Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 252226 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 626 PM EDT Sun Jun 25 2017 .SYNOPSIS... A large area of high pressure will slowly move through the eastern U.S. early this week before moving offshore Wednesday and Thursday. A series of surface lows will move from the northern and central plains to the Great Lakes and Northeast from mid to late week, ushering in warmer and more humid air late this week and the upcoming weekend. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... The mid and upper level shortwave trough now over the eastern Great Lakes region will lift northeast through the evening and early overnight hours. A few additional showers are possible across portions of the CWA through the early evening hours. However, after 00Z, the diurnally driven activity will dissipate. The latest run of the HRRR indicates that even the more widespread shower activity associated with the main trough axis now in western Pennsylvania will also dissipate by midnight. Temperature wise, as the drier air continues to filter into our area, expect efficient radiational cooling to result in lows slightly below normal, ranging from the lower 50s to mid 60s. && .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/... Main story through this period is a cold front at the surface which will approach our region late in the day. There may be enough lift with this to once again result in isolated convective initiation across the higher terrain of eastern PA and NW NJ. In the mid and upper levels, an approaching trough will result in decreasing 1000 to 500 mb thicknesses which should also translate to slightly lower temperatures than what we have seen lately. Highs are expected to range from the upper 60s in the higher terrain of the Poconos to the lower 80s across central Delmarva. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... Main forecast challenges for the long term include a deep trough passage around midweek and increasing warmth and precipitation chances late week into this weekend. A potent shortwave trough is poised to move through the larger- scale trough in eastern North America early this week, with passage through the northeastern U.S. in the Tuesday-Wednesday time frame. Large-scale ascent in advance of the attendant vorticity maximum will allow for increased cloud cover on Tuesday and fairly cool temperatures for this time of year. Forecast highs are around 5-10 degrees below seasonal averages. Forecast confidence is a little below average on Tuesday, however, given some lingering disagreement among the operational models regarding strength/orientation of the trough axis, extent of large-scale ascent in advance of the trough in the Mid-Atlantic, and associated frontal timing. The 12Z GFS/CMC are fairly dry across the area, but the 12Z ECMWF does generate some precipitation along the front and offshore. Though there is much improved agreement on the specifics of a developing low off the coast with the 12Z simulations, the QPF discrepancies do not bode much confidence in the sensible weather details, particularly in southern/eastern portions of the area -- which will be in closer proximity to the developing low and the last to be passed by the remnant surface boundary moving through the region. Decided not to stray much from forecast continuity for this period. Upstream ridging will move into the East on Wednesday, with a broad surface high migrating to the coast by 00Z Thursday. This should allow for a dry day with continued pleasant (slightly below average) temperatures. As the surface ridge moves offshore by Thursday, return flow commences. Temperatures are likely to rapidly warm in this regime, with substantial warm/moist advection occurring in advance of another potent vorticity maximum moving into the Great Lakes and adjacent southern/southeast Canada by this time. Midlevel ridging in advance of the vort max should permit the warm sector to surge well north of the area, which will prevent associated surface fronts from reaching the area likely through the end of the long term period. This means the main questions regarding the Friday-Sunday forecast revolve around heat/humidity and chances for precipitation, as the rapid warmth of Thursday will likely be followed by more gradual increases in temps/dew points thereafter. Additionally, with the Mid-Atlantic becoming more and more on the fringes of midlevel ridging, large-scale ascent (pronounced in advance of west-to-east moving vorticity maxima) will at least glance the area, especially the northern/western CWA. As such, the models (to varying degrees) suggest increasing chances for precipitation as a surface boundary sags southward from Canada through this period. There is pretty decent agreement on timing/location of the first vorticity maximum on Thursday (generally in New York/New England), so kept PoPs pretty low in the CWA during this time frame (only mentionable in the southern Poconos and vicinity). However, Friday through Sunday feature increasing chances for convection as faster southwest flow slowly edges toward the area, though pinpointing exact locations and timing (the latter of which will be closely tied to upstream vorticity maxima this weekend) is a dubious exercise at this time range given the somewhat unpredictable pattern (and associated poor phasing agreement among the ECMWF/GFS/CMC). Generally broad-brushed slight chance to chance PoPs through the CWA this weekend as a result, with a general blend of the above-mentioned models and heavy weighting to continuity/WPC guidance. Regarding heat/humidity, kept the forecast fairly tame during this period (and close to statistical guidance) given potential complications from proximity convection. However, if the ridge remains more prevalent across the area, forecast could be too cool this period. Regardless, a sultry weekend is anticipated. && .AVIATION /22Z SUNDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Mostly VFR conditions are expected through the TAF period. There is a slight chance showers and thunderstorms after 18Z Monday across the higher terrain (including KRDG and KABE). If any showers move over the TAF sites, MVFR or lower conditions are possible, but it is too uncertain to include in any of the TAFs at this time. Winds will stay mostly light out of the west or southwest through the TAF period. The one exception is KACY, where south southeasterly winds are expected to persist through at least 00Z in the wake of a sea breeze. There is a chance a sea breeze could redevelop after 18Z Monday and once again shift the winds at KACY. OUTLOOK... Monday night and Tuesday: Predominantly VFR, though isolated/scattered showers/storms are possible. Winds generally west or southwest 5 to 15 kts with gusts to 20 kts during the day. Confidence average. Tuesday night through Wednesday: VFR. Winds generally northwest 5 to 15 kts. Confidence above average. Wednesday night and Thursday night: VFR. Winds generally southwest under 10 kts at night and 10 to 20 kts during the day. Confidence above average. Thursday night and Friday: Generally VFR, though there is an increasing chance for showers/storms, especially north/west of KPHL during this period. Winds generally southwest 5 to 15 kts with gusts to 20 kts during the day. Confidence below average. && .MARINE... Winds and seas are expected to stay below small craft advisory criteria tonight and tomorrow. Westerly wind gusts up to 20 kt are possible on the Atlantic coastal waters overnight, but should stay below 25 kt. OUTLOOK... Monday night through Wednesday night: Sub-advisory conditions expected. A slight chance of showers/storms on Tuesday and Tuesday night. Thursday through Friday: Advisory winds/seas possible as stronger southwesterly winds become established. A slight chance of storms on Friday. .RIP CURRENTS... The outlook for tomorrow is that the low risk for the formation of dangerous rip currents will continue. && .TIDES/COASTAL FLOODING... Very high astronomical tides are expected to continue today and tomorrow. There is a chance that minor tidal flooding could occur especially along the northern NJ shore with the high tide cycle this evening. However, models over the last few days have had a high bias, and the winds have been light though the day today. Therefore, do not expect widespread tidal flooding today. Tomorrow water levels should be slightly lower as well. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...CMS Near Term...Johnson/Miketta Short Term...Johnson Long Term...CMS Aviation...CMS/Johnson Marine...CMS/Johnson/Miketta Tides/Coastal Flooding...Johnson is the U.S. government's official web portal to all federal, state and local government web resources and services.