&& .TIDES/COASTAL FLOODING...
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Based on observations yesterday evening and current projections for the high tide this evening, will hold off on issuing a coastal flood advisory this morning. Current model projections continue to indicate levels reaching near minor flood thresholds but consistently below advisory thresholds. Spotty minor flooding is probable this evening, but confidence is too low at this point to issue an advisory. Nonetheless, there is some concern that a more onshore component of the wind (south to possibly south-southeast) may contribute to somewhat higher levels than guidance suggests this evening. Should this be observed during the day, the threat for minor flooding would increase.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 3 PM Tuesday to 6 AM EDT Wednesday for ANZ450>455.
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&& $$ Synopsis...Klein Near Term...CMS Short Term...MPS Long Term...Klein Aviation...CMS/Johnson/Klein Marine...Johnson/Klein Tides/Coastal Flooding...

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