Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 141808 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 208 PM EDT Fri Jul 14 2017 .SYNOPSIS... A cold front will move through the region tonight into Saturday before stalling off the Carolina coast this weekend. High pressure will build across the region this weekend. An area of low pressure develops along the stalled boundary and starts to move northward into our area late Monday and Tuesday. High pressure will build into the region for Wednesday. Another cold front will slowly drop down from the north on Thursday. && .NEAR TERM /UNTIL MIDNIGHT TONIGHT/...
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Severe Thunderstorm Watch #414 is in effect for SE PA, including Philadelphia, as well as S NJ, E MD and DE through 9 PM. A line of strong storms are already starting to organize near the differential heating boundary/stationary front in southern PA. Strong heating south of the front has resulted in a moderately unstable airmass with MLCAPE ranging from around 1000 J/kg in far SE PA and adjacent S NJ to 2000+ J/kg over the mid Delmarva, per SPC mesoanalysis. It is these areas along and south of the warm front that will be conducive for severe thunderstorms. Expect to see pockets of damaging winds within the line segments that move across the Watch area. Similar to yesterday, there may be a brief spin up with storms that track along the stationary front, where winds are backed out of the SE and E. Two additional headlines are in effect. A Flash Flood Watch for portions of SE PA, S NJ, NE MD and DE through midnight. These locations will be closest the frontal boundary, where the focus for convection will be later this afternoon and evening. An increasingly stable boundary layer farther north into NE PA and N NJ will have a tough time supporting deeper convective updrafts needed for heavy rainfall rates and flash flooding. Poor drainage flooding is still a possibility though. A Heat Advisory is in effect for central and southern DE and adjacent eastern Maryland. Temperatures are close to forecast but dewpoints are several degrees higher (mid to upper 70s vs forecast in the low to mid 70s). Observed heat indices are in the 100-105F range as of 2 PM this afternoon. Discussion from this morning... Surface analysis this morning shows a 1013 mb surface low over Lake Ontario. A frontal boundary extends southward from the low into central PA and then curves back eastward near the Mason- Dixon line. This boundary did move southward as a cold front through our area yesterday before stalling over the Delmarva Peninsula last night. The front may drift back to the north and east through this afternoon in response to backed flow from the southwest ahead of the aforementioned low to our northwest. Another very active weather day is expected for our area with the front overhead. The main concerns for today will be locally heavy rainfall/flooding and severe thunderstorms. However, you can even add heat to the mix for Delmarva. Consideration for a Flash Flood Watch for a portion of the forecast area will be made by midday after some of the 12Z guidance starts to come in. A bands of locally heavy rain (1.5-2.5 inches) fell over the past 24 hours from Berks County eastward along the northern suburbs of Philadelphia and the I-195 corridor in central NJ. An additional swath of enhanced rain fell last night and early this morning just north of I-80 in NE PA and NW NJ. Flash Flood Guidance is considerably lower (less than 2 inches in three hours) in these locations due to the wet antecedent conditions. While there is still uncertainty regarding where the axis of heaviest rain sets up this afternoon and evening, it will likely be in the vicinity of the aforementioned frontal boundary. This would put SE PA, S NJ and Delmarva at highest risk. Some of the guidance does hint at another band of heavier rain farther north on the cool side of the boundary back toward the I-78 corridor. It is important to note that just about all of the model guidance yesterday was too far north with the storms and axis of heaviest rainfall. This may happen again today, so confidence for heavy rainfall that far north is considerably lower. SPC continues to highlight SE PA, the S half of NJ and Delmarva in a slight risk for severe storms. Visible satellite imagery shows s strong heating is underway south of the front from about the Mason- Dixon line southward. This differential heating boundary will be the focus for the development of numerous storms this afternoon with the greater risk for severe storms residing along and to the south of this heating boundary. This environment will be highly conducive for damaging wet microbursts with a delta theta-e of 25-30 C in the lowest 15-20 kft of the atmosphere. The timing for severe weather looks to be fairly similar to yesterday...about 4-8 PM. There will be a notable SW to NE temperature gradient across the region with highs ranging from only the low 70s north of the front in the southern Poconos and NW NJ to the mid 90s south of the front in eastern MD and interior southern Delaware. Expect differential heating near this front (breaks in the clouds leading to strong heating south of the front) to produce an impressive temperature gradient in a short distance (10+ degree temperature difference within 20 or 30 miles) around the greater Philadelphia metro and coastal NJ. With dewpoints in the 70s, heat indices look to peak in the 100-103F range for mid Delmarva. Any higher heat index values would warrant a Heat Advisory.
