Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 192009 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 409 PM EDT FRI AUG 19 2016 .SYNOPSIS... Our region will be under the influence of high pressure centered over the Canadian Maritimes through Saturday. Low pressure will move through the Great Lakes and into southern Canada over the weekend. Across the mid Atlantic, this will lead to a slow warm frontal passage Sunday, followed by a strong cool frontal passage Sunday night. High pressure over the Ohio Valley Monday will build into our region on Tuesday and Wednesday, then move offshore Thursday into Friday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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A broad area of high pressure was located over the eastern US this afternoon. However, there looks to be a very subtle surface trough forming this afternoon over the coastal plain. Although the flow is very light, surface convergence near the boundary has developed this afternoon as the sea-breeze circulation advances inland and the flow turns southeasterly to the east of the trough axis while remaining northwesterly farther inland. A few showers have developed along the sea-breeze front just inland of the Jersey shore. An additional pop-up shower or two may form over the ridge lines in northeast PA and northwestern NJ this afternoon but the main focus for additional convection late this afternoon and early this evening will be in the coastal plain of NJ, possibly reaching the I-95 corridor early this evening. Without a source of deeper, more organized lift, coverage should continue to be very low. SPC/LAPS mesoanalysis depicts a modestly unstable atmosphere over the coastal plain with MLCAPE values around 1250 J/kg. There is a mid-level subsidence inversion overhead that will hinder the potential for deeper convective updrafts.. Therefore, we expect the bulk of this activity to be showers though a rogue thunderstorm or two is possible through about sunset. This diurnally- driven convection should dissipate around or just after sunset. With a light pressure gradient overhead, expect winds to become calm at most locations late this evening and overnight. Good radiational cooling conditions are expected tonight with mainly clear skies and calm winds. A light onshore flow behind the sea-breeze fropa will advect slightly higher dewpoints inland through this evening, which will also help prime the boundary layer for the development of patchy fog tonight, especially toward daybreak Saturday morning. Low temperatures range from the low-mid 60s in northeast PA/ northwestern NJ to low 70s in the cities and along the coast.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
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The area will be situated between an old frontal boundary stalled to our south across VA and high pressure to our northeast across the Canadian Maritimes on Saturday. This high will nose southwestward down the eastern seaboard. With the surface ridge axis located off the mid-Atlantic coast, light onshore flow will persist across the region during the day. Similar to today, we expect to see isolated showers and storms develop during peak heating. The two potential sources for lift will be with the sea-breeze front and terrain circulations. Therefore, 20 PoPs were confined to the coastal plain and farther inland across the higher terrain. High temperatures will be similar to today... predominately in the mid to upper 80s with our warmest spots in the cities and interior Delmarva possibly reaching 90F. It will be slightly cooler (low 80s) near the coast and Poconos.
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&& .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
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The primary concern continues to be the potential for heavy rainfall and thunderstorms Sunday and Sunday night... A negatively tilted 500 hPa short wave, with heights around -3 Standard Deviations below normal, will traverse the Great Lakes over the weekend. The main uncertainty with this feature is the progress a warm front makes through our region Sunday, as there is good model agreement on a strong cool frontal passage Sunday night. Thereafter, a period of relatively fair weather expected Monday thru Thursday, while another cool front may approach our region by Friday. Temperatures will average near normal through the period for a change. In terms of the primary hazards Sunday... Heavy rainfall potential...A southerly low-level jet around 30 knots will transport anomalously moist air into the region, with Precipitable Water values over 2.0 inches, 2-3 Standard Deviations above normal. In addition, the area will be under the influence of the left-rear quadrant of the upper jet. The warm front will also provide a source of lift as it traverses the region. The heaviest rainfall potential will be northwest of I-95, where at least urban and poor drainage flooding is possible. Thunderstorm potential...This will be dependent on instability, as shear is not ideal. We are outlooked in general thunderstorms by SPC.
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&& .AVIATION /20Z FRIDAY THROUGH WEDNESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Mainly VFR through this evening. An isolated pop-up shower or storm possible in vicinity of I-95 TAF sites as well as MIV through about sunset. Light NW to NE winds currently reside inland while SE winds have developed closer to the coast with a sea-breeze moving inland. Expect winds to shift out of the SE at PHL and surrounding I-95 TAF sites within an hour or two of 00Z. Winds will generally have an onshore component to it tonight but they will be so light that the direction may be variable. Patchy fog and low clouds may develop early Saturday morning. Daytime mixing could produce a few hours of MVFR CIGS with stratocu developing during the mid to late morning. Mainly VFR in the afternoon Saturday although an isolated shower or storm is possible. This could bring very localized/brief restrictions if it moves over a terminal. OUTLOOK... Saturday night thru Sunday night...Ceilings are expected to lower in advance of a warm front, and these lower ceilings could persist into Sunday night. In particular, terminals northwest of I-95 have the greatest chance of MVFR and IFR ceilings. More widespread MVFR and IFR is possible as heavy showers and thunderstorms cross the region Sunday afternoon and early Sunday night. There is the potential for northwest wind gusts up to 20 knots at all TAF sites as the cold front crosses the region Sunday night. Monday thru Wednesday...Predominantly VFR conditions are expected.
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&& .MARINE...
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Light onshore flow (10 kt or less) that developed this afternoon will continue into tonight and Saturday. Wave heights will be in the 1-2 ft range thru tonight. It could increase to 2-3 ft in our coastal Atlantic waters in response to the persistent onshore flow. OUTLOOK... Sunday...Southeast flow across the waters in advance of a warm front could briefly reach Small Craft Advisory (SCA) criteria. Sunday night and Monday...As the cold front crosses the waters Sunday night, there is a better chance of northwest winds gusting to SCA criteria for a period of time, going into Monday morning. Tuesday and Wednesday...Winds and seas are expected to remain below SCA criteria. RIP CURRENTS...There is a low risk for the development of dangerous rip currents through the evening with a light onshore flow and relatively calm seas. The probable risk for the development of dangerous rip currents is low for Saturday. However, it may approach moderate in some spots during the afternoon with the establishment of a synoptic scale southeasterly wind around 10 mph.
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&& .CLIMATE... The heat wave of the past 7 to 8 days at many locations has ended. This lists the number of 90 degree or warmer days (as of August 18) and the mean number of 90 degree days for each climate site (computed from when the period of record began). We are above normal for 90 degree days at all climate sites. Site 90 deg days beginning of climate record 2016 mean POR ABE 28 17 1922 ACY 26 10 1874 GED 30 27 1998 PHL 33 21 1873 RDG 30 21 1999 TTN 27 20 1998 ILG 28 20 1894 && .EQUIPMENT...
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Due to equipment failure (A/C unit and computer), the KDIX WSR- 88D radar remains out of service. Return to service is still unknown at this time. Surrounding WSR-88D radars, including KCCX (State College) and KAKQ (Wakefield), are down with either equipment issues or maintenance. We will use TEWR and TPHL terminal doppler radars as well as surrounding WSR-88D radars that are available to help fill in the radar coverage gaps.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Franck Near Term...Klein Short Term...Klein Long Term...Franck Aviation...Franck/Klein Marine...Franck/Klein Climate...Drag Equipment...Klein

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