Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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756 FXUS61 KPHI 251810 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 210 PM EDT MON JUL 25 2016 .SYNOPSIS...
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A warm front will slowly push northward across the region today. Meanwhile, a cold front will approach from the west, moving into the region tonight and Tuesday, before stalling near or just to the south of the area through the midweek period. A couple of areas of low pressure may develop and move along this boundary as it remains over the area for the end of the week.
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&& .NEAR TERM /UNTIL 8 PM THIS EVENING/... 1 PM update...Severe Thunderstorm Watch 415 was just issued. The watch includes all of eastern PA and all of NJ except the four southern most counties in the state and is in effect until 8 PM. Updated hourly Pop and Weather grids through tonight were already published around noon. Added severe thunderstorm wording to the forecast, highlighting the potential for damaging winds and locally heavy rain. Also, added more timing details of the convection based on the expected downshear progression of the storms. The HRRR seems to have the best handle on convective initiation with the lee trough just to our west as well as the more organized convection that is tracking across southern NY. It should also be noted that the SVA 415 may need to be locally extended in time by an hour or two across the southern and eastern fringe of the watch box. Mid morning discussion... The MCS that moved through the area early this morning continues to move off the coast mid morning. Clouds and precipitation had delayed heating so far this morning as temps were still in the 70s in parts of central and eastern NJ as of 10 AM. However, subsidence in wake of the exiting mid-level shortwave trough has resulted in extensive clearing of the debris clouds farther to the west. Temperatures have quickly jumped into the mid 80s to near 90F across eastern PA and the Delmarva region where the clearing has already persisted for more than an hour. The humidity will be a factor into the excessive heat today with a very muggy airmass is in place. Dewpoints are currently in the low to mid 70s across the majority of the area (upper 60s in northeast PA and northwest NJ; upper 70s in the mid Delmarva region). There is quite a bit of model spread regarding how these dewpoints readings change over time . We think typical model biases are showing up in the moisture fields with the RAP/HRRR overdoing the amount of mixing of drier air aloft (they show dewpoints dropping into the mid 60s across most of the area this afternoon) while the GFS/NAM are too moist (the GFS has an 80F dewpoints in eastern PA this afternoon). We took a middle solution of these two extremes, which would result in dewpoints remaining near currently readings (maybe lowering a couple of degrees). It`s important to nail down the dewpoints to determine how extreme our heat index rises this afternoon as well as how much instability will be available to fuel afternoon storms. After updating the hourly grids at 930 AM, heat indices are similar to previous forecast so there are currently no plans to make changes with the heat headlines. We anticipate a lull in precipitation across the region thru at least midday before the boundary layer full destabilizes during peak heating. Modified OKX and IAD RAOBs for this afternoon shows about 1500-2500 J/kg MLCAPE for this afternoon). Convective initiation will likely occur closer to 1 or 2 PM along a pre- frontal/lee trough just to our west. If the convection is able to quickly grow upscale and organize along this lee trough, then this line of storms may race eastward into our far western zones in PA between about 2-4 PM, the Delaware Valley/I-95 corridor between about 4-6 PM and the coastal plain between about 6-8 PM. Morning runs of both the HRRR and HRRRx are indicating a second line of storms trailing back to our north and west that would be associated with a mid-level shortwave trough moving thru Upstate NY. If this were to happen, then another round of storms could threaten the same areas in eastern PA and NJ this evening. The coverage of storms is expected to be more isolated in the Delmarva initially until the line of storms makes it that far south closer to sunset (and may not until after dark in our farthest southern zones). The latest D1 outlook from SPC places all of our eastern PA and NJ zones as well as northeastern MD and northern DE in a slight risk for severe storms. A marginal risk extends farther south into the Delmarva due to lower coverage of storms. The primary threat is damaging winds. Even though mid-level lapse rates are weak, there isn`t an abundance of dry air aloft, and the wind field in the boundary layer is weak (less than 25 kt), the environment will be supportive of strong downbursts with almost an inverted V sounding below the LCL. Storms that form in such an anomalously hot, moist and unstable environment such as the one in place tend to produce severe episodes. The other concern will be locally heavy rainfall that could result in flash flooding as storms develop in a moisture-rich environment (PWATS 2-2.25), especially if multiple rounds of storms affect the same area. && .SHORT TERM /8 PM THIS EVENING THROUGH 6 AM TUESDAY/... The threat of severe thunderstorms should be trended downward after sunset although we will still have to watch out for additional strong to severe storms toward the Delaware Bay region (far southern NJ and Delmarva) during the late evening if the unstable airmass has yet to be worked over by prior convection. We`ll also have to watch how the orientation of the convective line evolves over time. If the strongest part of the line bows out, the trailing portion of it may become oriented more west to east and allow for storms to train along it. The threat would then transition from winds to flash flooding across these southern areas later this evening and maybe into the overnight. The cold frontal boundary will push through tonight with thunderstorms clearing the coast early in the evening. However, another muggy night is expected with lows still in the mid and upper 70`s for most of the region. && .LONG TERM /TUESDAY THROUGH SUNDAY/...
