Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 262003 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 403 PM EDT TUE JUL 26 2016 .SYNOPSIS...
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A weak cold front will continue to slowly drift south across the region tonight before stalling nearby to our south Wednesday into Thursday. Several waves of low pressure developing along this boundary will result in periods of showers and thunderstorms late in the week and this weekend.
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&& .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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The cold front has finally moved into the forecast area this afternoon. The front was located near the I-80 corridor at 19Z and was marked by a subtle wind shift from more westerly to more northerly and a slight drop in dewpoints. The front will weaken as it continues to progress slowly southward through the region tonight. A pre-frontal trough was become increasingly diffuse as the front catches up to it, but it was located near the Delmarva coast back into southeastern VA. Surface observations reveal a dewpoint boundary near this residual surface trough with an axis of higher dewpoints in the 70s and instability along and south of the Delaware Bay. The Heat Advisory continues through 7 PM for Kent and Sussex Counties in DE and Queen Annes, Carolina and Talbot Counties in MD, where current heat indices are in the 101-108F range. Mid afternoon objective mesoanalysis from SPC shows about MLCAPE 1000-1500 J/KG over the mid and lower portions of the Delmarva (it actually shows a pocket of 2000+ J/kg, but that is likely tainted by the spuriously high 81F dewpoint at KESN). A few showers and thunderstorms are developing across these areas in concert with peak heating. Locally heavy rainfall cannot be ruled out with the airmass remaining moist (PWATs still around 2 inches), slow storm motion (under 15 kt) and the potential for multiple storms to develop along the quasi-stationary surface trough. The low coverage of storms (mainly isolated) and sandy Delmarva soils will limit the overall flash flooding risk. One or two storms may also contain locally strong winds given they are forming in an environment conducive for wet microburst. Expect showers and storms to generally dissipate around or shortly after sunset with the loss of daytime heating. However, we`ll have to monitor radar trends across WV and northern VA this evening. The HRRR continues to advertise a mid-level shortwave trough and accompanied upstream convection moving into our Delmarva and far southern NJ zones overnight. Models generally show dewpoints and elevated instability increasing overnight across these far southern zones. This signal supports the idea that this convective activity could be maintained well into the night. Reintroduced scattered showers and storms into the forecast for the Delmarva late tonight. Farther north into southeastern PA and central NJ, high clouds will stream into the area. Ideal radiational cooling conditions under mainly clear skies and calm winds will allow for patchy fog to develop before sunrise and temperatures to fall into the lower 70s across northeastern PA and northwestern NJ.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
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Some of the model guidance shows the cold front moving south of Delaware Wednesday morning while others keep the front stalled over our southern zones. Given it`s the middle of the summer and zonal steering flow aloft, we favor a more northern solution with the front stalling in the Delmarva. Expect additional showers and storms to develop along the lingering boundary during peak heating hours tomorrow afternoon. Clouds near the front will likely keep temperatures down a few degrees in DE and eastern MD (upper 80s) while stronger heating under mostly sunny skies in eastern PA and NJ will allow for temperatures to climb back into the 90s. However, we are looking at low 90s for highs and humidity levels will be comfortable with dewpoints in the 60s.
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&& .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
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The front will remain fairly stationary over the Delmarva Wednesday night into Thursday before starting to push back north as a warm front Thursday night into Friday. This boundary will act as a focus for weak low pressure development. These waves will then ride along the boundary and will result in a flare-up of showers and thunderstorms. Specific timing for this can be tricky and subject to change, but current thinking is for Thursday night into early Friday and then again on Saturday. Outside of these times, expect generally diurnal driven climatological threat of showers and thunderstorms. After a bit of a reprieve with lower humidity levels mid-week, dew points will increase back to 70-75 over the Philadelphia metro area, southern New jersey and the Delmarva by Thursday and Thursday night. This will push the heat index values into the upper 90s, almost 100 on Thursday. After that, expect lower heat indices for Friday and the weekend as high temperatures will be suppressed by increased cloud cover. It appears that the weather will remain warm, muggy and unsettled through the start of the next week, before the frontal boundary finally sinks far enough south to allow a marginally cooler, but drier airmass the infiltrate the entire region.
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&& .AVIATION /20Z TUESDAY THROUGH SUNDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Mainly VFR through the valid TAF period. The one thing we`ll have to watch for is patchy fog to develop late tonight in the fog-prone terminals (e.g., RDG, MIV). Showers and storms Wednesday afternoon will likely stay south of the terminals but it will could be very close by to ILG-MIV-ACY). W to NW winds around 10 kt are gusting 15-20 kt this afternoon. The winds will weaken after sunset before becoming light and variable tonight. Winds generally W-NW 5-10 kt on Wednesday but the sea breeze will likely result in a wind shift out of the S-SE at ACY and MIV. The sea breeze front could make it to the I-95 terminals toward sunset. Outlook... Wednesday night through Sunday...Mainly VFR conditions expected. Scattered showers and thunderstorms Thursday, Friday and Saturday, with MVFR/IFR conditions possible with any shower and thunderstorm activity.
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&& .MARINE...
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NW winds will continue to shift out of the south late this afternoon and evening as the sea breeze moves toward the coast. The wind are then expected to turn around toward the north late tonight as a cold front moves southward. Expect a similar wind shift to occur again tomorrow. Speeds should remain under 20 knots. Wave heights on our ocean waters are expected be 2-3 foot through the period. Waves on Delaware Bay should be 2 feet or less. Outlook... Wednesday night through Sunday...Sub-advisory conditions are expected to continue on the area waters through the forecast period. Expect higher winds and waves in and near scattered showers and thunderstorms that will be more likely at the end of the week. Rip Currents... The risk for formation of dangerous rip currents is low this afternoon and again Wednesday.
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&& .CLIMATE...
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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 Wednesday and Thursday are below. Site 27th 28th ---- ---- ---- PHL... 101-1940 100-1941 ABE... 98-1955 97-1949 ACY... 99-2005 98-1999 ILG... 100-1894 101-1894 TTN... 100-1894 101-1894 GED... 98-2005 99-1949 RDG... 98-1955 99-1941 MPO... 91-1955 93-1949
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...Heat Advisory until 7 PM EDT this evening for DEZ002-003. MD...Heat Advisory until 7 PM EDT this evening for MDZ015-019-020. MARINE...None. && $$ Synopsis...JK Near Term...Klein Short Term...Klein Long Term...JK Aviation...Klein/JK Marine...Klein/JK Climate...Staff

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