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&& .SHORT TERM /MIDNIGHT THROUGH 6 AM SATURDAY/... The showers and thunderstorms will likely be ongoing in our region this evening with a continuing threat of heavy rain and strong winds. The precipitation is forecast to pass off the coast during the overnight hours but clouds and patchy fog should linger for much of the late night period. A mesoscale low is expected to develop around northern Virginia and Maryland late today and it should pass off the New Jersey coast overnight. A cold front approaching from the northwest is anticipated to arrive toward daybreak. The wind should become light and variable for much of the night, settling into the northwest around 5 to 10 MPH with the arrival of the cold front. Minimum temperatures are expected to favor the middle 60s to the lower 70s. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Saturday and Sunday...The cold front drops south and east of the area on Saturday and stalls off the Carolina coast, where it remains through the weekend. Weak high pressure will build into our area for the weekend, keeping us dry for at least a couple of days. Might be a bit cloudy to start the weekend off but overall skies will become mostly to partly sunny across the region. With drier air in place, it should feel pretty comfortable outside even with highs reaching into the mid to upper 80s. Monday through Tuesday...A low pressure system will develop along the stalled boundary to our south and will start to move northward on Monday. The models continue to show the low moving up through Virginia on Monday and then into Delmarva and through southern New Jersey on Tuesday. The low then pushes to the north and east of our area late Tuesday into Wednesday. A chance for showers and thunderstorms will exist as the surface low makes its way into our area. A surface trough also develops across our area again and this would provide a good source of lift, promoting convective activity. In addition, an upper level trough will build across the Mid- Atlantic. This will help to deepen the southerly flow into our region as the atmosphere becomes more unstable. Best timing for convection, at this juncture, looks to be mainly during the afternoon and evening hours. Wednesday through Thursday...High pressure will build in behind the departing low with clearing expected for Wednesday. The upper trough slides to the east during this time. A cold front will start to drop down from the north on Thursday. As the high builds in, we will see a return to more of a west to southwest flow, ushering in some warmer, and moister, air to the region. Highs will start to creep back up and into the upper 80s to lower 90s by Wednesday and then into the l ow to mid 90s for Thursday. Increasing moisture will also mean increased humidity across the area. Seems reasonable that some showers and thunderstorms may develop during the afternoon and evening on both days but location of activity is too difficult to pinpoint this far out. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. It is a very challenging aviation weather forecast for today with showers this morning, followed by another round of showers and storms mid to late afternoon and evening. Timing of the showers and thunderstorms is a low confidence forecast. Temporary restrictions in visibility below 1 SM is possible with these storms as they contain heavy rain. Locally strong wind gusts are also possible with a few storms, especially farther south and west toward RDG, PHL, ILG and MIV. CIGs are also a challenge as they lower quickly from NE to SW from VFR (ILG, MIV) to MVFR (PHL) to IFR (RDG, ABE) to LIFR (TTN, PNE). Expect this to continue today with some gradual improvement (perhaps more than gradual around RDG and Philly terminals later this morning as a warm front lifts to the N/E). A light winds over the area varies from NE to across ACY to E along the I-95 terminals to S at RDG. Variable flight conditions are forecast to persist through the nighttime hours. The wind is expected to become light and variable for tonight and patchy fog may develop. OUTLOOK... Saturday...VFR conditions expected. West winds around 10 knots. Sunday...VFR conditions expected. Light northwest winds early, becoming south to southeast around 10 knots or less. Monday and Tuesday...Mainly VFR conditions expected. Afternoon showers and thunderstorms with MVFR or lower conditions possible. South to southeast winds around 10 knots. && .MARINE...
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A stalled frontal boundary is forecast will remain over our southern waters through this afternoon with a cold front approaching from the northwest late tonight. The wind on the coastal waters of New Jersey and Delaware is expected to settle into the east and southeast around 10 knots for today and it should become south to southwest for tonight as the cold front approaches. Wave heights will favor 3 to 4 feet on our ocean waters today and tonight and they should be 1 to 3 feet on Delaware Bay. The main concern for today is strong thunderstorms with the risk greatest in the DE Bay and southern coastal waters from mid afternoon through mid evening (3:30-10 PM). A Severe Thunderstorm Watch is in effect for the coastal waters from Little Egg Inlet to Fenwick Island through 9 PM. Numerous SMWs could be needed for convective wind gusts in excess of 34 kt. OUTLOOK... Saturday through Tuesday...Sub-Small Craft Advisory conditions are expected on the area waters through Tuesday. Locally strong winds may accompany thunderstorms on Monday and Tuesday. RIP CURRENTS... There is a low risk for the development of dangerous rip currents along the coasts of New Jersey and Delaware today. A low risk is anticipated again for Saturday.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...Flash Flood Watch until midnight EDT tonight for PAZ060-070- 071-101>104. NJ...Flash Flood Watch until midnight EDT tonight for NJZ016>018- 021>025. DE...Flash Flood Watch until midnight EDT tonight for DEZ001-002. Heat Advisory until 8 PM EDT this evening for DEZ002-003. MD...Flash Flood Watch until midnight EDT tonight for MDZ008-012- 015. Heat Advisory until 8 PM EDT this evening for MDZ012-015-019- 020. MARINE...None. && $$ Synopsis...Meola Near Term...Klein Short Term...Iovino Long Term...Meola Aviation...Iovino/Klein/Meola Marine...Iovino/Klein/Meola

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