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The cold front will be through much of the area before it stalls in the vicinity of Delmarva Tuesday and Tuesday night. It will then remain nearly over the Delmarva through the middle of the week. Drier and less hot air will try to filter into the region from the north. Areas north of Philadelphia will see some relief from the heat and humidity with high temperatures trending down to the lower 90s by Wednesday with dew point temperatures dropping into the lower to mid 60s. Philadelphia metro and southern New Jersey will also see high temperatures drop into the low 90s, but will likely see dew point temperatures remain in the middle to upper 60s. Finally, further south across the Delmarva, the only relief will be the lower temperatures as dew point temperatures will remain in the 70s. This area will also likely continue to have a chance of a few showers and thunderstorms due to the more humid airmass and the proximity to the frontal boundary. Areas further north should stay dry for the middle of the week. As the week progresses, the frontal boundary will shift slightly northward resulting in a return to more humid conditions and a threat of showers and thunderstorms across the entire region. The models are starting to hint that a couple of areas of low pressure will develop and move along t VFR conditions early this afternoon. Showers and thunderstorms are currently developing to our north and west. These storms are expected to increase in coverage/strength and move southeast through the early evening. Tempo groups have been added to the TAF`s to account for this potential of lower ceilings and VSBYS. Storms should only last for an hour or two at any given location. However, the exact timing is still uncertain. There could be more than one round of storms as well. The first round of storms may affect the Phila area terminals between 20-22Z with another line possibly moving thru between about 00-02Z. Outside of the storms, southwesterly wind gusts up to 20 knots could occur in the afternoon as well. Mainly VFR tonight after thunderstorms end with winds shifting to westerly late under 10 knots. However, patchy fog may develop late, especially at the fog-prone terminals, if an extended period of clearing occurs. Outlook... Tuesday through Friday...Mainly VFR conditions expected. Scattered showers and thunderstorms Thursday and Friday, with MVFR/IFR conditions possible at times.he boundary. This will increase the amount of clouds, humidity levels and the chance of showers and thunderstorms for the end of the week and the weekend.
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&& .AVIATION /18Z MONDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of today...VFR conditions continue this afternoon. South to southwest winds around 10 knots or less with a few gusts up around 20 knots through the rest of this afternoon. Scattered showers and thunderstorms are developing to the north and west of the terminals and will continue to push to the east through this evening. Expect showers and thunderstorms to impact the terminals, mainly between 20-02Z, with MVFR/IFR conditions possible. Tempo groups have been added to indicate the most likely timing of storms at the terminals. Overnight...Behind the storms, winds will shift more to the west, slightly northwest, but remain under 10 knots. Patchy fog will develop in the more prone areas as winds go light and skies largely clear. Tuesday...Patchy fog early, clearing by around 15Z. Then VFR conditions expected for the remainder of the day. Westerly winds around 10 to 12 knots through the afternoon. Outlook... Tuesday night through Friday...Mainly VFR conditions expected. Scattered showers and thunderstorms Thursday and Friday, with MVFR/IFR conditions possible at times.
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&& .MARINE... Seas will start the day near two feet likely rising in the southern waters during the afternoon and evening hours getting close to four feet. Some southwest wind gusts around 20 knots could occur in the afternoon hours today. Outlook... Tuesday through Friday...Sub-advisory conditions are expected to continue on the area waters through much of the week. Expect higher winds and waves in and near scattered showers and thunderstorms that become more likely at the end of the week. Rip Currents...Today`s risk for formation of dangerous rip currents is low. However if the wind were to turn south 15 kt and a 4 ft 5 second southerly swell were to develop late in the day, we`d probably experience a low enhanced risk along the NJ coasts. A south to north longshore current will prevail. Beyond Monday: with the Atlantic Basin continuing quiet tropically, unless we get a strong onshore flow...RC risk this coming week should be generally low. Tstms and pockets of chilly upwelling water may the greater concerns. There is some sign of colder upwelling along the coasts this evening. && .CLIMATE... The forecast high temperature for Philadelphia today, July 25th, is currently 98F. The last time the temperature reached or exceeded 98F at PHL was three years ago on July 18, 2013. The last 100-degree day at PHL occurred four years ago on July 18, 2012. Record high temperatures for Monday through Thursday are below. We may tie or break daily record highs tomorrow at several of our climate sites. KABE and KRDG had record equaling highs on Sunday the 24th. Site 25th 26th 27th 28th ---- ---- ---- ---- ---- PHL... 96-1899 101-1892 101-1940 100-1941 ABE... 95-1999 98-1940 98-1955 97-1949 ACY... 99-2010 96-2011 99-2005 98-1999 ILG... 96-1987 99-1894 100-1894 101-1894 TTN... 97-1999 99-1892 100-1894 101-1894 GED... 99-2010 97-2012 98-2005 99-1949 RDG... 96-1999 99-1940 98-1955 99-1941 MPO... 90-1999 89-1949 91-1955 93-1949 && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Heat Advisory until 6 PM EDT this evening for PAZ054-055- 060>062-101-103-105. Excessive Heat Warning until 6 PM EDT this evening for PAZ070- 071-102-104-106. NJ...Heat Advisory until 6 PM EDT this evening for NJZ001-007>010- 012>014-016-020>027. Excessive Heat Warning until 6 PM EDT this evening for NJZ015- 017>019. DE...Heat Advisory until 6 PM EDT this evening for DEZ002>004. Excessive Heat Warning until 6 PM EDT this evening for DEZ001. MD...Heat Advisory until 6 PM EDT this evening for MDZ008-012-015- 019-020. MARINE...None. && $$ Synopsis...JK Near Term...Klein Short Term...Gaines/Klein Long Term...JK Aviation...Meola/JK Marine...Gaines/JK Climate...

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