Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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877 FXUS61 KPHI 201047 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 647 AM EDT WED JUL 20 2016 .SYNOPSIS... A front will remain stalled over the Delmarva region and southern New Jersey today before finally pushing south of the area tonight. Meanwhile, high pressure will build into the area. The high is expected to move offshore Thursday and Friday. A cold front may move into the area and dissipate Friday night into Saturday. Strong high pressure then re-establishes itself into next week, but another cold front will slowly approach from the northwest late in the weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... High pressure was located over the Great Lakes region early this morning. Meanwhile, a stationary front was analyzed over the Washington DC area, extending eastward into the Delmarva and southern NJ. The airmass south of the boundary is moist (dewpoints in the upper 60s-lower 70s) and unstable (MUCAPE 1500-2000 J/KG) while much drier air (dewpoints in the upper 50s to low 60s) resides north of the boundary. Strong to severe thunderstorms develop in the unstable warm sector during the night as an upper shortwave trough moved southeastward toward the Chesapeake and Delaware Bays. Additional showers and isolated storms will continue to develop early this morning in the northern Delmarva and southern NJ, where low-level convergence along the front will provide enough lift for parcels to their LFC. The environment is slightly more stable and thus not as favorable for severe storms this far north as it is over the lower Delmarva. There is quite a bit of model disagreement regarding how the front moves over the course of the day. There will be a brief wind surge out of the northeast this morning as high pressure builds in from the north, so this may cause the front to drift southward a bit through midday. However, the front may briefly waver back to the north during the first half of the afternoon as light southerly flow develops in response to a portion of the elongated surface high moving offshore. We are confident that areas north of the PA turnpike and I-195 should stay dry and see plenty of sunshine today. Locations farther south into far southern NJ and the Delmarva are at risk of seeing additional showers and storms that form during peak heating this afternoon. Confidence for afternoon storms is highest across the southern half of DE and adjacent locations in eastern MD, since these areas will more than likely reside in the warm sector south of the front. We think the 00/06Z NAM is a little too bullish on low-level moisture and instability (NAM forecast profiles show 2-3 kJ/kg of SBCAPE for this afternoon) across our far southern areas. However, even if it`s overdone by a factor of two (which is more in line with the RAP/HRRR), there will be a significant dry layer present above the moist boundary layer. Accordingly, there is a conditional threat (dependent on thunderstorms developing) for pulse severe storms in this thermodynamic environment that is conducive for wet-microbursts. PoPs mostly follow a blend of HRRR/RAP/SREF with slightly modifications to remove the noise caused by the hi-res model forecast grids. It will feel very comfortable out today for a middle of summer day in the mid-Atlantic region. Temperatures will be in the mid 80s across most of the area; cooler along the shore and across the higher terrain. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/... The stalled front will finally push south of the area tonight as high pressure moves overhead. Shower or storms that develop this afternoon across the Delmarva and far southern NJ should dissipate toward sunset, although cannot rule out an isolated shower or storm in our southern most zones after dark where elevated instability is forecast to reside. The setup will be favorable for radiational fog with clear skies and light winds under the high. Not expecting fog to be widespread across the entire area though given how short the night is right now. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Hot and humid conditions rebuilding across the area late this week into this weekend and early next week, with dangerously hot conditions likely. The warming trend begins Thursday, though Thursday looks to be the least warm day. Troughing across the northeast begins to move out of the area Thursday, while ridging begins to build in from the west. This will allow thicknesses and 925/850 mb temperatures to climb through early next week. With the building thicknesses and 925/850 mb temperatures, daytime highs are expected to climb into the upper 80s/low 90s Thursday, then the low to mid 90s Friday, followed by mid to upper 90s Saturday, Sunday, and Monday, back to low 90s again by Tuesday. Heat index values will begin approaching 100 degrees by Friday, but the dangerously high heat index values over 100 are expected through the weekend into Monday. Rain chances through the extended will be centered around a couple of frontal boundary passages. The first is late Friday into Friday night, and the second period being Monday into Monday night. Behind the frontal passage, conditions will still likely be quite warm Tuesday, but not as hot as the weekend or Monday. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Isolated showers and storms in VC of MIV early this morning. These should stay south of ACY and ILG. VFR expected for the TAF sites today with most of the showers and storms remaining south of the terminals. The convection could be close to ILG- MIV- ACY though but confidence not high enough to mention with 06Z TAFs. Light N-NE winds early this morning will veer more out of the NE or E-N after sunrise. A brief wind surge near 10 kt could accompany this wind shift. Winds then become light and variable around midday before the direction favors more out of the S or SE late in the day. There is a chance for fog late tonight into the early morning hours on Thursday. Winds will be light and variable tonight. OUTLOOK... Thursday-Thursday night...VFR. Friday-Friday night...Generally VFR. A chance of showers and thunderstorms which may briefly lead to lower conditions. Gusty southwest winds 15-20 knots. Saturday-Sunday. VFR. && .MARINE... Storms are moving into the coastal waters off DE. These storms may pose a threat to mariners (locally strong winds and higher seas as well as lightning) early morning as they move eastward. Otherwise, winds and seas will remain below Small Craft Advisory criteria. Nly winds will veer out of the NE early this morning. The could gust 15 to 20 kt this morning. Winds will slacken a bit early this afternoon and then shift out of the S-SE late in the day. OUTLOOK... Thursday-Thursday night...Sub-Small Craft Advisory conditions expected. Friday-Friday night...Small Craft Advisory conditions possible with increasing winds ahead of an approaching cold front. Saturday-Sunday...Sub-Small Craft Advisory conditions expected. RIP CURRENTS... The risk for the formation of dangerous rip currents today will be low at the DE and NJ beaches, but very localized moderate rips are possible, especially along the NJ coast where onshore flow will be strongest this morning. We have had some reports of upwelling along the coast, where temperatures have dropped into the low 60s. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Klein Near Term...Klein Short Term...Klein Long Term...Robertson Aviation...Klein/Robertson Marine...Klein/Robertson is the U.S. government's official web portal to all federal, state and local government web resources and services.